R Coronae Borealis Stars in M31 from the Palomar Transient Factory

TL;DR

This study reports the first discovery of R Coronae Borealis stars in the Andromeda galaxy using the Palomar Transient Factory, demonstrating the potential of synoptic surveys to find rare luminous variables in nearby galaxies.

Contribution

It presents the first identification of RCB stars in M31, including confirmed and candidate objects, expanding the known population beyond the Milky Way and Magellanic Clouds.

Findings

Discovered 2 confirmed and 2 candidate RCB stars in M31.

RCB stars in M31 show irregular declines >1.5 mag over weeks to months.

These RCBs are among the most luminous known, with M_R = -4 to -5.

Abstract

We report the discovery of R Coronae Borealis (RCB) stars in the Andromeda galaxy (M31) using the Palomar Transient Factory (PTF). RCB stars are rare hydrogen-deficient, carbon-rich supergiant variables, most likely the merger products of two white dwarfs. These new RCBs, including two confirmed ones and two candidates, are the first to be found beyond the Milky Way and the Magellanic Clouds. All of M31 RCBs showed >1.5 mag irregular declines over timescales of weeks to months. Due to the limiting magnitude of our data (R~21-22 mag), these RCB stars have R~19.5 to 20.5 mag at maximum light, corresponding to M_R= -4 to -5, making them some of the most luminous RCBs known. Spectra of two objects show that they are warm RCBs, similar to the Milky Way RCBs RY Sgr and V854 Cen. We consider these results, derived from a pilot study of M31 variables, as an important proof-of-concept for the study of rare bright variables in nearby galaxies with the PTF or other synoptic surveys.