On Measuring the Radial Velocity of White Dwarfs in Cataclysmic Binaries

Abstract

It is notoriously difficult to determine the semiamplitude of the white dwarf in a cataclysmic binary (Smak 1970). This is a result of the fact that spectral features arising in the photosphere of the white dwarf are rarely, if ever, seen in the spectra of these systems. The motion of the white dwarf must be inferred from radial velocity variations of the broad emission lines which originate in the surrounding accretion disk. This would not propose any particular problem if the disk emissivity was axially symmetric. Unfortunately, this is usually not the case, primarily because of enhanced emission in the vicinity of the shock front (hot spot) where the interstar mass transfer stream impacts the disk. The challenge, then, is to measure the velocity of the emission lines using a method which is insensitive to contamination from the hot spot. The purpose of this paper is twofold: (1) to review a simple method of measuring the velocity of emission lines in cataclysmic binaries and (2) to remind the reader of the disastrous consequences which can arise if care is not exercised when measuring the emission line velocities.