Absolute magnitudes of cataclysmic variables

Abstract

Absolute magnitudes <it>M</it><inf><it>v</it></inf> of the accretion discs (with allowance for effects of inclination) of stars in the principal classes of cataclysmic variable stars are derived from a variety of techniques. For dwarf novae, whose distances are found mostly from infrared observations of their secondaries, a tight relationship is found between <it>M</it><inf>v</inf>(max) at maximum of outburst and orbital period: <it>M</it><inf><it>v</it></inf>(max)=5.64 – 0.259 <it>P</it>(hr). Use of this equation provides <it>M</it><inf>v</inf>(min) at minimum light for further dwarf novae without known distances. For many of these stars it is possible also to derive <it>M</it><inf><it>v</it></inf>(mean), averaged over the outburst cycle. Analysis of this extensive data set discloses that <it>M</it><inf><it>v</it></inf>(min) and <it>M</it><inf><it>v</it></inf>(mean) are functions of orbital period <it>P</it> and mean time <it>T</it><inf>n</inf> between normal outbursts. <it>T</it><inf>n</inf> is shorter for the brighter systems. The Z Cam stars have <it>M</it><inf><it>v</it></inf>(mean) close to the theoretical value for stable (rather than episodic) mass transfer through accretion discs. Above that critical level all observed discs are stable. An amplitude – <it>T</it><inf>n</inf> (Kukarkin–Parenago) relationship is found to exist for the dwarf novae. Together with the above results it leads, through a simple model for disc instabilities, to a predicted relationship between duration of dwarf nova outbursts and <it>P</it> which is verified by observation. Absolute magnitudes of classical novae at minimum light, derived from application of the <it>M</it><inf><it>v</it></inf>(max) – rate of decline relationship, average <it>M</it><inf><it>v</it></inf>=4.4 which is close to the luminosity of dwarf novae during outburst. Nova-like variables have similar luminosities. Together these results suggest the possibility of an upper limit to the rate of mass transfer in an accretion disc. A scenario is given in which recent suggestions on cyclical evolution through classical novae, dwarf novae and nova-like variables are shown to agree qualitatively with the systematics found here. A relatively short interval ( ~ 103 yr) between successive nova outbursts appears likely. The Appendix lists cataclysmic variables which are nearer than 300 pc, which should be targets for trigonometrical parallaxes.