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Title:
On the Need for Deep-mixing in Asymptotic Giant Branch Stars of Low Mass

Authors:
Busso, M.; Palmerini, S.; Maiorca, E.; Cristallo, S.; Straniero, O.; Abia, C.; Gallino, R.; La Cognata, M.

Affiliation:
AA(Dipartimento di Fisica, Università di Perugia, Italy ; INFN, Sezione di Perugia, Italy ; ), AB(Dipartimento di Fisica, Università di Perugia, Italy ; INFN, Sezione di Perugia, Italy ), AC(Dipartimento di Fisica, Università di Perugia, Italy ; INFN, Sezione di Perugia, Italy ), AD(Departamento de Física Teórica y del Cosmos, Universidad de Granada, Spain ; INAF, Osservatorio di Teramo, Italy ), AE(INAF, Osservatorio di Teramo, Italy ; INFN, Sezione di Napoli, Italy ), AF(Departamento de Física Teórica y del Cosmos, Universidad de Granada, Spain ), AG(INAF, Osservatorio di Teramo, Italy ; Dipartimento di Fisica Generale, Università di Torino, Italy ), AH(Dipartimento di Metodologie Fisiche e Chimiche per l'Ingegneria, Università di Catania, Italy ; INFN, Laboratori Nazionali del Sud, Catania, Italy)

Publication:
The Astrophysical Journal Letters, Volume 717, Issue 1, pp. L47-L51 (2010). (ApJL Homepage)

Publication Date:
07/2010

Origin:
IOP

ApJ Keywords:
nuclear reactions, nucleosynthesis, abundances, stars: AGB and post-AGB, stars: carbon, stars: evolution, stars: low-mass

DOI:
10.1088/2041-8205/717/1/L47

Bibliographic Code:
2010ApJ...717L..47B

Abstract

The photospheres of low-mass red giants show CNO isotopic abundances that are not satisfactorily accounted for by canonical stellar models. The same is true for the measurements of these isotopes and of the ²⁶Al/²⁷Al ratio in presolar grains of circumstellar origin. Non-convective mixing, occurring during both red giant branch (RGB) and asymptotic giant branch (AGB) stages, is the explanation commonly invoked to account for the above evidence. Recently, the need for such mixing phenomena on the AGB was questioned, and chemical anomalies usually attributed to them were suggested to be formed in earlier phases. We have therefore re-calculated extra-mixing effects in low-mass stars for both the RGB and AGB stages, in order to verify the above claims. Our results contradict them; we actually confirm that slow transport below the convective envelope occurs also on the AGB. This is required primarily by the oxygen isotopic mix and the ²⁶Al content of presolar oxide grains. Other pieces of evidence exist, in particular from the isotopic ratios of carbon stars of type N, or C(N), in the Galaxy and in the LMC, as well as of SiC grains of AGB origin. We further show that, when extra-mixing occurs in the RGB phases of Population I stars above about 1.2 M _sun, this consumes ³He in the envelope, probably preventing the occurrence of thermohaline diffusion on the AGB. Therefore, we argue that other extra-mixing mechanisms should be active in those final evolutionary phases.

Title:		Fluorine Abundances in Galactic Asymptotic Giant Branch Stars
Authors:		Abia, C.; Cunha, K.; Cristallo, S.; de Laverny, P.; Domínguez, I.; Eriksson, K.; Gialanella, L.; Hinkle, K.; Imbriani, G.; Recio-Blanco, A.; Smith, V. V.; Straniero, O.; Wahlin, R.
Affiliation:		AA(Dpto. Física Teórica y del Cosmos, Universidad de Granada, 18071 Granada, Spain ), AB(National Optical Astronomy Observatory, P.O. Box 26732, Tucson, AZ 85726, USA ), AC(Dpto. Física Teórica y del Cosmos, Universidad de Granada, 18071 Granada, Spain ), AD(University of Nice-Sophia Antipolis, CNRS (UMR 6202), Cassiopée, Observatoire de la Côte d'Azur, B.P. 4229, 06304 Nice Cedex 4, France ), AE(Dpto. Física Teórica y del Cosmos, Universidad de Granada, 18071 Granada, Spain ), AF(Department of Physics & Astronomy, Uppsala University, Box 515, 751 20 Uppsala, Sweden ), AG(INFN Sezione di Napoli, Naples, Italy ), AH(National Optical Astronomy Observatory, P.O. Box 26732, Tucson, AZ 85726, USA ), AI(Dipt. di Scienze Fisiche, Universitá Federico II, Naples, Italy ), AJ(University of Nice-Sophia Antipolis, CNRS (UMR 6202), Cassiopée, Observatoire de la Côte d'Azur, B.P. 4229, 06304 Nice Cedex 4, France ), AK(National Optical Astronomy Observatory, P.O. Box 26732, Tucson, AZ 85726, USA ), AL(INAF-Osservatorio di Collurania, 64100 Teramo, Italy), AM(Department of Physics & Astronomy, Uppsala University, Box 515, 751 20 Uppsala, Sweden )
Publication:		The Astrophysical Journal Letters, Volume 715, Issue 2, pp. L94-L98 (2010). (ApJL Homepage)
Publication Date:		06/2010
Origin:		IOP
ApJ Keywords:		nuclear reactions, nucleosynthesis, abundances, stars: abundances, stars: AGB and post-AGB, stars: carbon
DOI:		10.1088/2041-8205/715/2/L94
Bibliographic Code:		2010ApJ...715L..94A

Abstract

An analysis of the fluorine abundance in Galactic asymptotic giant branch (AGB) carbon stars (24 N-type, 5 SC-type, and 5 J-type) is presented. This study uses the state-of-the-art carbon-rich atmosphere models and improved atomic and molecular line lists in the 2.3 μm region. Significantly lower F abundances are obtained in comparison to previous studies in the literature. This difference is mainly due to molecular blends. In the case of carbon stars of SC-type, differences in the model atmospheres are also relevant. The new F enhancements are now in agreement with the most recent theoretical nucleosynthesis models in low-mass AGB stars, solving the long-standing problem of F in Galactic AGB stars. Nevertheless, some SC-type carbon stars still show larger F abundances than predicted by stellar models. The possibility that these stars are of larger mass is briefly discussed.

Title:		s-Process in low-metallicity stars - I. Theoretical predictions
Authors:		Bisterzo, S.; Gallino, R.; Straniero, O.; Cristallo, S.; Käppeler, F.
Affiliation:		AA(Dipartimento di Fisica Generale, Università di Torino, Via P. Giuria 1, 10125 Torino, Italy), AB(Dipartimento di Fisica Generale, Università di Torino, Via P. Giuria 1, 10125 Torino, Italy), AC(INAF Osservatorio Astronomico di Collurania, via M. Maggini, 64100 Teramo, Italy), AD(INAF Osservatorio Astronomico di Collurania, via M. Maggini, 64100 Teramo, Italy; Departamento de Fisica Teorica y del Cosmos, Universidad de Granada, Campus de Fuentenueva, 18071 Granada, Spain), AE(Karlsruhe Institute of Technology, Campus Nord, Forschungszentrum Karlsruhe, Institut für Kernphysik, D-76021 Karlsruhe, Germany)
Publication:		Monthly Notices of the Royal Astronomical Society, Volume 404, Issue 3, pp. 1529-1544. (MNRAS Homepage)
Publication Date:		05/2010
Origin:		WILEY
MNRAS Keywords:		nuclear reactions, nucleosynthesis, abundances, stars: carbon, stars: Population II
DOI:		10.1111/j.1365-2966.2010.16369.x
Bibliographic Code:		2010MNRAS.404.1529B

Abstract

A large sample of carbon-enhanced metal-poor stars enriched in s-process elements (CEMP-s) have been observed in the Galactic halo. These stars of low mass (M ~ 0.9M_solar) are located on the main-sequence or the red-giant phase, and do not undergo third dredge-up (TDU) episodes. The s-process enhancement is most plausibly due to accretion in a binary system from a more massive companion when on the asymptotic giant branch (AGB) phase (now a white dwarf). In order to interpret the spectroscopic observations, updated AGB models are needed to follow in detail the s-process nucleosynthesis. We present nucleosynthesis calculations based on AGB stellar models obtained with Frascati Raphson-Newton Evolutionary Code (FRANEC) for low initial stellar masses and low metallicities. For a given metallicity, a wide spread in the abundance of the s-process elements is obtained by varying the amount of ¹³C and its profile in the pocket, where the ¹³C(α, n)¹⁶O reaction is the major neutron source, releasing neutrons in radiative conditions during the interpulse phase. We also account for the second neutron source ²²Ne(α, n)²⁵Mg, partially activated during convective thermal pulses. We discuss the surface abundance of elements from carbon to bismuth, for AGB models of initial masses M = 1.3-2M_solar, low metallicities ([Fe/H] from -1 down to -3.6) and for different ¹³C-pocket efficiencies. In particular, we analyse the relative behaviour of the three s-process peaks: light-s (ls at magic neutron number N = 50), heavy-s (hs at N = 82) and lead (N = 126). Two s-process indicators, [hs/ls] and [Pb/hs], are needed in order to characterize the s-process distribution. In the on-line material, we provide a set of data tables with surface predictions. Our final objective is to provide a full set of theoretical models of low-mass low-metallicity s-process-enhanced stars. In a forthcoming paper, we will test our results through a comparison with observations of CEMP-s stars.

Title:		Optical and near-infrared coverage of SN 2004et: physical parameters and comparison with other Type IIP supernovae
Authors:		Maguire, K.; di Carlo, E.; Smartt, S. J.; Pastorello, A.; Tsvetkov, D. Yu.; Benetti, S.; Spiro, S.; Arkharov, A. A.; Beccari, G.; Botticella, M. T.; Cappellaro, E.; Cristallo, S.; Dolci, M.; Elias-Rosa, N.; Fiaschi, M.; Gorshanov, D.; Harutyunyan, A.; Larionov, V. M.; Navasardyan, H.; Pietrinferni, A.; Raimondo, G.; di Rico, G.; Valenti, S.; Valentini, G.; Zampieri, L.
Affiliation:		AA(Astrophysics Research Centre, School of Maths and Physics, Queen's University Belfast, Belfast BT7 1NN), AB(INAF Osservatorio Astronomico Collurania di Teramo, Via Mentore Maggini, I-64100 Teramo, Italy), AC(Astrophysics Research Centre, School of Maths and Physics, Queen's University Belfast, Belfast BT7 1NN), AD(Astrophysics Research Centre, School of Maths and Physics, Queen's University Belfast, Belfast BT7 1NN), AE(Sternberg Astronomical Institute, University Ave. 13, 119992 Moscow, Russia), AF(INAF Osservatorio Astronomico di Padova, Vicolo dell' Osservatorio 5, I-35122 Padova, Italy), AG(Dipartimento di Fisica, Universitá di Roma Tor Vergata, Via della Ricerca scientifica 1, 00133 Roma, Italy; INAF Osservatorio Astronomico di Roma, Via di Frascati 33, 00040 Monte Porzio Catone, Italy), AH(Pulkovo Central Astronomical Observatory, Pulkovskoe shosse 65, 196140 St. Petersberg, Russia; Astronomical Institute of St Petersburg State University, Universitetskij Prospect 28, Petrodvorets, 198504 St. Petersburg, Russia), AI(ESA/ESTEC, Keplerlaab 1, 2200 AG Noordwijk, the Netherlands), AJ(Astrophysics Research Centre, School of Maths and Physics, Queen's University Belfast, Belfast BT7 1NN), AK(INAF Osservatorio Astronomico di Padova, Vicolo dell' Osservatorio 5, I-35122 Padova, Italy), AL(INAF Osservatorio Astronomico Collurania di Teramo, Via Mentore Maggini, I-64100 Teramo, Italy; Departamento de Fisica Teorica y del Cosmos, Universidad de Granada, Spain), AM(INAF Osservatorio Astronomico Collurania di Teramo, Via Mentore Maggini, I-64100 Teramo, Italy), AN(INAF Osservatorio Astronomico di Padova, Vicolo dell' Osservatorio 5, I-35122 Padova, Italy; Spitzer Science Center, California Institute of Technology, 1200 E. California Blvd., Pasadena, CA 91125, USA), AO(Dipartimento di Astronomia, Universitá di Padova, Vicolo dell Osservatorio 2, I-35122 Padova, Italy), AP(Pulkovo Central Astronomical Observatory, Pulkovskoe shosse 65, 196140 St. Petersberg, Russia), AQ(INAF Osservatorio Astronomico di Padova, Vicolo dell' Osservatorio 5, I-35122 Padova, Italy; Fundación Galileo Galilei - INAF, Apartado 565, E-38700 Santa Cruz de La Palma, Spain), AR(Pulkovo Central Astronomical Observatory, Pulkovskoe shosse 65, 196140 St. Petersberg, Russia; Astronomical Institute of St Petersburg State University, Universitetskij Prospect 28, Petrodvorets, 198504 St. Petersburg, Russia), AS(INAF Osservatorio Astronomico di Padova, Vicolo dell' Osservatorio 5, I-35122 Padova, Italy), AT(INAF Osservatorio Astronomico Collurania di Teramo, Via Mentore Maggini, I-64100 Teramo, Italy), AU(INAF Osservatorio Astronomico Collurania di Teramo, Via Mentore Maggini, I-64100 Teramo, Italy), AV(INAF Osservatorio Astronomico Collurania di Teramo, Via Mentore Maggini, I-64100 Teramo, Italy), AW(Astrophysics Research Centre, School of Maths and Physics, Queen's University Belfast, Belfast BT7 1NN), AX(INAF Osservatorio Astronomico Collurania di Teramo, Via Mentore Maggini, I-64100 Teramo, Italy), AY(INAF Osservatorio Astronomico di Padova, Vicolo dell' Osservatorio 5, I-35122 Padova, Italy)
Publication:		Monthly Notices of the Royal Astronomical Society, Volume 404, Issue 2, pp. 981-1004. (MNRAS Homepage)
Publication Date:		05/2010
Origin:		WILEY
MNRAS Keywords:		supernovae: general, supernovae: individual: 2004et, supernovae: individual: 2004A, supernovae: individual: 2006my
Abstract Copyright:		(c) Journal compilation © 2010 RAS
DOI:		10.1111/j.1365-2966.2010.16332.x
Bibliographic Code:		2010MNRAS.404..981M

