White dwarfs, exoplanets and fundamental physics

White dwarfs are the end products of the life cycles of more than 90% of all stars. This makes them important laboratories for studying stellar evolution and the behavior of matter at extremes of temperature and density. There is growing evidence that many white dwarfs are swallowing up debris from, now destroyed, planetary systems. Measuring the abundance of this material in the atmospheres of the white dwarfs can tell us about the composition of these planets. However, we first need to understand the evolution of white dwarfs, including the composition of their atmospheres. We are using data from a variety of space missions and telescopes to study the population of white dwarfs to provide this insight.

The work will involve the analysis of spectroscopic and imaging data from the Hubble Space Telescope (HST), the ESA Gaia mission and ground-based telescopes. There will also be a theoretical element to the work, using an existing computer programme that we run on the University's supercomputer to simulate stellar atmospheres, for comparison with the data.

Leicester are closely involved in the European Space Agency Gaia mission. Launched in December 2013, the satellite is carrying out a detailed survey of the entire visible galaxy to measure the positions, space motion and distances of approximately 1 billion stars. Many of these will be white dwarfs and the Gaia data will allow us to obtain measurements of their masses and radii with unprecedented accuracy and search for hidden stellar/exo-planet companions.

Measurement of the abundances of C and Si in a large sample of white dwarfs shows that in all cases C is depleted, indicating that the material has a rocky origin - similar to that of asteroids. A likely explanation is that this material is collected from extra-solar planetary debris and then retained in the hot white dwarf's envelope by the force of radiation. We now need to test this by adapting the atmospheric modeling computer programme to include these physical processes.

Recently, research has shown that high spectral resolution and signal-to-noise spectra of a single hot white dwarfs can be used to measure potential changes in the value of the fundamental physical constants, such as the fine structure constant and electron/proton mass ratio, in a strong gravitational field. The accuracy of the technique is limited by how well we know the wavelengths of atomic absorption features in the spectrum. We are currently acquiring improved atomic data and are obtaining more high quality spectra of other stars to continue this work.

Another project is a large survey of candidate hot white dwarfs with HST. These objects were identified as being relatively bright in the UV by the GALEX satellite. We are hoping to find the very hottest, youngest white dwarfs, born shortly after their parent stars end their normal lives. This will allow us to measure the relationship between the initial masses of the parent stars and the masses of the white dwarfs, understanding how heavy elements are recycled into interstellar space, as white dwarfs are born.