An Infrared Census of R Coronae Borealis Stars II—Spectroscopic Classifications and Implications for the Rate of Low-mass White Dwarf Mergers
Soria, Roberto; Clayton, Geoffrey C.; De, Kishalay; Kasliwal, Mansi M.; Ofek, Eran O.; Lau, Ryan M.; Bildsten, Lars; Smith, Roger M.; Anand, Shreya; McKenna, Dan; Moore, Anna; Tisserand, Patrick; Hankins, Matthew J.; Ashley, Michael C. B.; Karambelkar, Viraj R.; Soon, Jamie; Travouillon, Tony; Lamberts, Astrid; Crawford, Courtney C.; Earley, Nicholas; Hall, Xander
United States, France, Australia, Israel, China, Italy
Abstract
We present results from a systematic infrared (IR) census of R Coronae Borealis (RCB) stars in the Milky Way, using data from the Palomar Gattini IR (PGIR) survey. RCB stars are dusty, erratic variable stars presumably formed from the merger of a He-core and a CO-core white dwarf (WD). PGIR is a 30 cm J-band telescope with a 25 deg2 camera that surveys 18,000 deg2 of the northern sky (δ > ‑28°) at a cadence of 2 days. Using PGIR J-band lightcurves for ∼60 million stars together with mid-IR colors from WISE, we selected a sample of 530 candidate RCB stars. We obtained near-IR spectra for these candidates and identified 53 RCB stars in our sample. Accounting for our selection criteria, we find that there are a total of RCB stars in the Milky Way. Assuming typical RCB lifetimes, this corresponds to an RCB formation rate of 0.8–5 × 10‑3 yr‑1, consistent with observational and theoretical estimates of the He-CO WD merger rate. We searched for quasi-periodic pulsations in the PGIR lightcurves of RCB stars and present pulsation periods for 16 RCB stars. We also examined high-cadenced TESS lightcurves for RCB and the chemically similar, but dustless hydrogen-deficient carbon (dLHdC) stars. We find that dLHdC stars show variations on timescales shorter than RCB stars, suggesting that they may have lower masses than RCB stars. Finally, we identified 3 new spectroscopically confirmed and 12 candidate Galactic DY Per type stars—believed to be colder cousins of RCB star—doubling the sample of Galactic DY Per type stars.