Abstract

We present new optical and near-infrared (NIR) photometry and spectroscopy of the Type IIP supernova (SN), SN 2004et. In combination with already published data, this provides one of the most complete studies of optical and NIR data for any Type IIP SN from just after explosion to +500d. The contribution of the NIR flux to the bolometric light curve is estimated to increase from 15 per cent at explosion to around 50 per cent at the end of the plateau and then declines to 40 per cent at 300d. SN 2004et is one of the most luminous IIP SNe which has been well studied and characterized, and with a luminosity of logL = 42.3ergs^-1 and a ⁵⁶Ni mass of 0.06 +/- 0.04M_solar, it is two times brighter than SN 1999em. We provide parametrized bolometric corrections as a function of time since explosion for SN 2004et and three other IIP SNe that have extensive optical and NIR data. These can be used as templates for future events in optical and NIR surveys without full wavelength coverage. We compare the physical parameters of SN 2004et with those of other well-studied IIP SNe and find that the kinetic energies span a range of 10⁵⁰-10⁵¹erg. We compare the ejected masses calculated from hydrodynamic models with the progenitor masses and limits derived from pre-discovery images. Some of the ejected mass estimates are significantly higher than the progenitor mass estimates, with SN 2004et showing perhaps the most serious mass discrepancy. With the current models, it appears difficult to reconcile 100d plateau lengths and high expansion velocities with the low ejected masses of 5-6M_solar implied from 7-8M_solar progenitors. The nebular phase is studied using very late-time Hubble Space Telescope photometry, along with optical and NIR spectroscopy. The light curve shows a clear flattening at 600d in the optical and the NIR, which is likely due to the ejecta impacting on circumstellar material. We further show that the [OI] 6300, 6364Å line strengths in the nebular spectra of four Type IIP SNe imply ejected oxygen masses of 0.5-1.5M_solar.

Title:		The long-standing problem of ¹⁷⁶Lu/¹⁷⁶Hf branching: A new approach with full stellar evolutionary models
Authors:		Cristallo, S.; Piersanti, L.; Gallino, R.; Straniero, O.; Käppeler, F.; Domínguez, I.; Mohr, P.
Affiliation:		AA(D.pto Física Teorica y del Cosmo, Universidad de Granada, 18071 Granada, Spain ; Osservatorio Astronomico di Teramo – INAF, 64100 Teramo, Italy ), AB(Osservatorio Astronomico di Teramo – INAF, 64100 Teramo, Italy ), AC(D.pto Fisica Generale, Universitá di Torino, 10125 Torino, Italy ), AD(Osservatorio Astronomico di Teramo – INAF, 64100 Teramo, Italy ), AE(Forschungszentrum Karlsruhe, Institut für Kernphysik, D-76021 Karlsruhe, Germany ), AF(D.pto Física Teorica y del Cosmo, Universidad de Granada, 18071 Granada, Spain ), AG(Diakonie-Klinikum Schwäbisch Hall, D-74523 Schwäbisch Hall, Germany)
Publication:		Journal of Physics: Conference Series, Volume 202, Issue 1, pp. 012033 (2010).
Publication Date:		01/2010
Origin:		IOP
DOI:		10.1088/1742-6596/202/1/012033
Bibliographic Code:		2010JPhCS.202a2033C

Abstract

We present an analysis of the s-process branching at A=176, which is determined by the two s-only isotopes ¹⁷⁶Lu and ¹⁷⁶Hf. This work is motivated by the discrepancy between the current theoretical expectations and solar-system abundances of the two isotopes involved in the branching. Asymptotic Giant Branch models have been calculated with the FRANEC evolutionary code, coupling a full nuclear reaction network with the equation describing the physical evolution of the stellar structure. With respect to previous studies, we can better follow the complex interplay between mixing and burning, which both strongly influence this branching. We show that a substantial reduction of the mixing velocity within the convective zone generated by a He-shell flash (or thermal pulse) may alleviate the discrepancy between the theoretical expectations and the observed solar abundances. Nonetheless, we reckon that a definitive solution of this problem has to be searched in the nuclear coupling scheme between the thermally populated levels in ¹⁷⁶Lu.

Title:		The chemical composition of carbon stars. The R-type stars
Authors:		Zamora, O.; Abia, C.; Plez, B.; Domínguez, I.; Cristallo, S.
Affiliation:		AA(Departamento de Física Teórica y del Cosmos, Universidad de Granada, 18071 Granada, Spain ), AB(Departamento de Física Teórica y del Cosmos, Universidad de Granada, 18071 Granada, Spain), AC(GRAAL, Université Montpellier II, CNRS, 34095 Montpellier Cedex 5, France), AD(Departamento de Física Teórica y del Cosmos, Universidad de Granada, 18071 Granada, Spain), AE(Departamento de Física Teórica y del Cosmos, Universidad de Granada, 18071 Granada, Spain; INAF, Osservatorio Astronomico di Collurania, 64100 Teramo, Italy)
Publication:		Astronomy and Astrophysics, Volume 508, Issue 2, 2009, pp.909-922 (A&A Homepage)
Publication Date:		12/2009
Origin:		EDP Sciences
Keywords:		stars: abundances, stars: chemically peculiar, stars: carbon, stars: AGB and post-AGB
DOI:		10.1051/0004-6361/200912843
Bibliographic Code:		2009A&A...508..909Z

Abstract

Aims. The aim of this work is to shed some light on the problem of the formation of carbon stars of R-type from a detailed study of their chemical composition.
Methods: We use high-resolution and high signal-to-noise optical spectra of 23 R-type stars (both early- and late-types) selected from the Hipparcos catalogue. The chemical analysis is made using spectral synthesis in LTE and state-of-the-art carbon-rich spherical model atmospheres. We derive their CNO content (including the ¹²C/¹³C ratio), average metallicity, lithium, and light (Sr, Y, Zr) and heavy (Ba, La, Nd, Sm) s-element abundances. The observed properties of the stars (galactic distribution, kinematics, binarity, photometry and luminosity) are also discussed.
Results: Our analysis shows that late-R stars are carbon stars with identical chemical and observational characteristics as the normal (N-type) AGB carbon stars. The s-element abundance pattern derived can be reproduced by low-mass AGB nucleosynthesis models where the ¹³C(α, n)¹⁶O reaction is the main neutron donor. We confirm the results of the sole previous abundance analysis of early-R stars, namely that they are carbon stars with near solar metallicity showing enhanced nitrogen, low ¹²C/¹³C ratios and no s-element enhancements. In addition, we have found that early-R stars have Li abundances larger than expected for post RGB tip giants. We also find that a significant number (~40%) of the early-R stars in our sample are wrongly classified, probably being classical CH stars and normal K giants.
Conclusions: On the basis of the chemical analysis, we confirm the previous suggestion that late-R stars are just misclassified N-type carbon stars in the AGB phase of evolution. Their photometric, kinematic, variability and luminosity properties are also compatible with this. In consequence, we suggest that the number of true R stars is considerably lower than previously believed. This alleviates the problem of considering R stars as a frequent stage in the evolution of low-mass stars. We briefly discuss the different scenarios proposed for the formation of early-R stars. The mixing of carbon during an anomalous He-flash is favoured, although no physical mechanism able to trigger that mixing has been found yet. The origin of these stars still remains a mystery.

Title:		Fluorine Abundances in AGB Carbon Stars: New Results?
Authors:		Abia, C.; de Laverny, P.; Recio-Blanco, A.; Domínguez, I.; Cristallo, S.; Straniero, O.
Affiliation:		AA(Dpto. Física Teórica y del Cosmos, Universidad de Granada, 18071 Granada, Spain; ), AB(Observatoire de la Côte d'Azur, Dpt. Cassiopée UMR 6202, 06304 Nice Cedex 4, France), AC(Observatoire de la Côte d'Azur, Dpt. Cassiopée UMR 6202, 06304 Nice Cedex 4, France), AD(Dpto. Física Teórica y del Cosmos, Universidad de Granada, 18071 Granada, Spain), AE(INAF-Osservatorio di Collurania, 64100 Teramo, Italy), AF(INAF-Osservatorio di Collurania, 64100 Teramo, Italy)
Publication:		Publications of the Astronomical Society of Australia, Volume 26, Issue 3, pp. 351-353. (PASA Homepage)
Publication Date:		09/2009
Origin:		PASA
Keywords:		nuclear reactions, nucleosynthesis, abundances, stars: abundances, stars: AGB, stars: carbon, stars: evolution
DOI:		10.1071/AS08038
Bibliographic Code:		2009PASA...26..351A

Abstract

A recent reanalysis of the fluorine abundance in three Galactic Asymptotic Giant Branch (AGB) carbon stars (TX Psc, AQ Sgr and R Scl) by Abia et al. (2009) results in estimates of fluorine abundances systematically lower by ~0.8 dex on average, with respect to the sole previous estimates by Jorissen, Smith & Lambert (1992). The new F abundances are in better agreement with the predictions of full-network stellar models of low-mass (<3 M_solar) AGB stars.

Title:		montage: AGB Nucleosynthesis with Full s-Process Calculations
Authors:		Church, R. P.; Cristallo, S.; Lattanzio, J. C.; Stancliffe, R. J.; Straniero, O.; Cannon, R. C.
Affiliation:		AA(Centre for Stellar and Planetary Astrophysics, School of Mathematical Sciences, Monash University, Victoria 3800; ), AB(INAF, Osservatorio Astronomico di Collurania, 64100 Teramo, Italy), AC(Centre for Stellar and Planetary Astrophysics, School of Mathematical Sciences, Monash University, Victoria 3800), AD(Centre for Stellar and Planetary Astrophysics, School of Mathematical Sciences, Monash University, Victoria 3800), AE(INAF, Osservatorio Astronomico di Collurania, 64100 Teramo, Italy), AF(Textensor Limited, 37 McDonald Road, Edinburgh, EH7 4LY, Scotland)
Publication:		Publications of the Astronomical Society of Australia, Volume 26, Issue 3, pp. 217-224. (PASA Homepage)
Publication Date:		09/2009
Origin:		PASA
Keywords:		methods: numerical, nucleosynthesis, stars: abundances, stars: AGB and post-AGB
DOI:		10.1071/AS09008
Bibliographic Code:		2009PASA...26..217C

Abstract

We present montage, a post-processing nucleosynthesis code that combines a traditional network for isotopes lighter than calcium with a rapid algorithm for calculating the s-process nucleosynthesis of the heavier isotopes. The separation of those parts of the network where only neutron-capture and beta-decay reactions are significant provides a substantial advantage in computational efficiency. We present the yields for a complete set of s-process isotopes for a 3-M_solar, Z = 0.02 stellar model, as a demonstration of the utility of the approach. Future work will include a large grid of models suitable for use in calculations of Galactic chemical evolution.

Title:		Barium Stars: Theoretical Interpretation
Authors:		Husti, Laura; Gallino, Roberto; Bisterzo, Sara; Straniero, Oscar; Cristallo, Sergio
Affiliation:		AA(Dipartimento di Fisica Generale, Università degli Studi di Torino, via P. Giuria 1, 10125 Torino, Italia; Research Centre for Atomic Physics and Astrophysics, University of Bucharest, P.O. Box MG-6, RO-077125 Bucharest-Magurele, Romania; ), AB(Dipartimento di Fisica Generale, Università degli Studi di Torino, via P. Giuria 1, 10125 Torino, Italia), AC(Dipartimento di Fisica Generale, Università degli Studi di Torino, via P. Giuria 1, 10125 Torino, Italia), AD(INAF Osservatorio Astronomico di Collurania, via M. Maggini, 64100 Teramo, Italy), AE(INAF Osservatorio Astronomico di Collurania, via M. Maggini, 64100 Teramo, Italy)
Publication:		Publications of the Astronomical Society of Australia, Volume 26, Issue 3, pp. 176-183. (PASA Homepage)
Publication Date:		09/2009
Origin:		PASA
Keywords:		stars: abundances, stars: AGB and post-AGB, (stars:) binaries: general, stars: chemically peculiar, stars: mass loss,
DOI:		10.1071/AS08065
Bibliographic Code:		2009PASA...26..176H

Abstract

Barium stars are extrinsic Asymptotic Giant Branch (AGB) stars. They present the s-enhancement characteristic for AGB and post-AGB stars, but are in an earlier evolutionary stage (main sequence dwarfs, subgiants, red giants). They are believed to form in binary systems, where a more massive companion evolved faster, produced the s-elements during its AGB phase, polluted the present barium star through stellar winds and became a white dwarf. The samples of barium stars of Allen & Barbuy (2006) and of Smiljanic et al. (2007) are analysed here. Spectra of both samples were obtained at high-resolution and high S/N. We compare these observations with AGB nucleosynthesis models using different initial masses and a spread of ¹³C-pocket efficiencies. Once a consistent solution is found for the whole elemental distribution of abundances, a proper dilution factor is applied. This dilution is explained by the fact that the s-rich material transferred from the AGB to the nowadays observed stars is mixed with the envelope of the accretor. We also analyse the mass transfer process, and obtain the wind velocity for giants and subgiants with known orbital period. We find evidence that thermohaline mixing is acting inside main sequence dwarfs and we present a method for estimating its depth.

