Universität zu Köln

Matrozis, E. and Stancliffe, R. J. (2016). Radiative levitation in carbon-enhanced metal-poor stars with s-process enrichment. Astron. Astrophys., 592. LES ULIS CEDEX A: EDP SCIENCES S A. ISSN 1432-0746

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Abstract

A significant fraction of all metal-poor stars are carbon-rich. Most of these carbon-enhanced metal-poor (CEMP) stars also show enhancement in elements produced mainly by the s-process (CEMP-s stars), and evidence suggests that the origin of these nonstandard abundances can be traced to mass transfer from a binary asymptotic giant branch (AGB) companion. Thus, observations of CEMP-s stars are commonly used to infer the nucleosynthesis output of low-metallicity AGB stars. A crucial step in this exercise is understanding what happens to the accreted material after mass transfer ceases. Here we present models of the post-mass-transfer evolution of CEMP-s stars considering the physics of thermohaline mixing and atomic diffusion, including radiative levitation. We find that stars with typical CEMP-s star masses, M approximate to 0 : 85 M-circle dot, have very shallow convective envelopes (M-env less than or similar to 10(-7) M-circle dot). Hence, the surface abundance variations arising from the competition between gravitational settling and radiative levitation should be orders of magnitude larger than observed (e. g. [C/Fe] < 1 or [C/Fe] > + 4). Lower-mass stars (M approximate to 0.80 M-circle dot) retain thicker convective envelopes and thus show variations more in line with observations, but are generally too unevolved (log g > 4) when they reach the age of the Universe. We are therefore unable to reproduce the spread in the observed abundances with these models and conclude that some other physical process must largely suppress atomic diffusion in the outer layers of CEMP-s stars. We demonstrate that this could be achieved by some additional (turbulent) mixing process operating at the base of the convective envelope, as found by other authors. Alternatively, mass-loss rates around 10(-13) M-circle dot yr(-1) could also negate most of the abundance variations by eroding the surface layers and forcing the base of the convective envelope to move inwards in mass. Since atomic diffusion cannot have a substantial effect on the surface abundances of CEMP-s stars, the dilution of the accreted material, while variable in degree from one star to the next, is most likely the same for all elements.

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:38-267303
DOI:	10.1051/0004-6361/201628540
Journal or Publication Title:	Astron. Astrophys.
Volume:	592
Date:	2016
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 GALACTIC CHEMICAL EVOLUTION; LOW-METALLICITY STARS; DIGITAL SKY SURVEY; MASS-LOSS RATES; STELLAR ASTROPHYSICS MESA; 1ST SPECTROSCOPIC DATA; GIANT BRANCH MODELS; SUBDWARF B STARS; SOLAR-LIKE STARS; ABUNDANCE ANOMALIES Multiple languages Astronomy & Astrophysics Multiple languages
Refereed:	Yes
URI:	http://kups.ub.uni-koeln.de/id/eprint/26730