Carbon Stars from Gaia Data Release 3 and the Space Density of Dwarf Carbon Stars

Abstract

Carbon stars (with atmospheric C/O > 1) range widely in temperature and luminosity, from low-mass dwarfs to asymptotic giant branch (AGB) stars. The main-sequence dwarf carbon (dC) stars have inherited carbon-rich material from an AGB companion, which has since transitioned to a white dwarf. The dC stars are far more common than C giants, but no reliable estimates of the dC space density have been published to date. We present results from an all-sky survey for carbon stars using the low-resolution XP spectra from Gaia Data Release 3. We developed and measured a set of spectral indices contrasting C2 and CN molecular band strengths in carbon stars against common absorption features found in normal (C/O < 1) stars, such as Ca i, TiO, and Balmer lines. We combined these indices with the XP spectral coefficients as inputs to supervised machine learning algorithms trained on a vetted sample of known C stars from LAMOST. We describe the selection of the carbon candidate sample and provide a catalog of 43,574 candidates dominated by cool C giants in the Magellanic Clouds and at low Galactic latitude in the Milky Way. We report the confirmation of candidate C stars using intermediate-resolution (R ∼ 1800) optical spectroscopy from the Fred Lawrence Whipple Observatory and provide estimates of the sample purity and completeness. From a carefully vetted sample of over 600 dCs, we measure their local space density to be (about one dC in every local disk volume of radius 50 pc), with a relatively large disk scale height of pc.