Title:		Asymptotic Giant Branch Stars and Their Influence on the Isotopic Compositions of the Transition Elements
Authors:		Davis, A. M.; Gallino, R.; Cristallo, S.; Straniero, O.
Publication:		72nd Annual Meeting of the Meteoritical Society, held July 13-18, 2009 in Nancy, France. Published in Meteoritics and Planetary Science Supplement., p.5402
Publication Date:		09/2009
Origin:		LPI
Bibliographic Code:		2009M&PSA..72.5402D

Abstract

Not Available

Title:		Asymptotic-Giant-Branch Models at Very Low Metallicity
Authors:		Cristallo, S.; Piersanti, L.; Straniero, O.; Gallino, R.; Domínguez, I.; Käppeler, F.
Affiliation:		AA(Osservatorio Astronomico di Teramo (INAF), via Maggini 64100 Teramo, Italy; ), AB(Osservatorio Astronomico di Teramo (INAF), via Maggini 64100 Teramo, Italy), AC(Osservatorio Astronomico di Teramo (INAF), via Maggini 64100 Teramo, Italy), AD(Dipartimento di Fisica Generale, Universitá di Torino, via P. Giuria 1, 10125 Torino, Italy), AE(Departamento de Física Teórica y del Cosmos, Universidad de Granada, 18071 Granada, Spain), AF(Forschungszentrum Karlsruhe, Institut für Kernphysik Postfach 3460, D-76021 Karlsruhe, Germany)
Publication:		Publications of the Astronomical Society of Australia, Volume 26, Issue 3, pp. 139-144. (PASA Homepage)
Publication Date:		08/2009
Origin:		PASA
Keywords:		nuclear reactions, nucleosynthesis, abundances, stars: AGB and post-AGB,
DOI:		10.1071/AS09003
Bibliographic Code:		2009PASA...26..139C

Abstract

In this paper we present the evolution of a low-mass model (initial mass M = 1.5 M_solar) with a very low metal content (Z = 5 × 10^-5, equivalent to [Fe/H] = -2.44). We find that, at the beginning of the Asymptotic Giant Branch (AGB) phase, protons are ingested from the envelope in the underlying convective shell generated by the first fully developed thermal pulse. This peculiar phase is followed by a deep third dredge-up episode, which carries to the surface the freshly synthesized ¹³C, ¹⁴N and ⁷Li. A standard thermally pulsing AGB (TP-AGB) evolution then follows. During the proton-ingestion phase, a very high neutron density is attained and the s process is efficiently activated. We therefore adopt a nuclear network of about 700 isotopes, linked by more than 1200 reactions, and we couple it with the physical evolution of the model. We discuss in detail the evolution of the surface chemical composition, starting from the proton ingestion up to the end of the TP-AGB phase.

Title:		The ¹³C Pocket in Low-Mass AGB Stars
Authors:		Straniero, O.; Cristallo, S.; Gallino, R.
Affiliation:		AA(INAF-Osservatorio Astronomico di Teramo, Via Mentore Maggini s.n.c., 64100 Teramo, Italy; ), AB(INAF-Osservatorio Astronomico di Teramo, Via Mentore Maggini s.n.c., 64100 Teramo, Italy), AC(Dipartimento di Fisica Generale, Universitá di Torino, Via P. Giuria 1, 10125 Torino, Italy)
Publication:		Publications of the Astronomical Society of Australia, Volume 26, Issue 3, pp. 133-138. (PASA Homepage)
Publication Date:		08/2009
Origin:		PASA
Keywords:		nuclear reactions, nucleosynthesis, abundances, stars: AGB and post-AGB,
DOI:		10.1071/AS09002
Bibliographic Code:		2009PASA...26..133S

Abstract

It is well known that thermally pulsing Asymptotic Giant Branch stars with low mass play a relevant role in the chemical evolution. They have synthesized about 30% of the galactic carbon and provide an important contribution to the nucleosynthesis of heavy elements (A > 80). The relevant nucleosynthesis site is the He-rich intermediate zone (less than 10^-2 M_solar), where α(2α,γ)¹²C reactions and slow neutron captures on seed nuclei (essentially iron) take place. A key ingredient is the interplay between nuclear processes and convective mixing. It is the partial overlap of internal and external convective zones that allows the dredge-up of the material enriched in C and heavy elements. We review the progresses made in the last 50 years in the comprehension of the s process in AGB stars, with special attention to the identification of the main neutron sources and to the particular physical conditions allowing this important nucleosynthesis.

Title:		The puzzling dredge-up pattern in NGC 1978
Authors:		Lederer, M. T.; Lebzelter, T.; Cristallo, S.; Straniero, O.; Hinkle, K. H.; Aringer, B.
Affiliation:		AA(University of Vienna, Department of Astronomy, Türkenschanzstraße 17, 1180 Vienna, Austria ), AB(University of Vienna, Department of Astronomy, Türkenschanzstraße 17, 1180 Vienna, Austria ), AC(INAF, Osservatorio Astronomico di Collurania, 64100 Teramo, Italy ), AD(INAF, Osservatorio Astronomico di Collurania, 64100 Teramo, Italy ), AE(National Optical Astronomy Observatories (Operated by the Association of Universities for Research in Astronomy, under cooperative agreement with the National Science Foundation.) , PO Box 26732, Tucson, AZ 85726, USA ), AF(University of Vienna, Department of Astronomy, Türkenschanzstraße 17, 1180 Vienna, Austria ; INAF, Osservatorio Astronomico di Padova, Vicolo dell'Osservatorio 5, 35122 Padova, Italy)
Publication:		Astronomy and Astrophysics, Volume 502, Issue 3, 2009, pp.913-927 (A&A Homepage)
Publication Date:		08/2009
Origin:		EDP Sciences
Keywords:		stars: abundances, stars: AGB and post-AGB, stars: evolution
DOI:		10.1051/0004-6361/200911857
Bibliographic Code:		2009A&A...502..913L

Abstract

Context: Low-mass stars are element factories that efficiently release their products in the final stages of their evolution by means of stellar winds. Since they are large in number, they contribute significantly to the cosmic matter cycle. To assess this contribution quantitatively, it is crucial to obtain a detailed picture of the stellar interior, particularly with regard to nucleosynthesis and mixing mechanisms.
Aims: We seek to benchmark stellar evolutionary models of low-mass stars. In particular, we measure the surface abundance of ¹²C in thermally pulsing AGB stars with well-known mass and metallicity, which can be used to infer information about the onset and efficiency of the third dredge-up.
Methods: We recorded high-resolution near-infrared spectra of AGB stars in the LMC cluster NGC 1978. The sample comprised both oxygen-rich and carbon-rich stars, and is well-constrained in terms of the stellar mass, metallicity, and age. We derived the C/O and ¹²C/¹³C ratio from the target spectra by a comparison to synthetic spectra. Then, we compared the outcomes of stellar evolutionary models with our measurements.
Results: The M stars in NGC 1978 show values of C/O and ¹²C/¹³C that can best be explained with moderate extra-mixing on the RGB coupled to a moderate oxygen enhancement in the chemical composition. These oxygen-rich stars do not seem to have undergone third dredge-up episodes (yet). The C stars show carbon-to-oxygen and carbon isotopic ratios consistent with the occurrence of the third dredge-up. We did not find S stars in this cluster. None of the theoretical schemes that we considered was able to reproduce the observations appropriately. Instead, we discuss some non-standard scenarios to explain the puzzling abundance pattern in NGC 1978.

Title:		Evolution, Nucleosynthesis, and Yields of Low-Mass Asymptotic Giant Branch Stars at Different Metallicities
Authors:		Cristallo, S.; Straniero, O.; Gallino, R.; Piersanti, L.; Domínguez, I.; Lederer, M. T.
Affiliation:		AA(INAF-Osservatorio Astronomico di Collurania, 64100 Teramo, Italy ), AB(INAF-Osservatorio Astronomico di Collurania, 64100 Teramo, Italy ), AC(Dipartimento di Fisica Generale, Universitá di Torino, 10125 Torino, Italy ; Center for Stellar and Planetary Astrophysics, School of Mathematical Sciences, Monash University, P.O. Box 28, Victoria 3800, Australia ), AD(INAF-Osservatorio Astronomico di Collurania, 64100 Teramo, Italy ), AE(Departamento de Física Teórica y del Cosmos, Universidad de Granada, 18071 Granada, Spain ), AF(Institut für Astronomie, Türkenschanzstraße 17, A-1180 Wien, Austria)
Publication:		The Astrophysical Journal, Volume 696, Issue 1, pp. 797-820 (2009). (ApJ Homepage)
Publication Date:		05/2009
Origin:		IOP
ApJ Keywords:		nuclear reactions, nucleosynthesis, abundances, stars: AGB and post-AGB
DOI:		10.1088/0004-637X/696/1/797
Bibliographic Code:		2009ApJ...696..797C

Abstract

The envelope of thermally pulsing asymptotic giant branch (TP-AGB) stars undergoing periodic third dredge-up (TDU) episodes is enriched in both light and heavy elements, the ashes of a complex internal nucleosynthesis involving p, α, and n captures over hundreds of stable and unstable isotopes. In this paper, new models of low-mass AGB stars (2 M _sun), with metallicity ranging between Z = 0.0138 (the solar one) and Z = 0.0001, are presented. Main features are (1) a full nuclear network (from H to Bi) coupled to the stellar evolution code, (2) a mass loss-period-luminosity relation, based on available data for long-period variables, and (3) molecular and atomic opacities for C- and/or N-enhanced mixtures, appropriate for the chemical modifications of the envelope caused by the TDU. For each model, a detailed description of the physical and chemical evolutions is presented; moreover, we present a uniform set of yields, comprehensive of all chemical species (from hydrogen to bismuth). The main nucleosynthesis site is the thin ¹³C pocket, which forms in the core-envelope transition region after each TDU episode. The formation of this ¹³C pocket is the principal by-product of the introduction of a new algorithm, which shapes the velocity profile of convective elements at the inner border of the convective envelope: both the physical grounds and the calibration of the algorithm are discussed in detail. We find that the pockets shrink (in mass) as the star climbs the AGB, so that the first pockets, the largest ones, leave the major imprint on the overall nucleosynthesis. Neutrons are released by the ¹³C(α, n)¹⁶O reaction during the interpulse phase in radiative conditions, when temperatures within the pockets attain T ~ 1.0 × 10⁸ K, with typical densities of (10⁶-10⁷) neutrons cm^-3. Exceptions are found, as in the case of the first pocket of the metal-rich models (Z = 0.0138, Z = 0.006 and Z = 0.003), where the ¹³C is only partially burned during the interpulse: the surviving part is ingested in the convective zone generated by the subsequent thermal pulse (TP) and then burned at T ~ 1.5 × 10⁸ K, thus producing larger neutron densities (up to 10¹¹ neutrons cm^-3). An additional neutron exposure, caused by the ²²Ne(α, n)²⁵Mg during the TPs, is marginally activated at large Z, but becomes an important nucleosynthesis source at low Z, when most of the ²²Ne is primary. The final surface compositions of the various models reflect the differences in the initial iron-seed content and in the physical structure of AGB stars belonging to different stellar populations. Thus, at large metallicities the nucleosynthesis of light s-elements (Sr, Y, Zr) is favored, whilst, decreasing the iron content, the overproduction of heavy s-elements (Ba, La, Ce, Nd, Sm) and lead becomes progressively more important. At low metallicities (Z = 0.0001) the main product is lead. The agreement with the observed [hs/ls] index observed in intrinsic C stars at different [Fe/H] is generally good. For the solar metallicity model, we found an interesting overproduction of some radioactive isotopes, like ⁶⁰Fe, as a consequence of the anomalous first ¹³C pocket. Finally, light elements (C, F, Ne, and Na) are enhanced at any metallicity.

Title:		Fluorine in Asymptotic Giant Branch Carbon Stars Revisited
Authors:		Abia, C.; Recio-Blanco, A.; de Laverny, P.; Cristallo, S.; Domínguez, I.; Straniero, O.
Affiliation:		AA(Departamento de Física Teórica y del Cosmos, Universidad de Granada, 18071 Granada, Spain ), AB(Observatoire de la Côte d'Azur, Department Cassiopée UMR 6202, 06304 Nice Cedex 4, France ), AC(Observatoire de la Côte d'Azur, Department Cassiopée UMR 6202, 06304 Nice Cedex 4, France ), AD(INAF-Osservatorio di Collurania, 64100 Teramo, Italy), AE(Departamento de Física Teórica y del Cosmos, Universidad de Granada, 18071 Granada, Spain ), AF(INAF-Osservatorio di Collurania, 64100 Teramo, Italy)
Publication:		The Astrophysical Journal, Volume 694, Issue 2, pp. 971-977 (2009). (ApJ Homepage)
Publication Date:		04/2009
Origin:		IOP
ApJ Keywords:		nuclear reactions, nucleosynthesis, abundances, stars: abundances, stars: AGB and post-AGB, stars: carbon
DOI:		10.1088/0004-637X/694/2/971
Bibliographic Code:		2009ApJ...694..971A

Abstract

A re-analysis of the fluorine abundance in three Galactic asymptotic giant branch (AGB) carbon stars (TX Psc, AQ Sgr, and R Scl) has been performed from the molecular HF (1-0) R9 line at 2.3358 μm. High resolution (R ~ 50,000) and high signal-to-noise spectra obtained with the CRIRES spectrograph and the VLT telescope or from the NOAO archive (for TX Psc) have been used. Our abundance analysis uses the latest generation of MARCS model atmospheres for cool carbon-rich stars. Using spectral synthesis in local thermodynamic equilibrium, we derive for these stars fluorine abundances that are systematically lower by ~0.8 dex in average with respect to the sole previous estimates by Jorissen et al. The possible reasons of this discrepancy are explored. We conclude that the difference may rely on the blending with C-bearing molecules (CN and C₂) that were not properly taken into account in the former study. The new F abundances are in better agreement with the prediction of full network stellar models of low-mass AGB stars. These models also reproduce the s-process elements distribution in the sampled stars. This result, if confirmed in a larger sample of AGB stars, might alleviate the current difficulty to explain the largest [F/O] ratios found by Jorissen et al. In particular, it may not be necessary to search for alternative nuclear chains affecting the production of F in AGB stars.

