On the origin of carbon- and oxygen-rich hot subdwarfs: an sdB merger scenario

Abstract: Carbon- and oxygen-rich hot subdwarfs (CO-sdO stars) represent an extremely rare subclass of helium-rich hot subdwarfs. Recent observations of PG 1654+322, PG 1528+025, and UCAC4 108 reveal unusually large surface mass fractions of carbon and oxygen, which are difficult to explain within existing formation scenarios. We explore whether mergers involving sdB stars can provide a viable formation channel for these objects. Using the stellar evolution code MESA, we model the post-merger evolution of two merger channels: mergers between two sdB stars and mergers between a helium white dwarf and an sdB star. Carbon and oxygen synthesized during core helium burning in sdB stars may provide a natural chemical reservoir if efficient mixing occurs during the merger. The resulting evolutionary tracks are broadly consistent with the effective temperatures, surface gravities, and luminosities observed for the three stars, suggesting remnant masses of about 0.8-1.0 M_sun. Our calculations suggest that mergers involving sdB stars may represent a viable formation pathway for CO-rich hot subdwarfs.