Revised surface abundances of R Coronae Borealis stars

TL;DR

This study revises the surface abundances of R Coronae Borealis stars using observed C2 bands, providing insights into their metallicity, composition, and formation scenarios, and comparing them with extreme helium stars.

Contribution

It offers the first revised surface abundance measurements of RCB stars based on observed carbon data, addressing previous assumptions and comparing with related stellar types.

Findings

Revised carbon abundances show consistency with the known 'carbon problem' in RCBs.

Metallicity range of RCBs aligns with the metal-poor population in the galactic bulge.

Distinct differences in metallicity and element abundances between RCBs and extreme helium stars.

Abstract

Surface abundances of 14 (11 majority class and 3 minority class) R Coronae Borealis stars (RCBs) along with the final flash object, V4334 Sgr (Sakurai's object) are revised based on their carbon abundances measured from the observed C2 bands; note that the earlier reported abundances were derived using an assumed carbon abundance due to the well known ``carbon problem''. The hot RCB MV Sgr is not subject to a carbon problem; it is remarkable to note that MV Sgr's carbon abundance lies in the range that is measured for the majority and minority class RCBs. The revised iron abundances for the RCBs are in the range log E(Fe)=3.8 to log E(Fe)=5.8 with the minority class RCB V854 Cen at lower end and the majority class RCB R CrB at the higher end of this range. Indications are that the revised RCBs' metallicity range is roughly consistent with the metal poor population contained within the bulge. The revised abundances of RCBs are then compared with extreme helium stars (EHes), the hotter relatives of RCBs. Clear differences are observed between RCBs and EHes in their metallicity distribution, carbon abundances, and the abundance trends observed for the key elements. These abundances are further discussed in the light of their formation scenarios.