Innocent Bystanders: Carbon Stars from the Sloan Digital Sky Survey

TL;DR

This study significantly expands the catalog of carbon stars from the Sloan Digital Sky Survey, revealing their types, properties, and potential binary origins, and uncovers new stellar populations and phenomena.

Contribution

It presents a large, diverse sample of carbon stars, including new types and binary systems, and identifies potential new dwarf galaxy or tidal stream structures.

Findings

1220 C stars identified, five times more than previous catalogs

Discovered 167 DQ white dwarfs and 8 binary systems with AGB mass transfer evidence

First X-ray detection of a dC star with strong Balmer emission

Abstract

Among stars showing carbon molecular bands (C stars), the main sequence dwarfs, likely in post-mass transfer binaries, are numerically dominant in the Galaxy. Via spectroscopic selection from the Sloan Digital Sky Survey, we retrieve 1220 C stars, ~5 times more than previously known, including a wider variety than past techniques such as color or grism selection have netted, and additionally yielding 167 DQ white dwarfs. Of the C stars with proper motion measurements, we identify 69% as clearly dwarfs (dCs), while ~7% are giants. The dCs likely span absolute magnitudes M_i from ~6.5 to 10.5. "G-type" dC stars with weak CN and relatively blue colors are probably the most massive dCs still cool enough to show C_2 bands. We report Balmer emission in 22 dCs, none of which are G-types. We find 8 new DA/dC stars in composite spectrum binaries, quadrupling the total sample of these "smoking guns" for AGB binary mass transfer. Eleven very red C stars with strong red CN bands appear to be "N"-type AGB stars at large Galactocentric distances, one likely a new discovery in the dIrr galaxy Leo A. Two such stars within 30arcmin of each other may trace a previously unidentified dwarf galaxy or tidal stream at ~40 kpc. We explore the multiwavelength properties of the sample and report the first X-ray detection of a dC star, which shows strong Balmer emission. Our own spectroscopic survey additionally provides the dC surface density from a complete sample of dwarfs limited by magnitude, color, and proper-motion.