Carbon-enhanced stars with short orbital and spin periods

Abstract

ABSTRACT Many characteristics of dwarf carbon stars are broadly consistent with a binary origin, including mass transfer from an evolved companion. While the population overall appears to have old-disc or halo kinematics, roughly 2 per cent of these stars exhibit  Hα emission, which in low-mass main-sequence stars is generally associated with rotation and relative youth. Its presence in an older population therefore suggests either irradiation or spin-up. This study presents time-series analyses of photometric and radial-velocity data for seven dwarf carbon stars with  Hα emission. All are shown to have photometric periods in the range 0.2–5.2 d, and orbital periods of similar length, consistent with tidal synchronization. It is hypothesized that dwarf carbon stars with emission lines are the result of close-binary evolution, indicating that low-mass, metal-weak, or metal-poor stars can accrete substantial material prior to entering a common-envelope phase.