Mullard Space Science Laboratory

C. Hellier
Accretion flows in compact binary stars

1989 (supervisor: K. O. Mason)

The thesis describes an observational study of accretion flows occurring in close binary stars where one binary component is a degenerate star (white dwarf or neutron star). In particular it focuses upon the 'intermediate polar' sub-class in which the compact white dwarf possesses a substantial magnetic field able to control the accretion flow.

After an introductory chapter, I report the analysis of spectroscopy of the eclipsing intermediate polar EX Hya. To explain the data I propose a new model for the optical modulations at the spin period of the white dwarf.

The next chapter contains the first spectra of EX Hya obtained during one of its rare outbursts. The analysis suggests that this outburst was caused by an increase in the mass transfer rate from the secondary star, rather than by an instability in an accretion disc as in the alternative model for such outbursts.

I then describe the most extensive spectroscopic study of the intermediate polar FO Aqr to date. The data confirm the model for the spin period modulations proposed for EX Hya, which suggests the model's applicability to the whole intermediate polar class. The data show that this binary is also eclipsing, whereas previously it had been thought to be at low inclination. I also report a unique 'absorption S-wave' feature in the emission lines which is blue-shifted by 200 km/s from the line centres.

A final chapter explores another sub-class of interacting binary stars, the high inclination Low Mass X-ray Binaries (which contain a neutron star). In the system X 1822-371, the direct X-ray flux from the neutron star is obscured by material in the accretion disc. We see only X-rays scattered by a 'corona' around the disc. This provides an excellent opportunity to study the accretion flow which I exploit using X-ray data from the EXOSAT satellite.


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