Fluorine in R Coronae Borealis and Extreme Helium Stars

TL;DR

This study detects and measures fluorine in R Coronae Borealis stars, revealing significant overabundance and supporting fluorine synthesis, contrasting with its absence in a related stellar object, thus informing stellar evolution theories.

Contribution

First measurement of fluorine abundance in RCB stars using optical spectra, providing evidence for fluorine synthesis in these stars.

Findings

Fluorine is enriched by factors of 800 to 8000 in some RCBs.

Neutral fluorine lines are identified in RCB spectra at maximum light.

Sakurai's object shows no detectable fluorine lines, contrasting with RCBs.

Abstract

Neutral fluorine lines are identified in the optical spectra of several R Coronae Borealis stars (RCBs) at maximum light. These lines provide the first measurement of the fluorine abundance in these stars. Fluorine is enriched in some RCBs by factors of 800 to 8000 relative to its likely initial abundance. The overabundances of fluorine are evidence for the synthesis of fluorine. These results are discussed in the light of the scenario that RCBs are formed by accretion of an He white dwarf by a C-O white dwarf. Sakurai's object (V4334 Sgr), a final He-shell flash product, shows no detectable neutral fluorine lines.