Evolution and Nucleosynthesis in Low-Mass Asymptotic Giant Branch Stars. II. Neutron Capture and the s-Process (original) (raw)
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Nucleosynthesis and mixing on the asymptotic giant branch. I - MS and S stars with and without TC
The Astrophysical Journal, 1992
We present the results of s-process nucleosynthesis calculations for Asymptotic Giant Branch (AGB) stars of different metallicities and different initial stellar masses (1.5 and 3 M ⊙ ) and comparisons of them with observational constraints from high resolution spectroscopy of evolved stars over a wide metallicity range. The computations were based on previously published stellar evolutionary models that account for the third dredge up phenomenon occurring late on the AGB. Neutron production is driven by the 13 C(α,n) 16 O reaction during the interpulse periods in a tiny layer in radiative equilibrium at the top of the He-and C-rich shell. The neutron source 13 C is manufactured locally by proton captures on the abundant 12 C; a few protons are assumed to penetrate from the convective envelope into the radiative layer at any third dredge up episode, when a chemical discontinuity is established between the convective envelope and the He-and Crich zone. A weaker neutron release is also guaranteed by the marginal activation of the reaction 22 Ne(α,n) 25 Mg during the convective thermal pulses. Owing to the lack of a consistent model for 13 C formation, the abundance of 13 C burnt per cycle is allowed to vary as a free parameter over a wide interval (a factor of 50). The s-enriched material is subsequently mixed with the envelope by the third dredge up, and the envelope composition is computed after each thermal pulse. We follow the changes in the photospheric abundance of the Ba-peak elements (heavy s, or 'hs') and that of the Zr-peak ones (light s, or 'ls'), whose logarithmic ratio [hs/ls] has often been adopted as an indicator of the s-process efficiency (e.g. of the neutron exposure). Our model predictions for this parameter show a complex trend versus metallicity. Especially noteworthy is the prediction that the flow along the s path at low metallicities drains the Zr-peak and Ba-peak and builds an excess at the doubly-magic 208 Pb, at the termination of the s path. We then discuss the effects on the models of variations in the crucial parameters of the 13 C pocket, finding that they are not critical for interpreting the results.
s process in low-mass asymptotic giant branch stars
Nuclear Physics A, 2006
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 (10 5 yr) that elapses between two subsequent thermal pulses, a slow neutron flux is provided by the 13 C(α,n) 16 O reaction taking place within a thin 13 C 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 22 Ne(α,n) 25 Mg 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 13 C pocket. In order to illustrate the capabilities and the limits of the theory, an updated computation of a 2 M ⊙ 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.
Astrophysical Journal, 1997
A 10.5 model of Population I composition is evolved from the main sequence through the core M _ carbon-burning phase. As in 9 and 10 models studied in earlier papers of this series, carbon is M _ ignited o † center, but more carbon Ñashes occur before hydrogen is reignited and carbon burning dies out. Beginning with the second carbon Ñash, a carbon-burning Ñame propagates to the stellar center. The Ñame is divided into two parts by the Ñame "" front ÏÏ which is deÐned to coincide with the base of an associated convective shell. Ahead of the front is a "" precursor ÏÏ Ñame in which nuclear energy is converted into heat and the work of expansion at a rate comparable to the rate of release of nuclear energy in the convective shell. The width in mass of the precursor Ñame relative to the distance of the front from the center varies from D0.01 when the front is at D0.04
s ‐Process Nucleosynthesis in Asymptotic Giant Branch Stars: A Test for Stellar Evolution
The Astrophysical Journal, 2003
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 M ⊙ and solar metallicity star. First we investigate the formation and the efficiency of the main neutron source: the 13 C(α,n) 16 O reaction that occurs in radiative conditions. A tiny region rich in 13 C (the 13 C pocket) is created by proton captures on the abundant 12 C 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 protons over 12 C number ratio, p/ 12 C ∼ > 0.3, most of the 13 C nuclei produced are further converted by proton capture to 14 N. Besides, 14 N nuclei represent a major neutron poison. We find
