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

¹ Max-Planck Institut für Astrophysik, Karl-Schwarzschild-Str. 1, 85748 Garching bei München, Germany
e-mail: aldos@ice.csic.es
² Instituto de Ciencias del Espacio (CSIC-IEEC), Facultad de Ciències, Campus UAB, 08193 Bellaterra, Spain

Received: 30 April 2012
Accepted: 19 August 2013

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(\beta)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 ${\mathrm{12}}^{}\mathrm{C}{\mathit{/}}^{\mathrm{13}}\mathrm{C}$ 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.