Pre-supernova mixing in CEMP-no source stars
1 Geneva Observatory, University of Geneva, Maillettes 51, 1290 Sauverny, Switzerland
2 Astrophysics Group, Lennard-Jones Labs 2.09, Keele University, ST5 5BG, Staffordshire, UK
3 Kavli Institute for the Physics and Mathematics of the Universe (WPI), University of Tokyo, 5-1-5 Kashiwanoha, 277-8583 Kashiwa, Japan
4 UK Network for Bridging the Disciplines of Galactic Chemical Evolution (BRIDGCE), UK
Received: 2 December 2016
Accepted: 26 May 2017
Context. CEMP-no stars are long-lived low-mass stars with a very low iron content, overabundances of carbon and no or minor signs for the presence of s- or r-elements. Although their origin is still a matter of debate, they are often considered as being made of a material ejected by a previous stellar generation (source stars).
Aims. We place constraints on the source stars from the observed abundance data of CEMP-no stars.
Methods. We computed source star models of 20, 32, and 60 M⊙ at Z = 10-5 with and without fast rotation. For each model we also computed a case with a late mixing event occurring between the hydrogen and helium-burning shell ~200 yr before the end of the evolution. This creates a partially CNO-processed zone in the source star. We use the 12C/13C and C/N ratios observed on CEMP-no stars to put constraints on the possible source stars (mass, late mixing or not). Then, we inspect more closely the abundance data of six CEMP-no stars and select their preferred source star(s).
Results. Four out of the six CEMP-no stars studied cannot be explained without the late mixing process in the source star. Two of them show nucleosynthetic signatures of a progressive mixing (due e.g. to rotation) in the source star. We also show that a 20 M⊙ source star is preferred compared to one of 60 M⊙ and that likely only the outer layers of the source stars were expelled to reproduce the observed 12C/13C.
Conclusions. The results suggest that (1) a late mixing process could operate in some source stars; (2) a progressive mixing, possibly achieved by fast rotation, is at work in several source stars; (3) ~20 M⊙ source stars are preferred compared to ~60 M⊙ ones; and (4) the source star might have preferentially experienced a low energetic supernova with large fallback.
Key words: stars: abundances / stars: massive / stars: chemically peculiar / nuclear reactions, nucleosynthesis, abundances / stars: interiors
© ESO, 2017