Hot white dwarf candidates from the IGAPS-GALEX cross-match

Abstract

ABSTRACT White dwarf (WD) stars are often associated with the central stars of planetary nebulae (CSPNe) on their way to the cooling track. A large number of WD star candidates have been identified; thanks to optical large-scale surveys such as Gaia DR2 and EDR3. However, hot-WD/CSPNe stars are quite elusive in optical bands due to their high temperatures and low-optical luminosities. The Galaxy Evolution Explorer (GALEX) matched with the INT Galactic Plane Survey (IGAPS) allowed us to identify hot-WD candidates by combining the GALEX far-UV (FUV)and near-UV (NUV)with optical photometric bands (g, r, i, and Hα). After accounting for source confusion and filtering bad photometric data, a total of 236 485 sources were found in the GALEX and IGAPS footprint (GaPHAS). A preliminary selection of hot stellar sources was made using the GALEX colour cut on FUV−NUV> −0.53, yielding 74 hot-WD candidates. We analysed their spectral energy distribution (SED) by developing a fitting program for single- and two-body SED using an MCMC algorithm; 41 are probably binary systems (a binary fraction of ∼55 per cent was estimated). Additionally, we classified the WD star candidates using different infrared (IR) colours available for our sample, obtaining similar results as in the SED analysis for the single and binary systems. This supports the strength of the fitting method and the advantages of the combination of GALEX UV with optical photometry. Ground-based time-series photometry and spectra are required in order to confirm the nature of the WD star candidates.