Research

My primary interests are in the structure and evolution of stars, particularly very close (interacting) binary stars.

Interacting binary stars are systems in which two stars orbit around each other so closely that they strongly affect each other's structure and evolution. Often one component is a compact object: a white dwarf, neutron star or even black hole. As well as presenting some intrinsically fascinating phenomena, they provide a test-bed for theories of stellar evolution. In many of them, the interaction involves actual transfer of matter from one star to another, leading to the formation of an accretion disk around the compact star. Accretion is now believed to provide the power source for many astronomical objects, from proto-stars to quasars, and the phenomenon is most accessibly studied in interacting binaries. Our group has been involved in a number of observational and theoretical studies, concentrating particularly on the mass donor in cataclysmic variables, which are a sub-class of interacting binary in which a faint, cool star, hard to observe in the optical, is transferring mass through an accretion stream and disk onto a white dwarf.

(i) In the mid-1980s, our group performed the first major spectroscopic survey in the far red region of the spectrum, looking for evidence in the spectra of cataclysmic variables for the neutral sodium absorption lines near 820 nm that are a characteristic signature of cool stars. About a quarter of the systems surveyed showed these lines, trebling the number of systems in which the cool star could be detected. A follow-up survey in 1990 added more systems.

(ii) If the masses of the component stars in cataclysmic binary stars can be determined then interesting constraints can be placed on their evolutionary history, but this is fraught with difficulties. The dominant source of light from the binary is the accretion disk, which makes it very difficult to determine the orbital motions of the stars. The white dwarf is particularly hard to observe, since its emission is in the same part of the spectrum as the disk. We have concentrated on spectroscopic studies of the cool star, whose main emission is in the far red, where the disk is fainter, to determine its orbital motion as accurately as possible, and to find its rotational velocity. Combining these pieces of information enables the masses of both stars to be found with as few assumptions as possible.

(iii) Radiation from the disk heats the facing side of the cool star, distorting the spectra and complicating the determination of orbital parameters. Our group produced in 1992 some of the first maps of the irradiation pattern on the surfaces of these stars. Currently, we are using the technique of Roche Tomography to look for starspots, in collaboration with colleagues at Sheffield.

(iv) We have also used the technique of skew mapping (developed at Sussex) to detect faint secondary stars in cataclysmic variables, and completed a comprehensive study of 10 systems.

(v) With undergraduate project students, I have used the Sussex 460-mm telescope to collect CCD data on RR Lyrae variables, with the aim of improving published periods.