Constraints of the Physics of Low-Mass Agb Stars From Ch and Cemp Stars

Abstract

We analyze a set of published elemental abundances from a sample of CH stars which are based on high resolution spectral analysis of ELODIE and SUBARU/HDS spectra. All the elemental abundances were derived from local thermodynamic equilibrium analysis using model atmospheres, and thus, they represent the largest homogeneous abundance data available for CH stars up to date. For this reason, we can use the set to constrain the physics and the nucleosynthesis occurring in low mass AGB stars. CH stars have been polluted in the past from an already extinct AGB companion and thus show s-process enriched surfaces. We discuss the effects induced on the -2 -surface AGB s-process distributions by different prescriptions for convection and rotation. Our reference theoretical FRUITY set fits only part of the observations. Moreover, the s-process observational spread for a fixed metallicity cannot be reproduced. At [Fe/H]>-1, a good fit is found when rotation and a different treatment of the inner border of the convective envelope are simultaneously taken into account. In order to increase the statistics at low metallicities, we include in our analysis a selected number of CEMP stars and, therefore, we compute additional AGB models down to [Fe/H]=-2.85. Our theoretical models are unable to attain the large [hs/ls] ratios characterizing the surfaces of those objects. We speculate on the reasons for such a discrepancy, discussing the possibility that the observed distribution is a result of a proton mixing episode leading to a very high neutron density (the so-called i-process).

Chemical analysis of CH stars – III. Atmospheric parameters and elemental abundances

“…The abundance results presented here will add to the abundance data obtained from our previous studies in a homogeneous manner and compiled in Cristallo et al (2016), that can be used to constrain the physics and the nucleosynthesis occuring in low-mass AGB stars. Abundance results of CEMP-r/s stars will provide observational constrains for theoretical studies of i-process, suggested in the recent years for the origin of CEMP-r/s stars.…”

The s-process in stellar sites

“…As a consequence, the H-burning shell possibly switches off, allowing the convective envelope to penetrate in regions where heavy isotopes have been freshly synthesized. As already highlighted, the main neutron sources is the 13 C(α,n) 16 O reaction. In order to obtain a sufficient amount of 13 C for the activation of the s-process, protons from the envelope have to be mixed (with a low efficiency) with the 12 C-rich material in the underlying radiative zone.…”

“…The different s-process trends as a function of the initial metallicity are reported in the left panel of Figure 3. Major neutron sources in stars are the 13 C(α,n) 16 O and the 22 Ne(α,n) 25 Mg reactions. While the number of iron seeds directly scales with metallicity, the number of neutrons may have a primary behavior, that is it may be a direct product of H-and He-burning.…”

“…In those objects, freshly synthesized elements are carried out to the surface by means of recurrent mixing episodes, commonly known as Third Dredge Ups (TDUs) (see [9] and references therein; see also [14]). Interested readers may download TP-AGB surface abundances and yields from the FRUITY database 1 [15,16]. The structure of an AGB star is made of a partially degenerate carbon-oxygen core, an He shell separated from an H shell by a thin region, and by a convective envelope.…”

Heavy Element Nucleosynthesis

“…Several effects play a role in fixing the extension of the TDUP, like extra diffusion of protons termed "overshooting" (see discussion in ( [8]), [2], [4]), rotationally-induced mixing ( [11]), or a recent attempt considering the effect of mixing by magnetic buoyancy in the framework of magnetohydrodynamics ( [20]). More work is needed to better understand mixing processes during the AGB phases of stars.…”