The Population of Compact Double White Dwarf Binary Stars

There are around 1000 million white dwarfs in our Galaxy, and comparable fractions in other galaxies. A large number of these have formed within binary systems and are still part of binary systems now. This opens a host of exotic processes such as accretion, common envelope evolution, gravitational wave emission, and thermonuclear explosions denied to single stars. White dwarfs in binary systems are the host population of Type Ia supernovae used to detect dark energy and double white dwarfs are expected to be the dominant sources for the LISA mission for the space-based detection of gravitational waves due to fly in the 2030s. Compact double white binaries with orbital periods from days to minutes form the largest population of evolved binary stars in the Galaxy. They may be the long sought specific binary that forms Type Ia supernovae. Whether they are, and precisely what their contribution to the gravitational waves that LISA will detect depends upon their number. The aim of this project is to determine the number and properties of such double white dwarfs in our Galaxy.

The project will centre upon the detection of double white dwarfs from the Gaia survey. Gaia is an astrometric satellite that has revolutionised our view of the Galaxy through direct distance measurements to over a billion stars. It has already been transformative to the study of white dwarfs. Distances in Gaia are so good that one can use it to spot double white dwarfs through their position in the Hertzsprung-Russell diagram. The core of the project will be the follow-up of such stars to determine first whether they are binary, and second their orbital periods. The ultimate aim is to assess the fraction of all white dwarfs that are binary. The work is primarily observational (observations, data reduction, analysis), but modelling will be required to understand the spectra and to translate the observational constraints population limits. A second aspect of the project is to follow double white dwarfs of extremely short period (minutes). An increasing number of these are being discovered, and they allow the direct measurement of gravitational wave losses and have the potential to diagnose the effect of tides in white dwarf. Tides between white dwarfs play an important role in how they behave once mass transfer starts at orbital periods of a few minutes. At Warwick we operate several high-speed CCD cameras (HiPERCAM, ULTRACAM and ULTRASPEC) that allow the rapid observations needed to probe such effects.

[More to be added.]

Supervisor: Tom Marsh