Rotational mixing in carbon-enhanced metal-poor stars with s-process enrichment^⋆

Argelander-Institut für Astronomie (AIfA), University of Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
e-mail: elvijs@astro.uni-bonn.de

Received: 30 May 2017
Accepted: 23 July 2017

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 kms^-1. We find that only for j_a ≳ 10¹⁷ cm²s^-1 (v_rot > 20 kms^-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 ≲ 1 kms^-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.