Hot Stars in the GALEX Ultraviolet Sky Surveys (GUVcat_AISxSDSS_HS) and the Binary Fraction of Hot Evolved Stars

Abstract We present a catalog of 71,364 pointlike UV sources with Sloan Digital Sky Survey (SDSS) photometry and GALEX far-UV (FUV)−near-UV (NUV) ≤0.1 mag. The limit corresponds to stellar T eff ≳15,000−20,000 K, slightly depending on gravity but nearly reddening independent for Milky Way−type dust. Most sources are hot white dwarfs (WDs) and subdwarfs. Comparing the spectral energy distribution (SED; GALEX FUV, NUV, SDSS u,g,r,i,z ) of 35,294 sources having good photometry with colors of stellar models and known objects, we identify 12,404 ± <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow/> <mml:mrow> <mml:mn>1267</mml:mn> </mml:mrow> <mml:mrow> <mml:mn>1871</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> binary hot-compact stars with a cooler, less-evolved companion (with a possible 8%–15% contamination by low-redshift QSOs), and 22,848 ± <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow/> <mml:mrow> <mml:mn>3853</mml:mn> </mml:mrow> <mml:mrow> <mml:mn>1267</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> single-star candidates. Single-star counts are an upper limit because pairs of similar stars have single-star-like SEDs, and hot WDs with main-sequence companions of certain types (depending on the WD’s radius) are missed or counted as single in the available wavelength range and selection. The catalog offers unique leverage for identifying hot WDs, elusive at longer wavelengths when a cooler, larger companion dominates optical–IR fluxes: 51% of the binary-star and 20% of the single-star candidates are previously unknown objects. Gaia DR3 provides a parallax with error ≤20% for 34% of the binary-star candidates and 45% of single-star candidates, allowing T eff , E B − V , radius, and L bol to be derived from SED analysis. The binary-candidate sample usefully expands the overall current binary-WD census to subpopulations elusive to Gaia and to other searches. The binary fraction among this specific sample of hot compact objects, albeit with the mentioned biases, b f ≳ 46%, compared with that of their progenitors (&gt;80%–50% for mass range 8−1 M ⊙ , according to M. Moe), implies a lower merging rate than found for massive stars by H. Sana et al.