University of Leicester

This project aims to determine the hot white dwarf luminosity function as well as the Galactic distribution of hot white dwarfs. Both help to answer one of the fundamental questions in modern physics: What is the nature of dark matter?

One of the major, unsolved problems in physics relates to the nature of dark matter. This so-called “missing mass” of our universe outweighs visible matter roughly six to one, accounting for 27% of the content of the universe. However, it is extremely hard to spot. Possible candidates include weakly interacting particles, but also massive compact halo objects, such as white dwarfs.

About 95% of all stars end up as a white dwarf once their nuclear fusion has ceased. Nicole’s project utilizes 700 of the hottest white dwarfs recently detected to explore the nature of dark matter in two ways. Firstly, she will study the Galactic distribution of these stars, in order to pin down their contribution to the mass budget of the Galactic halo. The latter surrounds our Galaxy and which is where dark matter is located. Previous studies were restricted to only relatively cool and nearby white dwarfs.

Hot white dwarfs, however, can be observed up to regions where the Galactic halo dominates. Therefore, her sample offers the possibility of studying the Galactic distribution of a large number of white dwarfs far beyond the solar neighbourhood for the first time. Nicole expects to detect about 300 new Galactic halo members, allowing a precise estimate of their mass contribution to the Galactic dark matter halo.

Secondly, white dwarfs serve as excellent laboratories to investigate the characteristics of weakly interacting particles. This can be done by deriving the white dwarf luminosity function, which gives the number density of white dwarfs per luminosity interval and reflects quantitatively the cooling behaviour of the white dwarf population. The cooling process of hot and luminous white dwarfs is dominated by the radiation of particles, thought to be candidates for dark matter. Thus, the shape of the hot end of this luminosity function can be used to study the properties of weakly interacting particles (e.g., their masses or magnetic dipole moments). Her project will provide the first reliable observational basis for such investigations, by using the largest sample of hot white dwarfs ever studied as well as sophisticated models for the spectral analysis of these stars.