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 12C(p,γ)13N(β)13C(α,n)16O. 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 12C/^{13C} 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.
- Publication:
-
Astronomy and Astrophysics
- Pub Date:
- November 2013
- DOI:
- 10.1051/0004-6361/201219513
- arXiv:
- arXiv:1308.2224
- Bibcode:
- 2013A&A...559A...4C
- Keywords:
-
- stars: evolution;
- nuclear reactions;
- nucleosynthesis;
- abundances;
- stars: individual: HE1327-2326;
- stars: individual: HE0107-5240;
- stars: abundances;
- Astrophysics - Solar and Stellar Astrophysics
- E-Print:
- 12 pages, 8 figures, Accepted for publication by A&