Carbon-enhanced stars with short orbital and spin periods

TL;DR

This study investigates dwarf carbon stars with Hα emission, revealing they are in close binary systems with synchronized orbital and spin periods, suggesting a history of mass transfer and binary evolution.

Contribution

It provides the first detailed time-series analysis linking Hα emission in dwarf carbon stars to short-period binary systems and tidal synchronization.

Findings

All studied stars have photometric and orbital periods of 0.2–5.2 days.

The stars are in tidally synchronized binary systems.

Hα emission indicates past mass transfer and binary interaction.

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 H$\alpha$ 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 H$\alpha$ 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 synchronisation. It is hypothesised 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.