Title:		A study of AGB stars in LMC clusters
Authors:		Lebzelter, Thomas; Lederer, Michael T.; Cristallo, Sergio; Straniero, Oscar; Hinkle, Kenneth H.
Affiliation:		AA(Department of Astronomy, University of Vienna, Türkenschanzstraße 17, A-1180 Vienna, Austria ), AB(Department of Astronomy, University of Vienna, Türkenschanzstraße 17, A-1180 Vienna, Austria ), AC(INAF, Teramo, Italy ), AD(INAF, Teramo, Italy ), AE(NOAO, Tucson, USA )
Publication:		The Magellanic System: Stars, Gas, and Galaxies, Proceedings of the International Astronomical Union, IAU Symposium, Volume 256, p. 397-402
Publication Date:		03/2009
Origin:		CUP
Keywords:		stars: abundances, stars: AGB and post-AGB, stars: evolution, galaxies: individual (LMC), Magellanic Clouds, galaxies: star clusters
DOI:		10.1017/S1743921308028767
Bibliographic Code:		2009IAUS..256..397L

Abstract

LMC clusters offer an outstanding opportunity to investigate the late stages of stellar evolution of stars in the mass range between 1.5 and 2 M_ȯ. In this presentation we will focus on our results on mixing events during the evolution along the Asymptotic Giant Branch (AGB). Surface abundances have been determined for a number of cluster AGB stars from high resolution near infrared spectra. We show for the first time the evolution of C/O and ¹²C/¹³C ratios along a cluster AGB. The change of both quantities due to dredge up events is compared with model predictions. Our results indicate the late occurrence of a moderate extra-mixing in some cases.

Title:		Why galaxies care about Asymptotic Giant Branch Stars .
Authors:		Cristallo, S.; Straniero, O.; Gallino, R.; Piersanti, L.; Domínguez, I.; Lederer, M. T.
Affiliation:		AA( INAF - Osservatorio Astronomico di Collurania, via M. Maggini, 64100 Teramo, Italy), AB( INAF - Osservatorio Astronomico di Collurania, via M. Maggini, 64100 Teramo, Italy), AC(Dipartimento di Fisica Generale, Universitàdi Torino, via P. Giuria 1, 10125 Torino, Italy; Center for Stellar and Planetary Astrophysics, School of Mathematical Sciences, Monash University, PO Box 28, Victoria 3800 Australia), AD( INAF - Osservatorio Astronomico di Collurania, via M. Maggini, 64100 Teramo, Italy), AE(Center for Stellar and Planetary Astrophysics, School of Mathematical Sciences, Monash University, PO Box 28, Victoria 3800 Australia), AF(Departimento Fisica Teorica y del Cosmos, Universidad de Granada, 18071 Granada, Spain)
Publication:		Memorie della Società Astronomica Italiana, v.80, p.157 (2009)
Publication Date:		00/2009
Origin:		MmSAI
Keywords:		Stars: evolution, Stars: AGB and post-AGB, Stars: abundances
Bibliographic Code:		2009MmSAI..80..157C

Abstract

In the last 30 years much efforts have been devoted in modelling the longest phases characterizing the life of a star, i.e. core hydrogen burning and core helium burning. In the past, final phases of stellar evolution such as SuperNovae (SNe) phenomena (massive stars) or Asymptotic Giant Branch (AGB) (intermediate and low mass stars), have been quite often approached with oversimplified theories, due to evident difficulties in understanding and treating the involved physics. While SNe are fundamental tools for a correct comprehension of the expansion history of the Universe, AGB stars are as much precious objects for several reasons. Thanks to their very large luminosities, these stars are excellent tracers of halo structures and can be used to determine the extension of the systems they belong to; at the same time they give useful hints in revealing the presence of intermediate age stellar populations (see, e.g., the noisy case of the blue galaxy IZw18). Moreover, their emission in the Infra-Red (IR) band has strong consequences in determining the integrated colors of non resolved populations (about 80% of the IR emission comes in fact from this class of stars) and can be used in determining distances if fainter stars can not be resolved (through, e.g., the period-color-luminosity relation of MIRA stars in K band). Last but not least, AGB stars are important producers of light elements (such as carbon, nitrogen and fluorine) as well as heavy ones, being the production site of the main component of the slow neutron capture process (s-process). In this contribution, we will focus on this latter aspect.

Title:		52^nd Meeting of the Italian Astronomical Society
Authors:		Cantiello, M.; Cristallo, S.; Di Rico, G.; Pietrinferni, A.; Raimondo, G.
Publication:		Memorie della Società Astronomica Italiana, v.80, p.5 (2009)
Publication Date:		00/2009
Origin:		MmSAI
Bibliographic Code:		2009MmSAI..80....5C

Abstract

Not Available

Title:		AGB stars of the intermediate-age LMC cluster NGC 1846. II. Dredge up along the AGB
Authors:		Lebzelter, T.; Lederer, M. T.; Cristallo, S.; Hinkle, K. H.; Straniero, O.; Aringer, B.
Affiliation:		AA(Department of Astronomy, University of Vienna, Tuerkenschanzstrasse 17, A1180 Vienna, Austria ), AB(Department of Astronomy, University of Vienna, Tuerkenschanzstrasse 17, A1180 Vienna, Austria), AC(INAF, Osservatorio Astronomico di Collurania, 64100 Teramo, Italy), AD(National Optical Astronomy Observatories (Operated by the Association of Universities for Research in Astronomy, under cooperative agreement with the National Science Foundation.) , PO Box 26732, Tucson, AZ 85726, USA), AE(INAF, Osservatorio Astronomico di Collurania, 64100 Teramo, Italy), AF(Department of Astronomy, University of Vienna, Tuerkenschanzstrasse 17, A1180 Vienna, Austria; Dipartimento di Astronomia, Università di Padova, Vicolo dell'Osservatorio 3, 35122 Padova, Italy)
Publication:		Astronomy and Astrophysics, Volume 486, Issue 2, 2008, pp.511-521 (A&A Homepage)
Publication Date:		08/2008
Origin:		EDP Sciences
Keywords:		stars: AGB and post-AGB, stars: variables: general, galaxies, star clusters
DOI:		10.1051/0004-6361:200809363
Bibliographic Code:		2008A&A...486..511L

Abstract

Aims: We investigate the change in the surface abundance of ¹²C during the evolution along the AGB, aiming to constrain third dredge-up models.
Methods: High-resolution, near-infrared spectra of a sample of AGB stars in the LMC cluster NGC 1846 were obtained. A cluster sample ensures a high level of homogeneity with respect to age, metallicity, and distance. The C/O ratio and the ratio of ¹²C/¹³C were measured and compared with our evolutionary models.
Results: For the first time, we show the evolution of the C/O and ¹²C/¹³C ratios along a cluster AGB. Our findings allow us to check the reliability of the evolutionary models and, in particular, the efficiency of the third dredge up. The increase in both C/O and ¹²C/¹³C in the observed O-rich stars is reproduced by the models well. However, the low carbon isotopic ratios of the two C-stars in our sample indicate the late occurrence of moderate extra mixing. The extra mixing affects the most luminous AGB stars and is capable of increasing the abundance of ¹³C, while leaving unchanged the C/O ratio, which has been fixed by the cumulative action of several third dredge-up episodes. We find indications that the F abundance also increases along the AGB, supporting an in situ production of this element.

Title:		Fluorine in carbon-enhanced metal-poor stars: a binary scenario
Authors:		Lugaro, M.; de Mink, S. E.; Izzard, R. G.; Campbell, S. W.; Karakas, A. I.; Cristallo, S.; Pols, O. R.; Lattanzio, J. C.; Straniero, O.; Gallino, R.; Beers, T. C.
Affiliation:		AA(Sterrekundig Instituut, Universiteit Utrecht, PO Box 80000, 3508 TA Utrecht, The Netherlands ; Centre for Stellar and Planetary Astrophysics, School of Mathematical Sciences, Monash University, Victoria 3800, Australia), AB(Sterrekundig Instituut, Universiteit Utrecht, PO Box 80000, 3508 TA Utrecht, The Netherlands ), AC(Sterrekundig Instituut, Universiteit Utrecht, PO Box 80000, 3508 TA Utrecht, The Netherlands ), AD(Centre for Stellar and Planetary Astrophysics, School of Mathematical Sciences, Monash University, Victoria 3800, Australia; Academia Sinica Institute of Astronomy & Astrophysics, Taipei, Taiwan ), AE(Research School of Astronomy and Astrophysics, Mt. Stromlo Observatory, Cotter Rd., Weston, ACT 2611, Australia ), AF(INAF, Osservatorio Astronomico di Collurania, 64100 Teramo, Italy ), AG(Sterrekundig Instituut, Universiteit Utrecht, PO Box 80000, 3508 TA Utrecht, The Netherlands ), AH(Centre for Stellar and Planetary Astrophysics, School of Mathematical Sciences, Monash University, Victoria 3800, Australia ), AI(INAF, Osservatorio Astronomico di Collurania, 64100 Teramo, Italy ), AJ(Centre for Stellar and Planetary Astrophysics, School of Mathematical Sciences, Monash University, Victoria 3800, Australia ; Dipartimento di Fisica Generale, Universitá di Torino, Torino, Italy ), AK(Department of Physics and Astronomy, Center for the Study of Cosmic Evolution, and Joint Institute for Nuclear Astrophysics, Michigan State University, East Lansing, MI, USA )
Publication:		Astronomy and Astrophysics, Volume 484, Issue 3, 2008, pp.L27-L30 (A&A Homepage)
Publication Date:		06/2008
Origin:		EDP Sciences
Keywords:		stars: individual: HE 1305+0132, stars: AGB and, post-AGB , stars: abundances, nuclear reactions, nucleosynthesis, abundances
DOI:		10.1051/0004-6361:20079169
Bibliographic Code:		2008A&A...484L..27L

Abstract

Aims: A super-solar fluorine abundance was observed in the carbon-enhanced metal-poor (CEMP) star HE 1305+0132 ([F/Fe] = +2.90, [Fe/H] = -2.5). We propose that this observation can be explained using a binary model that involve mass transfer from an asymptotic giant branch (AGB) star companion and, based on this model, we predict F abundances in CEMP stars in general. We discuss wether F can be used to discriminate between the formation histories of most CEMP stars: via binary mass transfer or from the ejecta of fast-rotating massive stars.
Methods: We compute AGB yields using different stellar evolution and nucleosynthesis codes to evaluate stellar model uncertainties. We use a simple dilution model to determine the factor by which the AGB yields should be diluted to match the abundances observed in HE 1305+0132. We further employ a binary population synthesis tool to estimate the probability of F-rich CEMP stars.
Results: The abundances observed in HE 1305+0132 can be explained if this star accreted 3-11% of the mass lost by its former AGB companion. The primary AGB star should have dredged-up at least 0.2 {M}_ȯ of material from its He-rich region into the convective envelope via third dredge-up, which corresponds to AGB models of Z ≃ 0.0001 and mass ≃2 {M}_ȯ. Many AGB model uncertainties, such as the treatment of convective borders and mass loss, require further investigation. We find that in the binary scenario most CEMP stars should also be FEMP stars, that is, have [F/Fe] > +1, while fast-rotating massive stars do not appear to produce fluorine. We conclude that fluorine is a signature of low-mass AGB pollution in CEMP stars, together with elements associated with the slow neutron-capture process.

Title:		Analysis of Two Carbon Stars in the Carina Dwarf Spheroidal Galaxy
Authors:		Abia, C.; de Laverny, P.; Wahlin, R.; Domínguez, I.; Cristallo, S.; Straniero, O.
Affiliation:		AA(Dpto. Física Teórica y del Cosmos. Universidad de Granada, E-18071 Granada, Spain), AB(Observatoire de la Côte d'Azur, Dpt. Cassiopée UMR6202, 06304 Nice Cedex 4, France), AC(Department of Astronomy and Space Physics, Box 515, 75120 Uppsala, Sweden), AD(Dpto. Física Teórica y del Cosmos. Universidad de Granada, E-18071 Granada, Spain), AE(Osservatorio di Collurania-INAF, Teramo, Italy), AF(Osservatorio di Collurania-INAF, Teramo, Italy)
Publication:		IXTH TORINO WORKSHOP ON EVOLUTION AND NUCLEOSYNTHESIS IN AGB STARS AND THE IIND PERUGIA WORKSHOP ON NUCLEAR ASTROPHYSICS. AIP Conference Proceedings, Volume 1001, pp. 205-212 (2008). (AIPC Homepage)
Publication Date:		04/2008
Origin:		AIP
PACS Keywords:		Carbon stars, S stars, and related types, Population II stars, Stellar structure, interiors, evolution, nucleosynthesis, ages, Abundances, chemical composition, Dwarf galaxies, Spectroscopy and spectrophotometry
DOI:		10.1063/1.2916965
Bibliographic Code:		2008AIPC.1001..205A

Abstract

We present a chemical analysis of two metal-poor carbon stars belonging to the Carina Dwarf Spheroidal galaxy using high resolution optical spectra obtained with the VLT/UVES instrument. Their derived properties and chemical composition are compatible with these stars being in the TP-AGB phase undergoing third dredge-up episodes, although their extrinsic nature cannot be definitively discarded. The analysis indicates that these stars have large s-element enhancements, which are compatible with theoretical s-process nucleosynthesis predictions in low-mass AGB stars (~1.5M_solar), and also large carbon enhancements (C/O>=5), much larger than the values typically found in galactic AGB carbon stars (C/O~1-2). This is also in agreement with the theoretical prediction that carbon stars are formed more easily through third dredge-up episodes as the initial stellar metallicity drops. We speculate on the possibility that the abundance pattern found in the low-mass s-elements (Y, Zr) may be revealing the existence of some chemical depletion into dust grains.

