Oxygen isotopic ratios in cool R Coronae Borealis stars

TL;DR

This study measures oxygen isotope ratios in cool R Coronae Borealis stars to understand their relationship with hydrogen-deficient carbon stars, revealing differences in 18O content that inform stellar evolution theories.

Contribution

It provides precise 16O/18O ratios for RCB stars, highlighting differences from HdC stars and proposing a new evolutionary scenario involving late dredge-up processes.

Findings

RCB stars generally have higher 16O/18O ratios than HdC stars.

WX CrA is an exception with a very low 16O/18O ratio.

The results support a theory of stellar evolution involving late dredge-up of He-burning products.

Abstract

We investigate the relationship between R Coronae Borealis (RCB) stars and hydrogen-deficient carbon (HdC) stars by measuring precise 16O/18O ratios for five cool RCB stars. The 16O/18O ratios are derived by spectrum synthesis from high-resolution (R=50,000) K-band spectra. Lower limits to the 16O/17O and 14N/15}N ratios as well as Na and S abundances (when possible) are also given. RCB stars in our sample generally display less 18O than HdC stars - the derived 16O/18O ratios range from 3 to 20. The only exception is the RCB star WX CrA, which seems to be a HdC-like star with 16O/18O=0.3. Our result of a higher 16O/18O ratio for the RCB stars must be accounted for by a theory of the formation and evolution of HdC and RCB stars. We speculate that a late dredge-up of products of He-burning, principally 12C and 16O, may convert a 18O-rich HdC star into a 18O-poor RCB star as the H-deficient star begins its final evolution from a cool supergiant to the top of the white dwarf cooling track.