Radiative levitation in carbon-enhanced metal-poor stars withs-process enrichment

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 ≈ 0.85 M , have very shallow convective envelopes (M env 10 −7 M ). 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 ≈ 0.80 M ) 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 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.

Rotational mixing in carbon-enhanced metal-poor stars withs-process enrichment

“…In Matrozis & Stancliffe (2016) we showed that atomic diffusion should lead to very large abundance anomalies (e.g. [C/Fe] < −1) near the main sequence turn-off, a result clearly at odds with observational data.…”

“…In Matrozis & Stancliffe (2016) we showed how, in absence of other mixing processes, in most CEMP-s stars the carbon should settle out of the surface convection zone, while the surface abundance of iron should increase as a result of radiative levitation. Near the main sequence turn-off, before the convective envelope begins to move inwards in mass, the resulting abundances (e.g.…”

“…In particular, we use the yields from their models with initial masses M 1 = 0.9, 1.0, 1.25 and 1.5 M . The age, at which mass accretion starts is t mt = 9.1, 6.3, 3.06, and 1.8 Gyr, respectively (see table 1 in Matrozis & Stancliffe 2016). All models have a zero-age main sequence (ZAMS) metallicity of Z = 10 −4 (Asplund et al 2009, scaled to [Fe/H] = −2.14).…”

“…We refer to these changes as abundance anomalies. In models where atomic diffusion dominates, the abundance anomalies are usually largest around the main sequence turn-off, which is thus a convenient point of reference for comparing different systems (Matrozis & Stancliffe 2016). In models without diffusion, or when diffusion is inhibited, the abundances of most elements instead change monotonically throughout the main sequence and beyond, as the accreted material gets more and more diluted ( Fig.…”

“…Although in this study we have ignored radiative levitation, which can drastically alter the relative abundances of metals (e.g. the [C/Fe] ratio; Matrozis & Stancliffe 2016), levitation will only be important together with the other microscopic diffusion processes, that is, in the slowly rotating models (v rot 2 km s −1 ). If rotational mixing is indeed responsible for inhibiting atomic diffusion in metal-poor stars, the lack of stars with [C/H] < −2.5 between −2.5 < [Fe/H] < −2 seems to require that all stars rotate, even if very slowly (v rot 0.5 km s −1 ).…”