Title:		Abundances of Heavy Elements in PNe and AGB Model Predictions
Authors:		Pignatari, M.; Gallino, R.; Cristallo, S.; Straniero, O.
Affiliation:		AA(Keele Astrophysics Group, Keele University, ST5 5BG UK; Joint Institute for Nuclear Astrophysics, University of Notre Dame, IN 46556, USA), AB(Dipartimento di Fisica Generale, Universita' di Torino, Via Pietro Giuria 1, 10125, Torino, Italy), AC(Osservatorio Astronomico di Collurania, 64100 Teramo, Italy), AD(Osservatorio Astronomico di Collurania, 64100 Teramo, Italy)
Publication:		IXTH TORINO WORKSHOP ON EVOLUTION AND NUCLEOSYNTHESIS IN AGB STARS AND THE IIND PERUGIA WORKSHOP ON NUCLEAR ASTROPHYSICS. AIP Conference Proceedings, Volume 1001, pp. 154-159 (2008). (AIPC Homepage)
Publication Date:		04/2008
Origin:		AIP
PACS Keywords:		Stellar structure, interiors, evolution, nucleosynthesis, ages, Giant and subgiant stars, Abundances, chemical composition, Planetary nebulae
DOI:		10.1063/1.2916958
Bibliographic Code:		2008AIPC.1001..154P

Abstract

During the AGB phase of low and intermediate mass stars heavy s-process elements are produced and dredged-up in the envelope. In the following Planetary Nebula phase part of the AGB envelope will form the nebula around the hot central star, carrying the s-process signature unchanged. Recently, heavy elements have been observed is many Planetary Nebulae, providing a new tool to analyze the s-process nucleosynthesis working in the AGB stars. In this paper we present s-process predictions for a large spread of stellar masses, metallicities and ¹³C-pocket efficiencies. Theoretical results are compared with the most recent observations in PNe, finding a general good agreement.

Title:		Observing Third Dredge Up in NGC 1846
Authors:		Lebzelter, T.; Hinkle, K. H.; Lederer, M. T.; Cristallo, S.; Straniero, O.; Nowotny, W.; Wood, P.
Affiliation:		AA(Institute f. Astronomy, Tuerkenschanzstrasse 17, A1180 Vienna, Austria), AB(NOAO, P.O.Box 26732, Tucson, AZ 85726, USA), AC(Institute f. Astronomy, Tuerkenschanzstrasse 17, A1180 Vienna, Austria), AD(INAF, Osservatorio Astronomico di Collurania, 64100 Teramo, Italy), AE(INAF, Osservatorio Astronomico di Collurania, 64100 Teramo, Italy), AF(Institute f. Astronomy, Tuerkenschanzstrasse 17, A1180 Vienna, Austria), AG(RSAA, Australian National University, Weston Creek, ACT 2611, Australia)
Publication:		IXTH TORINO WORKSHOP ON EVOLUTION AND NUCLEOSYNTHESIS IN AGB STARS AND THE IIND PERUGIA WORKSHOP ON NUCLEAR ASTROPHYSICS. AIP Conference Proceedings, Volume 1001, pp. 56-62 (2008). (AIPC Homepage)
Publication Date:		04/2008
Origin:		AIP
PACS Keywords:		Stellar structure, interiors, evolution, nucleosynthesis, ages, Giant and subgiant stars, Globular clusters in external galaxies, Abundances, chemical composition
DOI:		10.1063/1.2916990
Bibliographic Code:		2008AIPC.1001...56L

Abstract

In this contribution we present the first measurements of the C/O ratio for a sample of AGB stars in the LMC globular cluster NGC 1846. From a variability study we set strong constraints on the mass of the cluster stars, which is an important quantity for comparison with evolutionary models. Variations of the surface abundance both with luminosity and pulsation properties were found and the various relevant factors are discussed. We find a good agreement with our synthetic models if we assume [O/Fe] = +/-0.2 dex. The F abundance seems to increase with the C/O ratio. As variability plays a key role on the AGB we give some preliminary results from our study of abundance determination in the dynamical case at the end of this paper.

Title:		Progresses in AGB Modelling
Authors:		Cristallo, S.; Straniero, O.; Gallino, R.; Lederer, M. T.; Piersanti, L.; Domínguez, I.
Affiliation:		AA(Osservatorio Astronomico di Teramo (INAF), via M. Maggini 64100 Teramo, Italy), AB(Osservatorio Astronomico di Teramo (INAF), via M. Maggini 64100 Teramo, Italy), AC(Dipartimento di Fisica Generale, Universitá di Torino e Sezione INFN di Torino, via P. Giuria 1, 10125 Torino, Italy), AD(Institut für Astronomie, Türkenschanzstraße 17, A-1180 Wien, Austria), AE(Osservatorio Astronomico di Teramo (INAF), via M. Maggini 64100 Teramo, Italy), AF(Departimento Fisica Teorica y del Cosmos, Universidad de Granada, 18071 Granada, Spain)
Publication:		IXTH TORINO WORKSHOP ON EVOLUTION AND NUCLEOSYNTHESIS IN AGB STARS AND THE IIND PERUGIA WORKSHOP ON NUCLEAR ASTROPHYSICS. AIP Conference Proceedings, Volume 1001, pp. 3-10 (2008). (AIPC Homepage)
Publication Date:		04/2008
Origin:		AIP
PACS Keywords:		Origin, formation, and abundances of the elements, Giant and subgiant stars, Carbon stars, S stars, and related types, Stellar structure, interiors, evolution, nucleosynthesis, ages, Abundances, chemical composition
DOI:		10.1063/1.2916987
Bibliographic Code:		2008AIPC.1001....3C

Abstract

The full understanding of final stellar evolution phases is a fundamental request to properly investigate the Universe at any temporal and spatial scale. While the theoretical scenarios of H- and He-burning have been deeply investigated in the last 30 years, the modelling of stellar evolution beyond the core-helium burning phase and the related nucleosynthesis still present problems related to the physics and to the numerical methods. Low mass AGB Stars (1<M/M_solar<3) are among the most important polluters of the Milky Way, because of the strong winds eroding their chemically enriched envelopes. In the AGB phase, the material processed during the alternating series of H and He burnings is recurrently mixed within the convective zones generated by Thermal Pulses and then partially mixed with the surface by the so called Third Dredge Up episodes. As a matter of fact, MS, S, C(N) and some post-AGB stars are enriched in C and s-process elements and the spectro-scopic detection of unstable Tc demonstrates that the s process is at workin these stars [1]. Within this context, we started a longstanding program devoted to the study of the physical and chemical evolution of low mass AGB stars by means on the FRANEC stellar evolutionary code [2, 3].

Among the most important results we achieved, we highlight the explanation of the mechanism of neutron production, which controls the s-process nucleosynthesis in low-mass AGB, the coupling of the code with a full nuclear network, from H up to Pb-Bi (at the termination point of the s-process path) and the introduction of C-enhanced low temperature opacities, whose effects are particularly important at low metallicities. In this paper, we describe the current state of the art of our modelling and we present detailed models of low mass AGB (M = 2M_solar) stars at different metallicities (from Z = Z_solar to Z = 10^-4). The final elemental distributions we find are representative of the ones expected for the intrinsic carbon stars observed in the disk and in the halo of the Milky Way.

Title:		Evolution and Nucleosynthesis of Low-Mass Metal-Poor AGB Models with C- and N-Enhanced Molecular Opacities.
Authors:		Cristallo, S.; Straniero, O.; Lederer, M. T.
Affiliation:		AA(Osservatorio Astronomico di Collurania (INAF), via M. Maggini Teramo-64100 Italy), AB(Osservatorio Astronomico di Collurania (INAF), via M. Maggini Teramo-64100 Italy), AC(Institut für Astronomie, Türkenschanzstraße 17, A-1180 Wien, Austria)
Publication:		FIRST STARS III: First Stars II Conference. AIP Conference Proceedings, Volume 990, pp. 320-324 (2008). (AIPC Homepage)
Publication Date:		03/2008
Origin:		AIP
PACS Keywords:		Giant and subgiant stars, Stellar structure, interiors, evolution, nucleosynthesis, ages, Abundances, chemical composition, Population II stars
DOI:		10.1063/1.2905571
Bibliographic Code:		2008AIPC..990..320C

Abstract

We use the COMA code to calculate radiative opacity coefficients for low temperature gas, where the abundances of C and N are enhanced up to a factor of 2000 with respect to a scaled solar composition. These opacities include the contributions of the most relevant atomic and molecular species. Then, by means of the FRANEC stellar evolutionary code, we compute low mass metal-poor ([Fe/H]<-2) AGB models by interpolating over the new opacity tables. In this way, we properly take into account the recurrent increase of the C abundance in the envelope caused by the third dredge up. We discuss changes induced by the use of these opacities on the physical and chemical properties of the models, with particular emphasis to the synthesis of carbon, nitrogen and s-process elements. We also consider the effects of a possible N enhancement, which may be caused by cool bottom process or by the engulfment of protons from the top of the convective zone generated by the first fully-developed thermal pulse. For a given metallicity, we find that it exists a lower mass limit for which normal AGB evolution occurs. On the contrary, stars with initial mass smaller than this limit, experience a proton engulfment during the first thermal pulse, which leads to a peculiar s-process nucleosynthesis, a low ¹²C/¹³C and to a significant synthesis of primary N. Finally, we compare our results with the observed chemical pattern of CEMP stars.

Title:		Constraints on AGB Models from s-Process Nucleosynthesis
Authors:		Cristallo, S.; Straniero, O.; Gallino, R.
Publication:		Why Galaxies Care About AGB Stars: Their Importance as Actors and Probes. ASP Conference Series, Vol. 378, proceedings of the conference held 7-11 August 2006 at University Campus, Vienna, Austria. Edited by F. Kerschbaum, C. Charbonnel, and R. F. Wing. San Francisco: Astronomical Society of the Pacific, 2007., p.119
Publication Date:		11/2007
Origin:		ASPC
Bibliographic Code:		2007ASPC..378..119C

Abstract

Thermally pulsing AGB stars with mass lower than about 3 M_ȯ play a fundamental role in the Galactic enrichment of heavy elements, by means of their s-process nucleosynthesis. The main neutron source is the ¹³C(α,n)¹⁶O reaction, operating during the interpulse phase. The amount of ¹³C required to explain the activation of the s-process can be obtained if, at the epoch of the third dredge-up, some protons are diffused from the H-rich envelope into the underlying radiative He-rich (and ¹²C-rich) intershell zone, thus allowing the formation of the so-called ¹³C pocket, through proton captures on carbon nuclei. Spectroscopic measurements of element overabundances in AGB stars and precise determinations of isotopic compositions in meteoritic SiC grains provide strong constraints on the amount of ¹³C held in the pocket. They generally show good agreement with nucleosynthesis predictions obtained by assuming M(¹³C) = 3×10^-6 M_ȯ, corresponding to the standard case defined by tet{ga98}, even if a certain spread is often required.

Title:		Third Dredge-up in Globular Cluster AGB Stars: Observation Versus Theory
Authors:		Lederer, Michael T.; Lebzelter, Thomas; Cristallo, Sergio; Straniero, Oscar; Aringer, Bernhard; Hinkle, Ken
Affiliation:		AA(Department of Astronomy, University of Vienna, Türkenschanzstraße 17, A-1180 Wien, Austria), AB(Department of Astronomy, University of Vienna, Türkenschanzstraße 17, A-1180 Wien, Austria), AC(INAF, Osservatorio Astronomico di Collurania, 64100 Teramo, Italy), AD(INAF, Osservatorio Astronomico di Collurania, 64100 Teramo, Italy), AE(Department of Astronomy, University of Vienna, Türkenschanzstraße 17, A-1180 Wien, Austria), AF(National Optical Astronomy Observatory, P.O. Box 26732, Tucson, AZ 85726-6732)
Publication:		UNSOLVED PROBLEMS IN STELLAR PHYSICS: A Conference in Honor of Douglas Gough. AIP Conference Proceedings, Volume 948, pp. 43-50 (2007). (AIPC Homepage)
Publication Date:		11/2007
Origin:		AIP
PACS Keywords:		Globular clusters in the Milky Way, Giant and subgiant stars, Stellar structure, interiors, evolution, nucleosynthesis, ages, Stellar atmospheres ; radiative transfer; opacity and line formation
DOI:		10.1063/1.2819007
Bibliographic Code:		2007AIPC..948...43L

Abstract

The changes in the surface abundances of Asymptotic Giant Branch (AGB) stars are due to ongoing nucleosynthesis in the stellar interior accompanied by strong mixing events. Quantitative details of the mixing mechanism - commonly dubbed Third Dredge-Up (TDU) - are still unclear. We derive the C/O ratio and the carbon isotopic ratio ¹²C/¹³C from high-resolution near-infrared spectra of a number of AGB stars in the globular cluster NGC 1846 in the Large Magellanic Cloud (LMC). Our sample is well defined in metallicity and mass which allows us to trace the TDU along the AGB as a function of luminosity. Supplementary, we compare the experimentally derived values to results from stellar evolution models. For these calculations we use a set of newly generated opacity coefficients which properly take into account the carbon (and nitrogen) enhancement caused by the third dredge-up.

Title:		Molecular Opacities for Low-Mass Metal-poor AGB Stars Undergoing the Third Dredge-up
Authors:		Cristallo, S.; Straniero, O.; Lederer, M. T.; Aringer, B.
Affiliation:		AA(INAF, Osservatorio Astronomico di Collurania, 64100 Teramo, Italy), AB(INAF, Osservatorio Astronomico di Collurania, 64100 Teramo, Italy), AC(Institut für Astronomie, Türkenschanzstraße 17, A-1180 Vienna, Austria), AD(Institut für Astronomie, Türkenschanzstraße 17, A-1180 Vienna, Austria)
Publication:		The Astrophysical Journal, Volume 667, Issue 1, pp. 489-496. (ApJ Homepage)
Publication Date:		09/2007
Origin:		UCP
ApJ Keywords:		Stars: AGB and Post-AGB, Stars: Atmospheres, Stars: Carbon, Stars: Evolution
DOI:		10.1086/520833
Bibliographic Code:		2007ApJ...667..489C

Abstract

The concomitant overabundances of C, N, and s-process elements are commonly ascribed to the complex interplay of nucleosynthesis, mixing, and mass loss taking place in asymptotic giant branch (AGB) stars. At low metallicity, the enhancement of C and/or N can be up to 1000 times larger than the original iron content and significantly affects the stellar structure and its evolution. For this reason, the interpretation of the already available and still growing amount of data concerning C-rich metal-poor stars belonging to our Galaxy as well as to dwarf spheroidal galaxies would require reliable AGB stellar models for low and very low metallicities. In this paper we address the question of calculation and use of appropriate opacity coefficients, which take into account the C enhancement caused by the third dredge-up. A possible N enhancement, caused by the cool bottom process or by the engulfment of protons into the convective zone generated by a thermal pulse and the subsequent huge third dredge-up, is also considered. Based on up-to-date stellar models, we illustrate the changes induced by the use of these opacity coefficients on the physical and chemical properties expected for these stars.

