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

TL;DR

This study develops a photometric method to identify hot subdwarf stars with F to K-type companions, revealing the prevalence of binary evolution in subdwarf formation through analysis of multi-wavelength data.

Contribution

It introduces a new selection technique combining ultraviolet, optical, and infrared data to find composite subdwarf-main sequence systems and analyzes their temperature and spectral type distributions.

Findings

Most subdwarfs have temperatures between 20,000 and 30,000 K.

F to K-type companions are most common, M-type companions are rarer.

Binary evolution via Roche-lobe overflow is a key formation channel.

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.