Modelling the nucleosynthetic properties of carbon-enhanced metal-poor RR Lyrae stars

TL;DR

This paper models the nucleosynthetic history of carbon-enhanced metal-poor RR Lyrae stars, exploring how accretion and mixing processes influence their surface compositions, with implications for early Universe nucleosynthesis.

Contribution

It provides detailed stellar models of CEMP RR Lyrae stars, analyzing accretion effects and mixing mechanisms to explain observed abundance patterns, including elements like carbon, sodium, strontium, and barium.

Findings

Successful fit for SDSS J1707+58's carbon and sodium abundances.

Indications of non-standard s-process contributions in SDSS J1707+58.

Better modeling results for TY Gru's abundance pattern.

Abstract

Certain carbon-enhanced metal-poor stars likely obtained their composition via pollution from some of the earliest generations of asymptotic giant branch stars and as such provide important clues to early Universe nucleosynthesis. Recently, Kinman et al. discovered that the highly carbon- and barium-enriched metal-poor star SDSS J1707+58 is in fact an RR Lyrae pulsator. This gives us an object in a definite evolutionary state where the effects of dilution of material during the Main Sequence are minimised owing to the object having passed through first dredge-up. We perform detailed stellar modelling of putative progenitor systems in which we accreted material from asymptotic giant branch stars in the mass range 1-2 solar masses. We investigate how the surface abundances are affected by the inclusion of mechanisms like thermohaline mixing and gravitational settling. While we are able to find a reasonable fit to the carbon and sodium abundances of SDSS J1707+58, suggesting accretion of around 0.1 solar masses from a 2 solar mass companion, the strontium and barium abundances remain problematic and this object may have experienced something other than a standard $s$ process. We have more success in fitting the abundances of the mildly carbon-enriched, metal-poor RR Lyrae pulsator TY Gru (CS 22881-071), which we suggest received 0.1 solar masses of material from a companion of around 1 solar mass.