Title:		Evolution and nucleosynthesis in low-mass asymptotic giant branch stars
Authors:		Cristallo, S.
Publication:		Il Nuovo Cimento B, vol. 122, Issue 6, p.697-706 (NCimB Homepage)
Publication Date:		06/2007
Origin:		WEB
DOI:		10.1393/ncb/i2007-10405-8
Bibliographic Code:		2007NCimB.122..697C

Abstract

People usually smile when astrophysicists assert that we are sons of the stars, but human life confirms this sentence: about 65% of the mass of our body is made up of oxygen, carbon occurs in all organic life and is the basis of organic chemistry, nitrogen is an essential part of amino acids and nucleic acids, calcium is a major component of our bones. Moreover, phosphorus plays a major role in biological molecules such as DNA and RNA (where the chemical codes of life is written) and our blood carries oxygen to tissues by means of the hemoglobin (an iron pigment of red blood cells). All these elements have been created in stars. I just list some examples related to human body, but also common element such as aluminum, nickel, gold, silver and lead come from a pristine generation of stars. The abundances in the Solar System are in fact due to the mixing of material ejected from stars that polluted the Universe in different epochs before the Sun formation, occurred about 5 billion years ago, after the gravitational contraction of the proto-solar cloud. Low mass AGB stars (1<M/Msun<3) are among the most important polluters of the Milky Way, because of the strong winds eroding their chemically enriched envelopes. They are responsible for the nucleosynthesis of the main component of the cosmic s-elements.

Title:		A method to derive the absolute composition of the Sun, the solar system, and the stars
Authors:		Piersanti, L.; Straniero, O.; Cristallo, S.
Affiliation:		AA(INAF-Osservatorio Astronomico di Collurania Teramo, via Mentore Maggini, snc, 64100 Teramo, Italy ), AB(INAF-Osservatorio Astronomico di Collurania Teramo, via Mentore Maggini, snc, 64100 Teramo, Italy ), AC(INAF-Osservatorio Astronomico di Collurania Teramo, via Mentore Maggini, snc, 64100 Teramo, Italy )
Publication:		Astronomy and Astrophysics, Volume 462, Issue 3, February II 2007, pp.1051-1062 (A&A Homepage)
Publication Date:		02/2007
Origin:		EDP Sciences
Keywords:		Sun: abundance, Sun: evolution, solar system: general, stars: abundances
DOI:		10.1051/0004-6361:20054505
Bibliographic Code:		2007A&A...462.1051P

Abstract

Context: The knowledge of isotopic and elemental abundances of the pristine solar system material provides a fundamental test of galactic chemical evolution models, while the composition of the solar photosphere is a reference pattern to understand stellar abundances. However, spectroscopic or meteoritic abundance determinations are only possible for an incomplete sample of the 83 elements detected in the solar system. Therefore, only relative abundances are experimentally determined, with respect to H or to Si for spectroscopic or meteoritic measurements, respectively. For this reason, the available compilations of solar abundances are obtained by combining spectroscopic and meteoritic determinations, a procedure requiring the knowledge of the chemical modification occurring in the solar photosphere.
Aims: We provide a method to derive the mass fractions of the 83 elements (and their most abundant isotopes) in the early solar system material and in the present-day solar surface.
Methods: By computing a solar model, we investigate physical processes responsible for the variation of the solar surface composition in the last 4.57 Gyr. An extended network, from H to U, is coupled to our stellar evolutionary code. The effects of microscopic diffusion, rotational-induced mixing in the tachocline and radiative acceleration are discussed.
Results: The abundances of the 83 elements are given for both the pristine solar system and the solar photosphere. Calculations are repeated by adopting the most widely adopted compilations of solar abundances. Since for a given [Fe/H], the total metallicity depends on (Z/X)_ȯ, a 30% reduction of Z is found when passing from the classical Anders & Grevesse to the most recent Lodders compilation. Some implications are discussed, such as the increase of about 700 Myr in the estimated age of Globular Clusters.
Conclusions: . Within the experimental errors, the complete set of relative solar abundances, as obtained by combining meteorite and photosphere measurements, are consistent with the variations implied by the quoted physical processes. The few deviations can be easily attributed to the decay of long-lived radioactive isotopes. The large lithium depletion is only partially explained by introducing a rotation-induced mixing in the tachocline.

Tables 3 and 4 are only available in electronic form at http://www.aanda.org

Title:		Primary production of light and s elements in very metal-poor Asymptotic Giant Branch stars.
Authors:		Husti, L.; Gallino, R.; Bisterzo, S.; Cristallo, S.; Straniero, O.
Affiliation:		AA( Dipartimento di Fisica Generale, Universitàdi Torino, Via Pietro Giuria 1, 10125 Torino, Italy; ), AB( Dipartimento di Fisica Generale, Universitàdi Torino, Via Pietro Giuria 1, 10125 Torino, Italy), AC( Dipartimento di Fisica Generale, Universitàdi Torino, Via Pietro Giuria 1, 10125 Torino, Italy), AD(INAF - Osservatorio di Teramo (INAF), Teramo, Italy), AE(INAF - Osservatorio di Teramo (INAF), Teramo, Italy)
Publication:		Memorie della Società Astronomica Italiana, v.78, p.523 (2007)
Publication Date:		00/2007
Origin:		MmSAI
Keywords:		Stars: abundances, Stars: AGB, Stars: metal poor, Nucleosynthesis: light elements
Bibliographic Code:		2007MmSAI..78..523H

Abstract

In AGB stars of low mass and very low metallicity, [Fe/H] < -2, a large abundance of primary ¹²C obtained by partial He burning in the convective thermal pulse is mixed with the envelope by each third dredge up episode. A progressive amount of primary ²²Ne is produced in the He intershell by the conversion of all CNO nuclei to ¹⁴N during H-burning shell, followed by double alpha capture in the thermal pulse. Despite its very low neutron cross section, ²²Ne contributes significantly to the production of primary ²³Na, ²⁴Mg, ²⁵Mg, ²⁶Mg. These last two are also obtained by alpha capture on ²²Ne. The neutron capture chain starting from ²²Ne continues up to iron, creating primary seeds for the s process. Further on, s elements are produced with high efficiency by neutron capture on the primary iron seeds. The third dredge up mixes with the envelope the freshly synthesized primary elements.

Title:		AGB Stars: Nucleosynthesis and Open Problems
Authors:		Domínguez, I.; Abia, C.; Cristallo, S.; de Laverny, P.; Straniero, O.
Publication:		Highlights of Spanish Astrophysics IV, Proceedings of the seventh Scientific Meeting of the Spanish Astronomical Society (SEA) held in Barcelona, Spain, September 12-15, 2006. Edited by F. Figueras, Departament d'Astronomia i Meteorologia, Universitat de Barcelona-IEEC; J.M. Girart,Institute de Ciències de l'Espai, CSIC-IEEC; M. Hernanz, Institute de Ciències de l'Espai, CSIC-IEEC; C. Jordi, Departament d'Astronomia i Meteorologia, Universitat de Barcelona-IEEC. ISBN: 978-1-4020-6000-7 (ebook). Published by Springer, Dordrecht, 2007, p.239
Publication Date:		00/2007
Origin:		ADS
Bibliographic Code:		2007hsa..conf..239D

Abstract

Not Available

Title:		s process in low-mass asymptotic giant branch stars
Authors:		Straniero, Oscar; Gallino, Roberto; Cristallo, Sergio
Affiliation:		AA(INAF Osservatorio Astronomico di Collurania and Università di Teramo, via M. Maggini, 64100 Teramo, Italy; INFN Sezione di Napoli, Complesso Universitario di Monte Sant'Angelo, via Cintia, 80126 Napoli, Italy), AB(Dipartimento di Fisica Generale, Università di Torino, INFN Sezione di Torino, via P. Giuria 1, 10125 Torino, Italy; Centre for Stellar and Planetary Astrophysics, School of Mathematical Sciences, Monash University, 3800 Victoria, Australia), AC(INAF Osservatorio Astronomico di Collurania and Università di Teramo, via M. Maggini, 64100 Teramo, Italy; INFN Sezione di Napoli, Complesso Universitario di Monte Sant'Angelo, via Cintia, 80126 Napoli, Italy)
Publication:		Nuclear Physics A, Volume 777, p. 311-339.
Publication Date:		10/2006
Origin:		ELSEVIER
DOI:		10.1016/j.nuclphysa.2005.01.011
Bibliographic Code:		2006NuPhA.777..311S

Abstract

The main component of the s process is produced by low mass stars (1.5⩽M/M⩽3), when they climb for the second time the red giant branch and experience a series of He shell flashes called thermal pulses. During the relatively long period (10⁵ yr) that elapses between two subsequent thermal pulses, a slow neutron flux is provided by the ¹³C(α,n)¹⁶O reaction taking place within a thin ¹³C pocket located in the He-rich and C-rich mantel of these stars. A second, marginal, neutron burst occurs during the thermal pulse and it is powered by the ²²Ne(α,n)²⁵Mg reaction. We review the present status of the nucleosynthesis models of low mass AGB stars. The advance in the knowledge of the complex coupling between convective mixing and nuclear process, which allows the surface enrichment of C- and s-process elements, is presented, together with the hypotheses concerning the physical mechanism driving the formation of the ¹³C pocket. In order to illustrate the capabilities and the limits of the theory, an updated computation of a 2M stellar structure with solar chemical composition is reported. This model has been obtained by including a full nuclear network (from H up to Bi, at the termination point of the s-process path) into the stellar evolution code. The predicted modification of the surface composition occurring during the AGB evolution is shown. The new challenge of AGB modeling, namely, the study of C-rich and s-rich very metal-poor stars, is discussed.

Title:		s-Process Nucleosynthesis in Low-Mass AGB Stars at Different Metallicities
Authors:		Cristallo, S.
Affiliation:		AA(Current address: Osservatorio Astronomico di Collurania, INAF, Via Mentore Maggini, I-64100 Teramo, Italy; Thesis work conducted at Osservatorio Astronomico di ColluraniaPh.D. thesis directed by O. Straniero; Ph.D. degree awarded 2006 July)
Publication:		The Publications of the Astronomical Society of the Pacific, Volume 118, Issue 847, pp. 1360-1360. (PASP Homepage)
Publication Date:		09/2006
Origin:		UCP
PASP Keywords:		dissertation summaries
DOI:		10.1086/507774
Bibliographic Code:		2006PASP..118.1360C

Abstract

Not Available

Title:		Short-lived isotopes and ²³Na production in low mass AGB Stars
Authors:		Cristallo, S.; Gallino, R.; Straniero, O.; Piersanti, L.; Domınguez, I.
Affiliation:		AA(Osservatorio Astronomico di Collurania, via Mentore Maggini, 64100 Teramo, Italy; ), AB(Dipartimento di Fisica Generale, Universitá di Torino e Sezione INFN di Torino, via P. Giuria 1, 10125 Torino, Italy), AC(Osservatorio Astronomico di Collurania, via Mentore Maggini, 64100 Teramo, Italy), AD(Osservatorio Astronomico di Collurania, via Mentore Maggini, 64100 Teramo, Italy), AE(Departamento Física Teórica y del Cosmos, Universidad de Granada, 18071 Granada, Spain)
Publication:		Memorie della Società Astronomica Italiana, v.77, p.774 (2006)
Publication Date:		00/2006
Origin:		MmSAI
Keywords:		AGB stars, nucleosynthesis, s-process, short-lived isotopes
Bibliographic Code:		2006MmSAI..77..774C

Abstract

We discuss the synthesis of some short-lived isotopes and of ²³Na in thermally pulsing AGB stars with initial mass of 2 M_ȯ and two different metallicities, Z=1.5×10^-2, corresponding to the metal amount in the present Sun, and Z=10^-4, representative of disk and halo stars, respectively. The different nucleosynthesis channels are illustrated in some details. As previously found, the ¹³C formed after each third dredge up episode is usually completely consumed by alpha captures before the onset of the subsequent thermal pulse, releasing neutrons. This is the most efficient neutron source in low mass AGB stars, and the resulting s-process nucleosynthesis is at the origin of the solar main component. However, in the solar metallicity model, we find that the temperature of the first formed ¹³C pocket remains too low during the interpulse and the ¹³C is not completely burnt, being partially engulfed in the convective zone generated by the following thermal pulse. Due to the rapid convective mixing in this zone, the ¹³C is exposed to a larger temperature and a nucleosynthesis characterized by a relatively high neutron density develops. The main effect is the strong enhancement of isotopes located beyond some critical branching in the neutron-capture path, like ⁶⁰Fe, otherwise only marginally produced during a standard s-process nucleosynthesis.

