S-process in extremely metal-poor, low-mass stars

Abstract

Context. Extremely metal-poor (EMP), low-mass stars experience an ingestion of protons into the helium-rich layer during the core He-flash, resulting in the production of neutrons through the reactions 12 C(p, γ) 13 N(β) 13 C(α, n) 16 O. This is a potential site for the production of s-process elements in EMP stars, which does not occur in more metal-rich counterparts. The signatures of s-process elements in the two most iron deficient stars observed to date, HE1327-2326 & HE0107-5240, still await for an explanation. Aims. We investigate the possibility that low-mass EMP stars could be the source of s-process elements observed in extremely iron deficient stars, either as a result of self-enrichment or in a binary scenario as the consequence of a mass transfer episode. Methods. We present evolutionary and post-processing s-process calculations of a 1 M stellar model with metallicities of Z = 0, 10 −8 , and 10 −7 . We assess the sensitivity of nucleosynthesis results to uncertainties in the input physics of the stellar models with particular regard to the details of convective mixing during the core He-flash. Results. Our models provide the possibility of explaining the C, O, Sr, and Ba abundance for the star HE0107-5240 as the result of mass-transfer from a low-mass EMP star. The drawback of our model is that nitrogen would be overproduced and the 12 C/ 13 C abundance ratio would be underproduced in comparison to the observed values if mass would be transferred before the primary star enters the asymptotic giant branch phase. Conclusions. Our results show that low-mass EMP stars cannot be ruled out as companion stars that might have polluted HE1327-2326 and HE0107-5240 and produced the observed s-process pattern. However, more detailed studies of the core He-flash and the proton ingestion episode are needed to determine the robustness of our predictions.

Binarity among CEMP-no stars: an indication of multiple formation pathways?

“…These five stars are HE 0107−5240, SDSS J0929+0238, G77−61, HE 1150−0428 and CS 22957−027 (in order of increasing [Fe/H]). For HE 0107−5240, the very low [Mg/C] ratio was also reproduced in the mass-transfer model by Cruz et al (2013). G77−61 most likely has a white dwarf as a companion (Dearborn et al 1986), so past mass transfer is also not unlikely.…”

The lowest detected stellar Fe abundance: the halo star SMSS J160540.18−144323.1

“…Alternative explanations are unsatisfactory. The elevated abundance of carbon could be due to pollution from an intermediatemass companion star, but models predict that this also leads to similar enhancement of nitrogen and neutron-capture elements (Campbell & Lattanzio 2008;Campbell et al 2010;Cruz et al 2013). An initially metal-free, or perhaps metal-poor but carbonnormal, star could also be polluted by accretion from the ISM.…”

The Dawes Review 2: Nucleosynthesis and Stellar Yields of Low- and Intermediate-Mass Single Stars

“…Iwamoto (2009) AGB stars but no tabulated yields. No yields of s-process elements are available from very low-metallicity AGB models at the present (although see Campbell et al 2010;Cruz et al 2013).…”