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

TL;DR

This paper introduces a catalog of hot ultraviolet sources, primarily white dwarfs and sub-dwarfs, identifying binary systems and providing insights into their properties and binary fraction, enhancing understanding of hot star populations.

Contribution

The study presents a large, new catalog of hot UV sources with detailed binary classification and analysis, expanding the known population of hot white dwarfs and sub-dwarfs, and assessing their binary fraction.

Findings

Identified 12,404+1871-1267 binary hot-compact stars.

Detected 22,848+1267-3853 single-star candidates, many previously unknown.

Binary fraction among hot-compact objects exceeds 46%.

Abstract

We present a catalog of 71,364 point-like UV sources with SDSS photometry and GALEX FUV-NUV less or equal 0.1mag. The limit corresponds to stellar Teff greater than 15,000 to 20,000K, slightly depending on gravity but nearly reddening-independent for Milky-Way-type dust. Most sources are hot white-dwarfs (WDs) and sub-dwarfs (SDs). Comparing the 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+1871-1267 binary hot-compact stars with a cooler, less-evolved companion (with a possible 8% to 15% contamination by low-redshift QSOs), and 22,848+1267-3853 single-star candidates. Single-star counts are an upper limit because pairs of similar stars have single-star-like SED, and hot-WDs with main-sequence companions of certain types (depending on 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- and 20% of the single-star candidates are previously un-known objects. Gaia DR3 provides a parallax with error less than 20% for 34% of the binaries- and 45% of single-star candidates, allowing Teff, E(B-V), radius and Lbol 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 (>80%-50% for mass range 8-1Msun, Moe (2019)), implies a lower merging rate than found for massive stars by Sana et al (2017).