The Carbon Star DY Persei May Be a Cool R Coronae Borealis Variable

Abstract

Optical and near-IR photometry suggests that the carbon star DY Persei exhibits fadings similar to those of R Coronae Borealis (RCB) variables. Photometric surveys of the Galaxy and Magellanic Clouds uncovered new DY Per variables with infrared photometry identifying them with cool carbon stars, perhaps, with an unusual tendency to shed mass. In an attempt to resolve DY Per’s identity crisis—a cool carbon giant or a cool RCB variable?—we analyze a high-resolution IGRINS H &K-band spectrum of DY Per. The CO first-overtone bands in the K band of DY Per show a high abundance of ^18 O such that ^16 O/ ^18 O = 4 ± 1, a ratio sharply at odds with published results for regular cool carbon giants with ^16 O/ ^18 O ∼ 1000 but this exceptionally low ratio is characteristic of RCB variables and HdC stars. This similarity suggests that DY Per indeed may be a cool RCB variable. Current opinion considers RCB variables to result from the merger of a He onto a CO white dwarf; observed abundances of these H-deficient stars including the exceptionally low ^16 O/ ^18 O ratios are in fair accord with predicted compositions for white dwarf merger products. An H-deficiency for DY Per is not directly observable but is suggested from the strength of an HF line and an assumption that F may be overabundant, as observed and predicted for RCB stars.

Keywords

• Chemically peculiar stars
• R Coronae Borealis variable stars
• Chemical abundances
• Stellar atmospheres
• Late-type stars
• Stellar evolution