The Astrophysical Journal, 1996
A 10.5 model of Population I composition is evolved from the main sequence through the core M _ carbon-burning phase. As in 9 and 10 models studied in earlier papers of this series, carbon is M _ ignited o † center, but more carbon Ñashes occur before hydrogen is reignited and carbon burning dies out. Beginning with the second carbon Ñash, a carbon-burning Ñame propagates to the stellar center. The Ñame is divided into two parts by the Ñame "" front ÏÏ which is deÐned to coincide with the base of an associated convective shell. Ahead of the front is a "" precursor ÏÏ Ñame in which nuclear energy is converted into heat and the work of expansion at a rate comparable to the rate of release of nuclear energy in the convective shell. The width in mass of the precursor Ñame relative to the distance of the front from the center varies from D0.01 when the front is at D0.04
The new s-process in low mass TP-AGB stars
Nuclear Physics A, 1997
The s-process nucleosynthesis occurring in the Asymptotic Giant Branch phases has been computed through a post-process calculation, following the results of stellar evolutionary models obtained with the FRANEC code for low mass stars (1-3 M®) with metallicity Z = 0.02. Mass loss with the Reimers (1975)'s parameterization was taken into account (r/= 3). Since the development of the third dredge up, we assumed that a laC-pocket is formed at the H/He discontinuity when the H-shell is inactive. Neutrons are released by the 13C(a,n)160 source in radiative conditions in the interpulse period in a thin layer of a few 10 .4 M®. The 22Ne(a,n)2SMg reaction operates marginally in convective pulses, over a material that was previously s-processed in the radiative phase and mixed with fresh Fe and H-burning ashes. The conditions under which the solar main s-process component is reproduced are discussed.
s-Process Nucleosynthesis in Advanced Burning Phases of Massive Stars
Astrophysical Journal - ASTROPHYS J, 2006
We present a detailed study of s-process nucleosynthesis in massive stars of solar-like initial composition and masses 15, 20,25, and 30 Msun. We update our previous results of s-process nucleosynthesis during the core He-burning of these stars and then focus on an analysis of the s-process under the physical conditions encountered during the shell-carbon burning. We show that the recent compilation of the Ne22(alpha,n)Mg25 rate leads to a remarkable reduction of the efficiency of the s-process during core He-burning. In particular, this rate leads to the lowest overproduction factor of Kr80 found to date during core He-burning in massive stars. The s-process yields resulting from shell carbon burning turn out to be very sensitive to the structural evolution of the carbon shell. This structure is influenced by the mass fraction of C12 attained at the end of core helium burning, which in turn is mainly determined by the C12(alpha,gamma)O16 reaction. The still present uncertainty in t...
s ‐Process Nucleosynthesis in Carbon Stars
The Astrophysical Journal, 2002
We present the first detailed and homogeneous analysis of the s-element content in Galactic carbon stars of N-type. Abundances of Sr,Y, Zr (low-mass selements, or ls) and of Ba, La, Nd, Sm and Ce (high-mass s-elements, hs) are derived using the spectral synthesis technique from high-resolution spectra. The N-stars analyzed are of nearly solar metallicity and show moderate s-element enhancements, similar to those found in S stars, but smaller than those found in the only previous similar study , and also smaller than those found in supergiant post-AGB stars. This is in agreement with the present understanding of the envelope s-element enrichment in giant stars, which is increasing along the spectral sequence M→MS→S→SC→C during the AGB phase. We compare the observational data with recent s-process nucleosynthesis models for different metallicities and stellar masses. Good agreement is obtained between low mass AGB star models (M 3M ⊙ ) and s-elements observations. In low mass AGB stars, the 13 C(α, n) 16 O reaction is the main source of neutrons for the s-process; a moderate spread, however, must exist in the abundance of 13 C that is burnt in different stars. By combining information deriving from the detection of Tc, the infrared colours and the theoretical relations between stellar mass, metallicity and the final C/O ratio, we conclude that most (or maybe all) of the N-stars studied in this work are intrinsic, thermally-pulsing AGB stars; their abundances are the consequence of the operation of third dredge-up and are not to be ascribed to mass transfer in binary systems.