White dwarf plus FGK star binaries in Gaia

White Dwarf (WD)s with an F, G or K type companion (or stars which evolved from such a companion) are the last common ancestor to a zoo of exotic phenomena. These systems can emerge from either a period of Stable Mass Transfer (SMT), or from a phase of Common Envelope Evolution (CEE).

We set about establishing a sample of such systems in Chapter 3 (Gaia orbits for known UV excess binaries) using Gaia Data Release (DR)3 data with previous Ultraviolet (UV) excess catalogues, finding a sample of 206 candidate WD + FGK systems, complete with orbital periods and a measure or estimation of the WD mass. We find that WD + FGK binaries can have a wide range of orbital periods. A large number of these systems are likely post-SMT systems based on their mass/period relationships, while others are difficult to explain either via SMT or standard CEE.

We also work to construct a new UV excess sample from a Gaia-Galaxy Evolution Explorer (GALEX) crossmatch, compiling a sample of 2597 candidate WD + FGK binary systems within 350 pc in Chapter 4 (The Gaia-GALEX UV Excess Sample for systems within 350 pc). We identify these systems by their UV excess as compared to what MESA Isochrones & Stellar Tracks (MIST) isochrones predict give the systems Gaia DR3 stellar parameters, after applying corrections to systematic uncertainties found within the UV predictions of said MIST isochrones. We estimate the physical properties of the WD by fitting the excess UV flux to models, or through interpolation where there was only a clear excess in one UV band. We estimate the age of the system where possible by comparing against wide resolved WD companions, or by checking if the system is a known member of a cluster.