Evolution and Nucleosynthesis in Low‐Mass Asymptotic Giant Branch Stars. II. Neutron Capture and thes‐Process

Abstract

We present a new analysis of neutron capture occurring in low-mass asymptotic giant branch (AGB) stars su †ering recurrent thermal pulses. We use dedicated evolutionary models for stars of initial mass in the range 1 to 3 and metallicity from solar to half solar. Mass loss is taken into account with the M _ Reimers parameterization. The third dredge-up mechanism is self-consistently found to occur after a limited number of pulses, mixing with the envelope freshly synthesized 12C and s-processed material from the He intershell. During thermal pulses, the temperature at the base of the convective region barely reaches being the temperature in units of 108 K), leading to a marginal activation of T 8 D 3 (T 8 the 22Ne(a, n)25Mg neutron source. The alternative and much faster reaction 13C(a, n)16O must then play the major role. However, the 13C abundance left behind by the H shell is far too low to drive the synthesis of the s-elements. We assume instead that at any third dredge-up episode, hydrogen downÑows from the envelope penetrate into a tiny region placed at the top of the 12C-rich intershell, of the order of a few 10~4At H reignition, a 13C-rich (and 14N-rich) zone is formed. Neutrons by the major 13C M _ . source are then released in radiative conditions at during the interpulse period, giving rise to an T 8 D 0.9 efficient s-processing that depends on the 13C proÐle in the pocket. A second small neutron burst from the 22Ne source operates during convective pulses over previously s-processed material diluted with fresh Fe seeds and H-burning ashes. The main features of the Ðnal s-process abundance distribution in the material cumulatively mixed with the envelope through the various third dredge-up episodes are discussed. Contrary to current expectations, the distribution cannot be approximated by a simple exponential law of neutron irradiations. The s-process nucleosynthesis mostly occurs inside the 13C pocket ; the form of the distribution is built through the interplay of the s-processing occurring in the intershell zones and the geometrical overlap of di †erent pulses.The 13C pocket is of primary origin, resulting from proton captures on newly synthesized 12C. Consequently, the s-process nucleosynthesis also depends on Fe seeds, a lower metallicity favoring the production of the heaviest elements. This allows a wide range of s-element abundance distributions to be produced in AGB stars of di †erent metallicities, in agreement with spectroscopic evidence and with the Galactic enrichment of the heavy s-elements at the time of formation of the solar system. AGB stars of metallicity are the best candidates for the buildup of the main component, i.e., for the s-Z^1 2 Z _ distribution of the heavy elements from the Sr-Y-Zr peak up to the Pb peak, as deduced by meteoritic and solar spectroscopic analyses. A number of AGB stars may actually show in their envelopes an sprocess abundance distribution almost identical to that of the main component. Eventually, the astrophysical origin of mainstream circumstel...

Partial mixing and the formation of 13C pockets in AGB stars: effects on the s-process elements

“…In AGB stars the s process occurs in the intershell, both within the convective regions generated by the TPs and during the periods between TPs (the so-called interpulse) in radiative conditions (Straniero et al 1997;Gallino et al 1998;Busso et al 1999;Goriely & Mowlavi 2000;Lugaro et al 2003a;Cristallo et al 2009;Bisterzo et al 2010;Lugaro et al 2012). In the convective regions the neutron source is the 22 Ne(α,n) 25 Mg reaction.…”

The 13C Neutron Source and s-Processing in AGB Stars

“…Our 14 N concentration allows for a negligible 19 F production, as required by recent observations [14]. Elements below A = 90 are now produced more efficiently than in previous s-process models [12]; in particular, 86,87 Sr appear now to derive at almost 90% from AGB stars [7].…”

“…For A > 90, the accuracy is the same as possible before, in parameterized models, i.e. 10% [12]; but our 13 C reservoir is now based on sound physical principles. Although the mechanism is different, our results are rather similar to those by [13].…”

Evolution and Nucleosynthesis of Low and Intermediate-Mass Stars

“…Stellar yields are an essential ingredient of chemical evolution models. Karakas & Lattanzio (2014) [23] recently reviewed the stellar yields available from AGB stars, focusing on tabulated yields available in the literature (which do not include the many studies of the s-process including [3,4,8,16,27,34]). The main deficiencies identified were stellar yields for very metal-poor AGB stars and stellar yields of heavy elements for all metallicities for a significant range of initial mass.…”