Evolution, Nucleosynthesis, and Yields of Low-Mass Asymptotic Giant Branch Stars at Different Metallicities

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 lowmass AGB stars (2 M ), 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 13 C pocket, which forms in the core-envelope transition region after each TDU episode. The formation of this 13 C 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 13 C(α, n) 16 O reaction during the interpulse phase in radiative conditions, when temperatures within the pockets attain T ∼ 1.0 × 10 8 K, with typical densities of (10 6 -10 7 ) 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 13 C 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 × 10 8 K, thus producing larger neutron densities (up to 10 11 neutrons cm −3 ). An additional neutron exposure, caused by the 22 Ne(α, n) 25 Mg during the TPs, is marginally activated at large Z, but becomes an important nucleosynthesis source at low Z, when most of the 22 Ne 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, w...

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

“…Among the elements lighter than Fe, Na is the only one whose abundance changes significantly for the different profiles tested here (Table 6). This is because 23 Na (the only stable isotope of Na) is produced in the H-burning shell and in the top layers of the PMZ by proton captures on 22 Ne and in the presence of neutrons in the 13 C pocket and in the TPs via neutron captures on 22 Ne (Goriely & Mowlavi 2000;Cristallo et al 2009;Bisterzo et al 2011;Lugaro et al 2012). As a consequence both the mixing profile itself and its effect on the intershell abundance of 22 Ne play a significant role.…”

“…More recently, Herwig (2000) and Cristallo et al (2009) successfully employed convective overshoot to reproduce the mixing mechanism responsible for the PMZ. Herwig (2000) modelled the overshoot via the diffusion coefficient while Cristallo et al (2009) via the convective velocity, both using an exponential decay function of the form:…”

“…Cristallo et al (2009Cristallo et al ( , 2011 presented and discussed detailed s-process models and results based on this scheme (the FRUITY database, http://fruity.oa-teramo.inaf.it/). The results are very similar to those obtained using the parametric model that we present below, where the PMZ is introduced artificially using an exponential mixing profile (for a detailed comparison between the two sets of models see Lugaro et al 2012;Fishlock et al 2014;.…”

“…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.…”

Asymptotic Giant Branch Evolution and the Initial-Final Mass Relation of Single CO White Dwarfs

“…During the third dredge-up the base of the convective envelope reaches across the extinct H-shell and penetrates into the inter-shell region where the He-flash nucleosynthesis has taken place. The net consequence is the enrichment in the surface abundances of 4 He, primary 12 C and 16 O, 19 F, 22 Ne, 25 Mg, and s-process elements (Iben & Truran 1978;Cristallo et al 2009). At the same time, the third dredge-up causes a reduction of the core mass, the entity of which is traditionally quantified via the efficiency parameter, λ = ΔM dred /ΔM c , given by the ratio between the dredged-up mass and the core mass increment over the preceding quiescent inter-pulse period.…”