R Coronae Borealis Stars are Viable Factories of Pre-solar Grains

TL;DR

This paper estimates the birthrate and dust production of R Coronae Borealis stars, suggesting they are significant sources of pre-solar carbonaceous grains in the Galaxy, with implications for understanding stellar dust contributions.

Contribution

It provides a new, lower theoretical estimate of RCB star birthrate and demonstrates their comparable dust production to other stellar sources, supporting their role in pre-solar grain formation.

Findings

RCB star birthrate is approximately 1.8 x 10^-3 per year.

Estimated 180-540 RCB stars in the Galaxy.

RCB dust production is comparable to novae and born-again AGB stars.

Abstract

We present a new theoretical estimate for the birthrate of R Coronae Borealis (RCB) stars that is in agreement with recent observational data. We find the current Galactic birthrate of RCB stars to be $\approx$ 25% of the Galactic rate of Type Ia supernovae, assuming that RCB stars are formed through the merger of carbon-oxygen and helium-rich white dwarfs. Our new RCB birthrate ($1.8 \times 10^{-3}$ yr$^{-1}$) is a factor of 10 lower than previous theoretical estimates. This results in roughly 180--540 RCB stars in the Galaxy, depending on the RCB lifetime. From the theoretical and observational estimates, we calculate the total dust production from RCB stars and compare this rate to dust production from novae and born-again asymptotic giant branch (AGB) stars. We find that the amount of dust produced by RCB stars is comparable to the amounts produced by novae or born-again post-AGB stars, indicating that these merger objects are a viable source of carbonaceous pre-solar grains in the Galaxy. There are graphite grains with carbon and oxygen isotopic ratios consistent with the observed composition of RCB stars, adding weight to the suggestion that these rare objects are a source of stardust grains.