Accretion Disks in Cataclysmic Variable Stars: Recent Observational Developments

Abstract

I review three current research topics in the observational study of the accretion disks in cataclysmic variable stars: the star-disk connection, eclipse mapping developments, and HST observations. First, an R ^-3/2 emissivity is inferred from the double-peaked velocity profiles of H I and Ca II emission lines from the chromospheres of quiescent accretion disks. There appears to be a direct analogy between these quiescent disk chromospheres and the dynamo-generated chromospheres of rotating stars. In both cases the H I and Ca II line emissivities scale roughly linearly with rotation rate. This observational evidence suggests that dynamo action powers the emission lines from quiescent accretion disks. Second, a new eclipse-mapping study of 6 long-period nova-like variables reveals that their optically-thick disks have temperature-radius profiles that are flatter than expected for a steady-state disk, and become progressively flatter as the binary period decreases toward 3 hours. As these nova-like systems are thought to be in a steady state, the anomalous eclipse maps hint that an important piece of physics is missing in our understanding of these disks. A likely candidate is non-radiative cooling related to driving of a wind from the inner disk. Eclipse mapping techniques are also now being pressed to a new frontier by mapping disks at hundreds of wavelengths and thereby recovering spectra from each position on the disk surface with sufficient resolution to see fines transitioning from absorption to emission. This offers vastly improved constraints on models of disk atmospheres that should make it possible to probe vertical structure. Finally, partly-digested results from HST observations of the cataclysmic variables OY Car, AE Aqr, IP Peg, and GP Com are presented to illustrate some of the new puzzles we are confronting as we begin to use high time resolution in the ultraviolet regime to study the hotter parts of accretion flows