The Unseen Population of F to K-type Companions to Hot Subdwarf Stars

Abstract

We present a method to select hot subdwarf stars with A to M-type companions using photometric selection criteria. We cover a wide range in wavelength by combining GALEX ultraviolet data, optical photometry from the SDSS and the Carlsberg Meridian telescope, near-infrared data from 2MASS and UKIDSS. We construct two complimentary samples, one by matching GALEX, CMC and 2MASS, as well as a smaller, but deeper, sample using GALEX, SDSS and UKIDSS. In both cases, a large number of composite subdwarf plus main-sequence star candidates were found. We fit their spectral energy distributions with a composite model in order to estimate the subdwarf and companion star effective temperatures along with the distance to each system. The distribution of subdwarf effective temperature was found to primarily lie in the 20,000 - 30,000 K regime, but we also find cooler subdwarf candidates, making up ~5-10 per cent. The most prevalent companion spectral types were seen to be main-sequence stars between F0 and K0, while subdwarfs with M-type companions appear much rarer. This is clear observational confirmation that a very efficient first stable Roche-lobe overflow channel appears to produce a large number of subdwarfs with F to K-type companions. Our samples thus support the importance of binary evolution for subdwarf formation.

30 pages, 10 figures, 11 tables. Accepted for publication in MNRAS

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• New binaries among UV-selected, hot subdwarf stars and population properties
• A Radial Velocity Study of Composite-Spectra Hot Subdwarf Stars with the Hobby-Eberly Telescope
• The population of hot subdwarf stars studied with Gaia I. The catalogue of known hot subdwarf stars
• A search for new hot subdwarf stars by means of Virtual Observatory tools II
• Interesting Features in the Combined GALEX and Sloan Color Diagrams of Solar-like Galactic Populations
• Photometry and spectroscopy of faint candidate spectrophotometric standard DA white dwarfs
• Identification of new hot subdwarf binary systems by means of Virtual Observatory tools
• Perfecting our set of spectrophotometric standard DA white dwarfs
• Hot Subdwarf Formation: Confronting Theory with Observation
• Hot subdwarf stars from the Hamburg Quasar Survey