Universität zu Köln

Matrozis, E. and Stancliffe, R. J. (2017). Rotational mixing in carbon-enhanced metal-poor stars with s-process enrichment. Astron. Astrophys., 606. LES ULIS CEDEX A: EDP SCIENCES S A. ISSN 1432-0746

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Abstract

Carbon-enhanced metal-poor (CEMP) stars with s-process enrichment (CEMP-s) are believed to be the products of mass transfer from an asymptotic giant branch (AGB) companion, which has long since become a white dwarf. The surface abundances of CEMP-s stars are thus commonly assumed to reflect the nucleosynthesis output of the first AGB stars. We have previously shown that, for this to be the case, some physical mechanism must counter atomic diffusion (gravitational settling and radiative levitation) in these nearly fully radiative stars, which otherwise leads to surface abundance anomalies clearly inconsistent with observations. Here we take into account angular momentum accretion by these stars. We compute in detail the evolution of typical CEMP-s stars from the zero-age main sequence, through the mass accretion, and up the red giant branch for a wide range of specific angular momentum j(a) of the accreted material, corresponding to surface rotation velocities, v(rot), between about 0.3 and 300 km s(-1). We find that only for j(a) greater than or similar to 10(17) cm(2) s(-1) (v(rot) > 20 km s(-1), depending on mass accreted) angular momentum accretion directly causes chemical dilution of the accreted material. This could nevertheless be relevant to CEMP-s stars, which are observed to rotate more slowly, if they undergo continuous angular momentum loss akin to solar-like stars. In models with rotation velocities characteristic of CEMP-s stars, rotational mixing primarily serves to inhibit atomic diffusion, such that the maximal surface abundance variations (with respect to the composition of the accreted material) prior to first dredge-up remain within about 0.4 dex without thermohaline mixing or about 0.5-1.5 dex with thermohaline mixing. Even in models with the lowest rotation velocities (v(rot) less than or similar to 1 km s(-1)), rotational mixing is able to severely inhibit atomic diffusion, compared to non-rotating models. We thus conclude that it offers a natural solution to the problem posed by atomic diffusion and cannot be neglected in models of CEMP-s stars.

Item Type:	Journal Article
Creators:	Creators Email ORCID ORCID Put Code Matrozis, E. UNSPECIFIED UNSPECIFIED UNSPECIFIED Stancliffe, R. J. UNSPECIFIED UNSPECIFIED UNSPECIFIED
URN:	urn:nbn:de:hbz:38-214441
DOI:	10.1051/0004-6361/201731272
Journal or Publication Title:	Astron. Astrophys.
Volume:	606
Date:	2017
Publisher:	EDP SCIENCES S A
Place of Publication:	LES ULIS CEDEX A
ISSN:	1432-0746
Language:	English
Faculty:	Unspecified
Divisions:	Unspecified
Subjects:	no entry
Uncontrolled Keywords:	Keywords Language INTERNAL GRAVITY-WAVES; ANGULAR-MOMENTUM TRANSPORT; LOW-METALLICITY STARS; MAIN-SEQUENCE; DIFFERENTIAL ROTATION; STELLAR EVOLUTION; MICROSCOPIC DIFFUSION; MAGNETIC ACTIVITY; COOL STARS; MODELS Multiple languages Astronomy & Astrophysics Multiple languages
Refereed:	Yes
URI:	http://kups.ub.uni-koeln.de/id/eprint/21444