Smithsonian/NASA ADS Astronomy Abstract Service

Title:
The s-Process in Rotating Asymptotic Giant Branch Stars
Authors:
Herwig, Falk; Langer, Norbert; Lugaro, Maria
Affiliation:
AA(Department of Physics and Astronomy, University of Victoria, 3800 Finnerty Road, Victoria, BC V8P 1A1, Canada; ), AB(Astronomical Institute, Universiteit Utrecht, P.O. Box 80000, NL-3508 TA Utrecht, Netherlands; ), AC(Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK; )
Journal:
The Astrophysical Journal, Volume 593, Issue 2, pp. 1056-1073. (ApJ Homepage)
Publication Date:
08/2003
Origin:
UCP
ApJ Keywords:
Nuclear Reactions, Nucleosynthesis, Abundances, Stars: AGB and Post-AGB, Stars: Evolution, Stars: Interiors, Stars: Rotation
Abstract Copyright:
(c) 2003: The American Astronomical Society
DOI:
10.1086/376726
Bibliographic Code:
2003ApJ...593.1056H

Abstract

We model the nucleosynthesis during the thermal pulse phase of a rotating, solar metallicity, asymptotic giant branch (AGB) star of 3 Msolar, which was evolved from a main-sequence model rotating with 250 km s-1 at the stellar equator. Rotationally induced mixing during the thermal pulses produces a layer (~2×10-5 Msolar) on top of the CO core where large amounts of protons and 12C coexist. With a postprocessing nucleosynthesis and mixing code, we follow the abundance evolution in this layer, in particular that of the neutron source 13C and of the neutron poison 14N. In our AGB model mixing persists during the entire interpulse phase because of the steep angular velocity gradient at the core-envelope interface, thereby spreading 14N over the entire 13C-rich part of the layer. We follow the neutron production during the interpulse phase and find a resulting maximum neutron exposure of taumax=0.04 mbarn-1, which is too small to produce any significant s-process. In parametric models, we then investigate the combined effects of diffusive overshooting from the convective envelope and rotationally induced mixing. Just adding the overshooting and leaving the rotational mixing unchanged results in a small maximum neutron exposure (0.03 mbarn-1). Models with overshoot and weaker interpulse mixing-as perhaps expected from more slowly rotating stars-yield larger neutron exposures. In a model with overshooting without any interpulse mixing a neutron exposure of up to 0.72 mbarn-1 is obtained, which is larger than required by observations. We conclude that the incorporation of rotationally induced mixing processes has important consequences for the production of heavy elements in AGB stars. While through a distribution of initial rotation rates, it may lead to a natural spread in the neutron exposures obtained in AGB stars of a given mass in general-as appears to be required by observations-it may moderate the large neutron exposures found in models with diffusive overshoot in particular. Our results suggest that both processes, diffusive overshoot and rotational mixing, may be required to obtain a consistent description of the s-process in AGB stars that fulfills all observational constraints. Finally, we find that mixing due to rotation within our current framework does increase the production of 15N in the partial mixing zone. However, this increase is not large enough to boost the production of fluorine to the level required by observations.

Title:
s-Process Nucleosynthesis in Asymptotic Giant Branch Stars: A Test for Stellar Evolution
Authors:
Lugaro, Maria; Herwig, Falk; Lattanzio, John C.; Gallino, Roberto; Straniero, Oscar
Affiliation:
AA(Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK; .), AB(Department of Physics and Astronomy, University of Victoria, Box 3055, Victoria, BC V8W 3P6, Canada; .), AC(School of Mathematical Sciences, P.O. Box 28M, Monash University, Victoria 3800, Australia; .), AD(Dipartimento di Fisica Generale, Università di Torino, Via Pietro Giuria 1, 10125 Torino, Italy; .), AE(Osservatorio Astronomico di Collurania, Teramo, Italy; .)
Journal:
The Astrophysical Journal, Volume 586, Issue 2, pp. 1305-1319. (ApJ Homepage)
Publication Date:
04/2003
Origin:
UCP
ApJ Keywords:
Nuclear Reactions, Nucleosynthesis, Abundances, Stars: AGB and Post-AGB, Stars: Evolution
Abstract Copyright:
(c) 2003: The American Astronomical Society
DOI:
10.1086/367887
Bibliographic Code:
2003ApJ...586.1305L

Abstract

We study the slow neutron capture process (s-process) in asymptotic giant branch (AGB) stars using three different stellar evolutionary models computed for a 3 Msolar, solar metallicity star. First we investigate the formation and the efficiency of the main neutron source: the 13C(alpha, n)16O reaction that occurs in radiative conditions. A tiny region rich in 13C (the 13C pocket) is created by proton captures on the abundant 12C in the top layers of the He intershell, the zone between the H shell and the He shell. We parametrically vary the number of protons mixed from the envelope. For high local proton-to-12C number ratios, p/12C>~0.3, most of the 13C nuclei produced are further converted by proton capture to 14N. Besides, 14N nuclei represent a major neutron poison. We find that a linear relationship exists between the amount of 12C in the He intershell and the maximum value of the time-integrated neutron flux. Then we generate detailed s-process calculations on the basis of stellar evolutionary models constructed with three different codes, all of them self-consistently finding the third dredge-up, although with different efficiency. One of the codes includes a mechanism at each convective boundary that simulates time-dependent hydrodynamic overshoot. This mechanism depends on a free parameter f and results in partial mixing beyond convective boundaries, the most efficient third dredge-up, and the formation of the 13C pocket. For the other two codes, an identical 13C pocket is introduced in the postprocessing nucleosynthesis calculations. The models typically produce enhancements of heavy elements of about 2 orders of magnitude in the He intershell and of up to 1 order of magnitude at the stellar surface, after dilution with the convective envelope, thus generally reproducing spectroscopic observations. The results of the cases without overshoot are remarkably similar, pointing out that the important uncertainty in s-process predictions is the 13C pocket and not the intrinsic differences among different codes when no overshoot mechanism is included. The code including hydrodynamic overshoot at each convective boundary finds that the He intershell convective zone driven by the recurrent thermal instabilities of the He shell (thermal pulses) penetrates the C-O core, producing a He intershell composition near that observed in H-deficient central stars of planetary nebulae. As a result of this intershell dredge-up, the neutron fluxes have a higher efficiency, both during the interpulse periods and within thermal pulses. The s-element distribution is pushed toward the heavier s-process elements, and large abundances of neutron-rich isotopes fed by branching points in the s-process path are produced. Several observational constraints are better matched by the models without overshoot. Our study needs to be extended to different masses and metallicities and in the space of the free overshoot parameter f.

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