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 _ { \textrm { a } } of the accreted material , corresponding to surface rotation velocities , v _ { \textrm { rot } } , between about 0.3 and 300 \textrm { km } \thinspace \textrm { s } ^ { -1 } .
We find that only for j _ { \textrm { a } } \gtrsim 10 ^ { 17 } \text { cm } ^ { 2 } \thinspace \text { s } ^ { -1 } ( v _ { \text { rot } } > 20 \text { km } \thinspace \text { 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 \text { dex } without thermohaline mixing or about 0.5 \text { - - } 1.5 dex with thermohaline mixing .
Even in models with the lowest rotation velocities ( v _ { \text { rot } } \lesssim 1 \text { km } \thinspace \text { 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 can not be neglected in models of CEMP- s stars .