Light element abundances in carbon-enhanced metal-poor stars

Abstract

We model the evolution of the abundances of light elements in carbon-enhanced metal-poor (CEMP) stars, under the assumption that such stars are formed by mass transfer in a binary system. We have modelled the accretion of material ejected by an asymptotic giant branch star on to the surface of a companion star. We then examine three different scenarios: one in which the material is mixed only by convective processes, one in which thermohaline mixing is present and a third in which both thermohaline mixing and gravitational settling are taken in to account. The results of these runs are compared to light element abundance measurements in CEMP stars (primarily CEMP-s stars, which are rich in $s$-processes elements and likely to have formed by mass transfer from an AGB star), focusing on the elements Li, F, Na and Mg. None of the elements is able to provide a conclusive picture of the extent of mixing of accreted material. We confirm that lithium can only be preserved if little mixing takes place. The bulk of the sodium observations suggest that accreted material is effectively mixed but there are also several highly Na and Mg-rich objects that can only be explained if the accreted material is unmixed. We suggest that the available sodium data may hint that extra mixing is taking place on the giant branch, though we caution that the data is sparse.

Accepted for publication in MNRAS. 9 figures, 1 table

References in corpus (18)

• Carbon Enhanced Metal-Poor Stars. I. Chemical Compositions of 26 Stars
• Thermohaline mixing: A physical mechanism governing the photospheric composition of low-mass giants
• Stellar Models and Yields of Asymptotic Giant Branch Stars
• Deep Mixing of He-3: Reconciling Big Bang and Stellar Nucleosynthesis
• Atomic Diffusion and Mixing in Old Stars I. VLT/FLAMES-UVES Observations of Stars in NGC 6397
• The Frequency of Carbon-Enhanced Metal-Poor Stars in the Galaxy from the HERES sample
• Carbon-Enhanced Metal-Poor Stars. III. Main-Sequence Turn-Off Stars from the SDSS/SEGUE Sample
• Thermohaline mixing and gravitational settling in carbon-enhanced metal-poor stars
• CS22964-161: A Double-Lined Carbon- and s-Process-Enhanced Metal-Poor Binary Star
• Atomic diffusion and mixing in old stars II. Observations of stars in the globular cluster NGC 6397 with VLT/FLAMES-GIRAFFE
• The Hobby-Eberly Telescope "Chemical Abundances of Stars in the Halo" (CASH) Project. I. The Lithium-, s-, and r-Enhanced Metal-Poor Giant HKII 17435-00532
• CH stars at High Galactic Latitudes
• Mass loss and yield uncertainty in low-mass asymptotic giant branch stars
• Fluorine in carbon-enhanced metal-poor stars: a binary scenario
• 3He-Driven Mixing in Low-Mass Red Giants: Convective Instability in Radiative and Adiabatic Limits
• The Impact of Carbon Enhancement on Extra Mixing in Metal-Poor Stars
• Does simultaneous solution matter for stellar evolution codes?
• The impact of the 18F(a,p)21Ne reaction on asymptotic giant branch nucleosynthesis