Title:		22Ne a primary source of neutron for the s-process and a major neutron poison in CEMP AGB stars
Authors:		Gallino, Roberto; Bisterzo, S.; Husti, L.; Käppeler, F.; Cristallo, S.; Straniero, O.
Publication:		Proceedings of the International Symposium on Nuclear Astrophysics - Nuclei in the Cosmos - IX. 25-30 June 2006, CERN., p.100.1
Publication Date:		00/2006
Origin:		POS
Bibliographic Code:		2006isna.confE.100G

Abstract

In AGB stars of low mass and very low metallicity, [Fe/H] 2, a large abundance of pri- 12 mary C is mixed with the envelope by each third dredge up episode. The subsequent activation of the H shell converts almost all CNO nuclei into 14 N. Thus the H burning ashes contain 14 N from the original CNO nuclei, plus an increasing amount of primary 14 N. During the subse- quent convective thermal instability in the He shell, all the 14 N nuclides are converted to 22 Ne ¤ by 14 N(α,γ)18 F(β ν)18 O, followed by 18 O(α,γ)22 Ne. At the peak temperature reached at the base of the thermal pulse, the 22 Ne(α,n)25 Mg reaction is partly activated, giving rise to a small neutron exposure. At the same time, although the neutron capture cross section of 22 Ne is very small (σ (22 Ne, 30keV)=0.059¥ 0.0057 mbarn, Beer et al. 1991), the very large amount of pri- mary 22 Ne acts as a major poison against the s process. This poison effect is substantial also in case of addition of a 13 C-pocket. Some fraction of primary 16 O is also made in the thermal pulse by α-capture on 12 C (with mass fraction X(16 O) 0.003, while X(12 C) 0.20). Besides 12 C and ¦ ¦ 22 Ne, a number of light isotopes are largely produced in a primary way, among which one finds 19 F (from neutron capture on 18 O), 23 Na, 25 Mg, 26 Mg. An effort should be devoted to improve the accuracy of the cross sections of these light isotopes, in order to better constrain the s-process efficiency and their production in low metallicity AGB stars. At very low metallicity, iron is di- rectly made starting from neutron captures on 22 Ne and then used as a bridge for the build-up of the s elements.

Title:		Light and heavy elements nucleosynthesis in low mass AGB Stars
Authors:		Cristallo, Sergio; Straniero, Oscar; Gallino, Roberto; Piersanti, Luciano; Dominguez, Inma
Publication:		Proceedings of the International Symposium on Nuclear Astrophysics - Nuclei in the Cosmos - IX. 25-30 June 2006, CERN., p.62.1
Publication Date:		00/2006
Origin:		POS
Bibliographic Code:		2006isna.confE..62C

Abstract

We present a new set of low mass AGB star models having the same mass (2 M ) and different metallicities. For each metallicity we follow the evolution from the Pre Main Sequence up to the end of the Thermally Pulsing AGB Phase. By means of an exponential decay of the convective velocities at the inner border of the convective envelope we obtain the formation of a tiny 13 C pocket after Third Dredge Up episodes, whose extension in mass decreases along the AGB path. Detailed pulse by pulse surface enrichments and final yields at different metallicities, computed by using a full nuclear network coupled to the FRANEC stellar evolutionary code, are presented and discussed. We follow the production of both light and heavy elements describing nuclear chains responsible for their production and show new results for the synthesis of radioactive isotopes such as 26 Al and 60 Fe.

Title:		Lead Stars at Low Metallicity: Observation versus Theory
Authors:		Delaude, D.; Gallino, R.; Cristallo, S.; Straniero, O.
Affiliation:		AA(Dipartimento di Fisica Generale dell’Universitá di Torino), AB(Dipartimento di Fisica Generale dell’Universitá di Torino), AC(Osservatorio Astronomico di Collurania), AD(Osservatorio Astronomico di Collurania)
Publication:		Chemical Abundances and Mixing in Stars in the Milky Way and its Satellites, ESO ASTROPHYSICS SYMPOSIA. ISBN 978-3-540-34135-2. Springer-Verlag, 2006, p. 126
Publication Date:		00/2006
Origin:		SPRINGER
Bibliographic Code:		2006cams.book..126D

Abstract

The s process in AGB stars is mainly driven by the ¹³C(α,n)¹⁶O reaction. During a third dredge up episode, penetration of a small amount of protons from the envelope into the top layers of the ¹²C-rich and He-rich zone gives rise to the formation of a so-called ¹³C pocket [3]. At any given metallicity, a large range of ¹³C-pocket efficiencies is required for the interpretation of the s-process distributions observed in s-enhanced stars in the Galactic disk [4]. Stellar models predict wide ranges of [hs/Fe], [ls/Fe], [hs/ls], where ls=ls(Y, Zr) represents the first s-peak at neutron magic N = 50 and hs=hs(Ba, La, Nd, Sm) the second s-peak at neutron magic N = 82.

Title:		S-process nucleosynthesis in low mass AGB Stars: do we really need an improved determination of the ¹³C(α, n)¹⁶O reaction rate?
Authors:		Cristallo, S.; Straniero, O.; Gallino, R.
Affiliation:		AA(Osservatorio Astronomico di Collurania (INAF), via Mentore Maggini, 64100 Teramo, Italy), AB(Osservatorio Astronomico di Collurania (INAF), via Mentore Maggini, 64100 Teramo, Italy), AC(Dipartimento di Fisica Generale, Università di Torino, via P. Giuria 1, 10125 Torino, Italy)
Publication:		Nuclear Physics A, Volume 758, p. 509-512.
Publication Date:		07/2005
Origin:		ELSEVIER
DOI:		10.1016/j.nuclphysa.2005.05.181
Bibliographic Code:		2005NuPhA.758..509C

Abstract

Thermally pulsing Asymptotic Giant Branch stars are responsible for the nucleosynthesis of the main component of the cosmic s-elements. The most important neutron source is the ¹³C(α, n)¹⁶O reaction. Owing to the presence of a subthreshold resonance, the low energy extrapolation is a rather complex task. The rate quoted in the literature differ up to a factor of 4 at typical stellar energies. The latest improvements in computer power allows us to calculate the evolution of TP-AGB stars coupled with a full nuclear network, extending from hydrogen to lead. Here we discuss the effects of the variation of the ¹³C(α, n)¹⁶O rate on the predicted neutron capture nucleosynthesis.

Title:		Predictions of s-process Lead in low-metallicity stars compared with spectroscopic observations
Authors:		Gallino, R.; Delaude, D.; Husti, L.; Cristallo, S.; Straniero, O.; Ryan, S.
Affiliation:		AA(Dipartimento di Fisica Generale, Universitá di Torino, Via P. Giuria 1, 10125 Torino, Italy), AB(Dipartimento di Fisica Generale, Universitá di Torino, Via P. Giuria 1, 10125 Torino, Italy), AC(University of Bucharest, Romania), AD(Osservatorio Astronomico di Collurania, Teramo), AE(Osservatorio Astronomico di Collurania, Teramo), AF(Open University, Walton Hall, Milton Keynes, UK)
Publication:		Nuclear Physics A, Volume 758, p. 485-488.
Publication Date:		07/2005
Origin:		ELSEVIER
DOI:		10.1016/j.nuclphysa.2005.05.089
Bibliographic Code:		2005NuPhA.758..485G

Abstract

Not Available

Title:		Chemical pollution from AGB Stars
Authors:		Cristallo, S.; Straniero, O.; Gallino, R.; Piersanti, L.; Dominguez, I.
Publication:		From Lithium to Uranium: Elemental Tracers of Early Cosmic Evolution, IAU Symposium Proceedings of the international Astronomical Union 228, Held in Paris, France, May 23-27, 2005. edited by Hill, V.; François, P.; Primas, F. Cambridge: Cambridge University Press, 2005., pp.483-484
Publication Date:		00/2005
Origin:		CUP
DOI:		10.1017/S1743921305006265
Bibliographic Code:		2005IAUS..228..483C

Abstract

Low mass AGB Stars are the main contributors to the Galactic s-process enrichment. We present new theoretical results obtained by adopting a full network from H to Bi coupled with the physical evolution of the stellar structure. We describe the formation of a 13C pocket as a consequence of H diffusion from the envelope into the He-rich intershell. Such 13C is burnt during the interpulse phase and provides the main neutron source in these stars. We computed two models with the same total mass (that is 2 Mȯ) but two different initial chemical composition, namely (Y=0.269 - Z=0.015) and (Y=0.245 - Z=0.0001), representative of disk and halo stars respectively. We evaluate the differences in the final s-process surface composition and compare the results with the available observational data.

Title:		First detection of a lithium rich carbon star in the Draco dwarf galaxy: Evidence for a young stellar population
Authors:		Domínguez, I.; Abia, C.; Straniero, O.; Cristallo, S.; Pavlenko, Ya. V.
Affiliation:		AA(Dpto. Física Teórica y del Cosmos, Universidad de Granada, 18071 Granada, Spain ), AB(Dpto. Física Teórica y del Cosmos, Universidad de Granada, 18071 Granada, Spain), AC(INAF-Osservatorio Astronomico di Collurania, 64100 Teramo, Italy), AD(INAF-Osservatorio Astronomico di Collurania, 64100 Teramo, Italy), AE(Main Astronomical Observatory, National Academy of Sciences, Zabolotnoho 27, Kiev-127 03680, Ukranie)
Publication:		Astronomy and Astrophysics, v.422, p.1045-1052 (2004) (A&A Homepage)
Publication Date:		08/2004
Origin:		A&A
A&A Keywords:		stars: carbon, stars: abundances, stars: evolution, stars: AGB and post-AGB, galaxies: local group
DOI:		10.1051/0004-6361:20040289
Bibliographic Code:		2004A&A...422.1045D

Abstract

We present a spectroscopic study of D461, a giant star belonging to Draco dwarf spheroidal galaxy. From spectral synthesis in LTE we derive a lithium abundance of log ɛ(Li) = 3.5±0.4 and a C/O ratio between 3 and 5. This is the first detection of a lithium rich C-star in a dwarf spheroidal galaxy. Basing on stellar models of appropriate chemical composition, we show that a similar C enrichment is compatible with that expected for a low mass low metallicity thermally pulsing AGB star, undergoing few third dredge up episodes. The position in the log g-log T_eff diagram of D461 is also compatible with this theoretical scenario. In particular, the low effective temperature, lower than that expected for a low metallicity giant star, is a consequence of the huge increase of the envelope opacity occurring after the carbon dredge up. The Li enrichment may be explained if a deep circulation would take place during the interpulse period, the so called cool bottom process. In spite of the low resolution of our spectra, we derive a lower limit for the carbon isotopic ratio, namely ¹²C/¹³C>40, and a constraint for the Ba abundance, namely 0.5<[Ba/Fe]< 2. The proposed scenario also fits these further constraints. Then, we estimate that the mass of D461 ranges between 1.2 and 2 M_ȯ, which corresponds to an age ranging between 1 and 3 Gyr. We conclude that this star is more massive and younger than the typical stellar population of Draco.

Title:		Lead and mathbf s-process elements in stars of various metallicities: AGB predictions compared with observation
Authors:		Delaude, D.; Gallino, R.; Cristallo, S.; Straniero, O.; Husti, L.; Ryan, S.
Affiliation:		AA(Dipartimento di Fisica dell'Universitá di Torino, via P.Giuria 1, 10125 Torino, Italy), AB(Dipartimento di Fisica dell'Universitá di Torino, via P.Giuria 1, 10125 Torino, Italy), AC(Osservatorio Astronomico di Collurania, 64100 Teramo, Italy), AD(Osservatorio Astronomico di Collurania, 64100 Teramo, Italy), AE(University of Bucharest, Romania), AF(Open University, Walton Hall, Milton Keynes, UK)
Publication:		Memorie della Società Astronomica Italiana, v.75, p.706 (2004)
Publication Date:		00/2004
Origin:		MmSAI
Keywords:		AGB stars, AGB nucleosynthesis, Lead stars, s-process
Bibliographic Code:		2004MmSAI..75..706D

Abstract

We present AGB predictions for all heavy elements within a large range of ¹³C-pocket efficiencies for stars of different metallicities, and compare them in detail with a number of spectroscopic observations of s-rich and lead-rich in the Galaxy. The current concept of the s-process efficiency, specified by the [hs/ls] index, is shown to be inappropriate for the metal poor AGB stars and a second independent index, [Pb/hs] or [Pb/ls], needs to be introduced. The state-of-the-art concerning the interpretation of lead stars allows a very large spread of [Pb/hs] in metal poor stars, as typically observed. We discuss agreements and discrepancies for a large range of elements.

Title:		A full network coupled to the evolutionary code: detailed s-process nucleosynthesis in low mass AGB Stars
Authors:		Cristallo, S.; Straniero, O.; Gallino, R.
Affiliation:		AA(Osservatorio Astronomico di Collurania, via Mentore Maggini, 64100 Teramo, Italy; ), AB(Osservatorio Astronomico di Collurania, via Mentore Maggini, 64100 Teramo, Italy; ), AC(Dipartimento di Fisica Generale, Universitá di Torino e Sezione INFN di Torino, via P. Giuria 1, 10125 Torino, Italy)
Publication:		Memorie della Società Astronomica Italiana, v.75, p.676 (2004)
Publication Date:		00/2004
Origin:		MmSAI
Keywords:		AGB stars, nucleosynthesis, {s
Bibliographic Code:		2004MmSAI..75..676C

Abstract

In the FRANEC code, a full set of equations describing the physical evolution of a star is coupled with the nuclear processes fixing the temporal variation of the nuclear species. Mixing induced by convection is calculated by means of a time dependent algorithm, where the efficiency of the process is taken proportional to the convective velocity. In the computation of AGB stellar models, we assume an exponentially decaying profile of the velocity at the inner border of the convective envelope, which allows the formation of a tiny ¹³C-pocket after a third dredge up episode. The {s}-process nucleosynthesis occurring in an AGB star of initial mass M=2 M_ȯ and solar metallicity is presented.

