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Warwick Astronomy PhD Projects

The Properties and Evolution of White Dwarfs in Cataclysmic Variables

Single white dwarfs are relatively simple objects that have thoroughly been studied for many decades. Deprived of an energy source, they are condemned to cool (the coldest known white dwarfs may provide an estimate of the age of our Galaxy). Because of their high surface gravity their atmospheres are stratified, with a thin layer of hydrogen floating ontop. Thus, their spectra are extremely simple, being entirely dominated by Balmer and Lyman lines. In contrast to their field relatives, the white dwarfs in cataclysmic variables continously accrete mass, angular momentum, and energy from their companion stars, and, as a consequence, their masses, temperatures, photospheric compositions and rotation rates differ from those of single white dwarfs. The interrelations between the white dwarf properties, the accretion process, and the evolution of the entire CV are extremely complex. Observational studies of CV white dwarfs have to be carried out in the ultraviolet, as the optical emission of CVs is dominated by the accretion flow and/or the donor star. I am leading two large projects on the Hubble Space Telescope (Cycle 11 & Cycle 12) that will obtain ultraviolet spectroscopy for many CVs, and you will participate during your PhD in the analysis of this data set, hopefully leading to a significant progress in our understanding of the properties of CV white dwarfs and the evolution of CVs in general. Supervisor: Boris Gänsicke

Boundary Layer

 

Accretion column

A close-up of the white dwarf in a non-magnetic cataclysmic variable. An accretion disc feeds matter on the equator of the white dwarf, which is heated to a few 100000K and emits soft X-rays.  

A close-up pf the white dwarf in a strongly magnetic (>10000T) cataclysmic variable. The material falls along the magnetic field lines towards the magnetic pole of the white dwarf, where it reaches velocities of a few 1000km/s. It is decelerated in a shock, and heated to about 100 million K - subsequently cooling by the emission of hard X-rays and cyclotron emission

 

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