A spectroscopic analysis of three cataclysmic variable stars

Abstract

Cataclysmic variable stars (CVs) are binary systems in which matter is transferred from a low mass star to a white dwarf via an accretion disc.  My thesis is a spectroscopic study of three of these objects:  U Gem, GD 552 and GY Cnc.

I present high-resolution optical spectra of U Gem taken during quiescent.  For U Gem, the radial velocity semi-amplitude of the white dwarf, K₁, is accurately known thanks to a direct observation by Long et al. (1999), I find that even with these data the optical measurements are seriously distorted compared to the known value, which is not recovered to better than ˜ 20%.  Doppler tomograms show emission at low velocity, close to the centre of mass, and a transient and sharp absorption feature is seen in the Balmer lines close to eclipse.  I suggest that stellar prominences may explain part of these features.  I study two features detected in He II4686.75 Å.  They seem to be produced in the bright spot.  The narrower feature has a velocity close to that of the accretion disc in the impact region.  I present evidence of weak spiral structure, which may support explanations for "spiral shocks " based upon 3-body effects.  I apply a method of isophote fitting to search for evidence of stream-disc overflow, but fail to uncover any.  I detect evidence of irradiation of the mass donor with shielding by the disc:  I estimate an H/R ratio between 0.15 and 0.20.

For GD 552 I present spectroscopy taken with the aim of detecting emission from the mass donor.  I fail to do so at a level which allows me to rule out the presence of a near-main-sequence star donor. Given GD 552’s orbital period of 103 minutes, this suggests instead that it may be a system that has evolved through the ˜ 80-minute orbital period minimum of CVs and now has a brown dwarf mass donor.

Finally, I give a first look at high-resolution data for GY Cnc whose dynamical parameters make it a near-perfect twin of U Gem.  I find several surprising features:  the bright spot is completely absent from the Balmer lines, although visible in other lines; emission from the secondary star seems to arise from the whole of its Roche lobe;  and low velocity emission is detected near the centre of mass as in U Gem.  I argue that GY Cnc provides further evidence of the presence of prominence-like structures on CVs.

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