Title:		Nucleosynthesis in very metal poor AGB stars
Authors:		Straniero, O.; Cristallo, S.; Gallino, R.; Dominguez, I.
Affiliation:		AA(Osservatorio Astronomico di Collurania, via Mentore Maggini, 64100 Teramo, Italy ), AB(Osservatorio Astronomico di Collurania, via Mentore Maggini, 64100 Teramo, Italy), AC(Dipartimento di Fisica, Universitá di Torino e Sezione INFN di Torino, via P. Giuria 1, 10125 Torino, Italy), AD(Dpto. de Fisica Teorica y del Cosmos, Universidad de Granada, 18071 Granada, Spain)
Publication:		Memorie della Società Astronomica Italiana, v.75, p.665 (2004)
Publication Date:		00/2004
Origin:		MmSAI
Keywords:		AGB stars, nucleosynthesis, s-process
Bibliographic Code:		2004MmSAI..75..665S

Abstract

High resolution spectroscopy of very metal poor C- and s-rich stars provides a fossil record of the nucleosynthesis that occurred several Gyr ago in halo AGB stars. They are dwarfs or giants, whose surface composition was polluted by the wind of an AGB companion. Then, the C and s-element enhancements are ashes of the nucleosynthesis occurred in the He-rich inter-shell of a low metallicity AGB star. We present a calculation of a Z=5×10^-5 low mass (M=1.5 M_ȯ) AGB stellar model. A full network including all the relevant isotopes up to the end point of the s-process path has been used. After a few weak thermal pulses, the convective region generated by the He burning engulfs protons from the overlying H-rich envelope into the hot He-rich region. This occurrence induces a violent H-flash, followed by a quiescent double shell burning and, later on, by a deep dredge up. During the H-flash the density of the neutrons released by the ¹³C(alpha ,n)¹⁶O becomes quite large (>10¹³ cm^-3), but only for a few days. Then, after the deep dredge up episode, the envelope composition is strongly enhanced in C and N, but not in s-elements. At that time, the mass fraction of CNO in the envelope is as large as 10^-3 and the subsequent evolution becomes similar to that of more metal-rich AGBs. Then, each thermal pulse is regularly followed by a third dredge up episode. Near to the region of the maximum penetration of the convective envelope, a ¹³C-pocket forms, whose burning produces neutrons and allows the s-process nucleosynthesis. A secondary burst of s-process, resulting from the activation of the ²²Ne(alpha ,n)²⁵Mg reaction, takes place within the convective zone generated by the thermal pulse. The evolution of the surface composition is presented in some details.

Title:		First chemical analysis of extragalactic carbon stars
Authors:		Abia, C.; de Laverny, P.; Domínguez, I.; Straniero, O.; Cristallo, S.; Plez, B.
Affiliation:		AA(Dpto. de Física Teórica y del Cosmos, Universidad de Granada, 18071 Granada, Spain, ), AB(Observatoire de la Côte d'Azur, Dpt. Cassiopée UMR6202, 06304 Nice Cedex 4, France), AC(Dpto. de Física Teórica y del Cosmos, Universidad de Granada, 18071 Granada, Spain), AD(INAF-Osservatorio di Collurania, 64100 Teramo, Italy), AE(INAF-Osservatorio di Collurania, 64100 Teramo, Italy), AF(Université de Montpellier II, Montpellier, France)
Publication:		Memorie della Società Astronomica Italiana, v.75, p.607 (2004)
Publication Date:		00/2004
Origin:		MmSAI
Keywords:		AGB stars, carbon stars, nucleosynthesis, dwarf galaxies
Bibliographic Code:		2004MmSAI..75..607A

Abstract

We have performed the chemical analysis of extragalactic carbon stars from VLT/UVES spectra. The derived individual abundances of metals and s-elements as well as the well known distance of the selected stars in the Small Magellanic Cloud and the Sagittarius dwarf galaxy permit us to test current models of stellar evolution and nucleosynthesis during the asymptotic giant branch phase in low metallicity environments.

Title:		Self-consistent ¹³C pocket in low mass AGB stars and related nucleosynthesis
Authors:		Cristallo, S.; Gallino, R.; Straniero, O.
Affiliation:		AA(Dipartimento di Fisica, Universitá di Torino e Sezione INFN di Torino, via P. Giuria 1, 10125 Torino, Italy), AB(Dipartimento di Fisica, Universitá di Torino e Sezione INFN di Torino, via P. Giuria 1, 10125 Torino, Italy), AC(Osservatorio Astronomico di Collurania, via Mentore Maggini, 64100 Teramo, Italy)
Publication:		Memorie della Società Astronomica Italiana, v.75, p.174 (2004)
Publication Date:		00/2004
Origin:		MmSAI
Keywords:		s-process, AGB stars, nucleosynthesis
Bibliographic Code:		2004MmSAI..75..174C

Abstract

We present new results concerning the self-consistent formation of a tiny ¹³C pocket in a star with initial mass M = 3 M_ȯ and solar metallicity during the Asymptotic Giant Branch phase. The introduction of an exponentially decaying profile of velocity below the convective envelope during third dredge up episodes allows a small amount of protons to diffuse in the ⁴He- and ¹²C- rich zone, leading to the formation of a ¹³C-rich layer, whose extension varies from pulse to pulse. The resulting s-process nucleosynthesis is discussed and compared with previous models. Moreover, new models show an enhancement factor of about 50% in the efficiency of Third dredge up, which starts when the core mass M_H≃ 0.58 M_ȯ, in better agreement with the observed C-star luminosity function.

Title:		Minute steps on the quest of the s-process
Authors:		Gallino, R.; Arnone, E.; Cristallo, S.; Masera, S.; Travaglio, C.; Lambert, D. L.; Lugaro, M.; Käppeler, F.; van Winckel, H.; Reyniers, M.; Straniero, O.; Davis, A. M.
Affiliation:		AA(Dipartimento di Fisica Generale, Universita' di Torino Via. P. Giuria 1, 10125 Torino I-10125, Italy), AB(Dipartimento di Fisica Generale, Universita' di Torino Via. P. Giuria 1, 10125 Torino I-10125, Italy), AC(Dipartimento di Fisica Generale, Universita' di Torino Via. P. Giuria 1, 10125 Torino I-10125, Italy), AD(Dipartimento di Fisica Generale, Universita' di Torino Via. P. Giuria 1, 10125 Torino I-10125, Italy), AE(Max-Planck Institut für Astrophysik, Karl-Schwarzschild-Str. 1, D-85748, Garching, Germany), AF(Department of Astronomy, University of Texas, Austin, TX 78712, USA), AG(Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB30HA, United Kingdom), AH(Forschungszentrum Karlsruhe, Institut für Kernphysik Postfach 3640, D-76021 Karlsruhe, Germany), AI(Instituut voor Sterrenkunde, K.U. Leuven, Celestijnenlaan 200B, B-3001 Leuven, Belgium), AJ(Instituut voor Sterrenkunde, K.U. Leuven, Celestijnenlaan 200B, B-3001 Leuven, Belgium), AK(Osservatorio Astronomico di Collurania, Teramo I-64100, Italy), AL(Enrico Fermi Institute and Department of the Geophysical Sciences, University of Chicago, 5640 S. Ellis Avenue, Chicago, IL 60637, USA)
Publication:		Nuclear Physics A, Volume 718, p. 181-188.
Publication Date:		05/2003
Origin:		ELSEVIER
DOI:		10.1016/S0375-9474(03)00711-5
Bibliographic Code:		2003NuPhA.718..181G

Abstract

Not Available

Title:		The s-process in meteoritic interstellar grains, a continuos interaction between Nuclear Physics and Astrophysics
Authors:		Gallino, R.; Kaeppeler, F.; Reifarth, R.; Straniero, O.; Cristallo, S.; Davis, A. M.; Dominguez, I.
Publication:		EGS - AGU - EUG Joint Assembly, Abstracts from the meeting held in Nice, France, 6 - 11 April 2003, abstract #13716
Publication Date:		04/2003
Origin:		EGU
Bibliographic Code:		2003EAEJA....13716G

Abstract

The results of s-process nucleosynthesis calculations for Asymptotic Giant Branch stars (AGB) of different initial mass and different metallicity are discussed in the framework of stellar evolutionary models using an updated network of neutron capture cross sections. They are applied to the astrophysical interpretation of the stellar origin of meteoritic interstellar SiC grains of different populations. The neutron production is driven by the ¹³C(α,n)¹⁶O reaction taking place in radiative conditions during the interpulse periods in a tiny layer (¹³C pocket) at the top of the He- and C-rich region. A second neutron exposure is released by the reaction ²²Ne(α,n)²⁵Mg during recurrent convective thermal pulses. This neutron source is marginal in low mass AGB stars. However, it affects the final distribution of isotopes involved in branching in the s-flow. The s-enriched material is subsequently mixed with the envelope according to the efficiency of the third dredge up, and then ejected in the interstellar medium by stellar winds. Noble gases are most likely implanted in the already formed SiC grains in the protoplanetary phase. We provide strong constraints for the Ne-E(H), Kr-S and Xe-S meteoritic components as compared with stellar models. With decreasing the initial metallicity, from solar to around 1/10 solar, the s-process abundance distribution in AGB stars first feeds the Zr-peak, then the Zr-peak is partly bypassed in favor of the Ba-peak (at metallicities around one third solar), eventually the s-process distribution is peaked at the double-magic nucleus ²⁰⁸Pb, at the termination point of the s-path. The predicted s-process isotopic composition of all elements is strongly dependent on the metallicity, especially for isotopes involved in branchings in the s-flow. This has important impacts in discriminating the origin of subclasses Y and Z SiC grains with future NANOSIMS OR CHARISMA measurements.

Title:		Low-Mass AGB Stellar Models for 0.003 <= Z <= 0.02: Basic Formulae for Nucleosynthesis Calculations
Authors:		Straniero, O.; Domínguez, I.; Cristallo, S.; Gallino, R.
Affiliation:		AA(INAF, Osservatorio Astronomico di Teramo, Italy ), AB(Universidad de Granada, Spain ), AC(INAF, Osservatorio Astronomico di Teramo, Italy ), AD(Dip. Fisica Generale Università di Torino and Sez. INFN Torino, Italy ; Center for Stellar and Planetary Astrophysics, Monash University, Melbourne 3800, Australia)
Publication:		Publications of the Astronomical Society of Australia, Volume 20, Issue 4, pp. 389-392. (PASA Homepage)
Publication Date:		00/2003
Origin:		PASA
Keywords:		stars: abundances, stars: AGB, stars: evolution, nucleosynthesis
DOI:		10.1071/AS03041
Bibliographic Code:		2003PASA...20..389S

Abstract

We have extended our published set of low-mass AGB stellar modelsto lower metallicities. Different mass-loss rates have been explored. We provide interpolation formulae for the luminosity, effective temperature, core mass, mass of dredge up material and maximum temperature in the convective zone generated by thermal pulses. Finally, we discuss the resultant modification of these quantities when we use an appropriate treatment of the inward propagation of the convective instability, as caused by the steeprise in radiative opacity when the convective envelope penetratesthe H-depleted region.

Title:		Pb/eu/ba/zr AGB Predictiions for Globulars and the Halo.
Authors:		Gallino, Roberto; Pignatari, Marco; Cristallo, Sergio; Straniero, Oscar; Travaglio, Claudia; Busso, Maurizio
Affiliation:		AA(Dip. Fisica Generale Universita' di Torino)
Publication:		Astrophysical Impact of Abundances in Globular Cluster Stars, 25th meeting of the IAU, Joint Discussion 4, 16-17 July 2003, Sydney, Australia
Publication Date:		00/2003
Origin:		IAU
Bibliographic Code:		2003IAUJD...4E..27G

Abstract

We present predictions of neutron-capture elements produced from AGB stars at various metallicities. We concentrate on s-process in low-metallicity stars. We show that the usual r and s process separation using the classsical analysis of the s-process fails at low metallicicty where the s-flow preferentially feeds Lead. We compare our prediction with a number of 'lead stars' recently observed..

Title:		The formation of the ¹³C pocket in Asymptotic Giant Branch stars and related nucleosynthesis
Authors:		Cristallo, S.; Straniero, O.; Gallino, R.; Herwig, F.; Chieffi, A.; Limongi, M.; Busso, M.
Publication:		Nuclear Physics A, Volume 688, Issue 1-2, p. 217-220.
Publication Date:		05/2001
Origin:		ELSEVIER
DOI:		10.1016/S0375-9474(01)00701-1
Bibliographic Code:		2001NuPhA.688..217C

Abstract

Not Available

Title:		On the Need for Deep-mixing in Asymptotic Giant Branch Stars of Low Mass
Authors:		Busso, M.; Palmerini, S.; Maiorca, E.; Cristallo, S.; Straniero, O.; Abia, C.; Gallino, R.; La Cognata, M.
Affiliation:		AA(Dipartimento di Fisica, Università di Perugia, Italy ; INFN, Sezione di Perugia, Italy ; ), AB(Dipartimento di Fisica, Università di Perugia, Italy ; INFN, Sezione di Perugia, Italy ), AC(Dipartimento di Fisica, Università di Perugia, Italy ; INFN, Sezione di Perugia, Italy ), AD(Departamento de Física Teórica y del Cosmos, Universidad de Granada, Spain ; INAF, Osservatorio di Teramo, Italy ), AE(INAF, Osservatorio di Teramo, Italy ; INFN, Sezione di Napoli, Italy ), AF(Departamento de Física Teórica y del Cosmos, Universidad de Granada, Spain ), AG(INAF, Osservatorio di Teramo, Italy ; Dipartimento di Fisica Generale, Università di Torino, Italy ), AH(Dipartimento di Metodologie Fisiche e Chimiche per l'Ingegneria, Università di Catania, Italy ; INFN, Laboratori Nazionali del Sud, Catania, Italy)
Publication:		The Astrophysical Journal Letters, Volume 717, Issue 1, pp. L47-L51 (2010). (ApJL Homepage)
Publication Date:		07/2010
Origin:		IOP
ApJ Keywords:		nuclear reactions, nucleosynthesis, abundances, stars: AGB and post-AGB, stars: carbon, stars: evolution, stars: low-mass
DOI:		10.1088/2041-8205/717/1/L47
Bibliographic Code:		2010ApJ...717L..47B