Temporal Variations of the White Dwarf and Disk in OY Carinae Following the 1992 Superoutburst

Abstract

Hubble Space Telescope observations of the eclipsing dwarf nova OY Carinae after its 1992 April superoutburst are used to isolate ultraviolet spectra (1150-2500 A at 9.2 A FWHM resolution) of the white dwarf, the accretion disk, and the bright spot. The white dwarf spectra have a Stark-broadened photospheric Lyα absorption feature but are veiled by a forest of absorption features that we attribute to absorption by intervening disk material (a curtain). All the spectral fits required supersonic turbulence in the curtain material with Mach numbers of 6-8. All curtain temperatures were between 10,000 and 11,000 K. There was a curtain temperature increase ~3 months after the superoutburst. We find that the white dwarf temperature changed from 19,700 K just 27 days after the end of the superoutburst to 18,000 K roughly 3 months after the superoutburst; the exponential (e-folding) decay time of the white dwarf temperature was 66 days. We present evidence that the heating of the white dwarf was more extensive during the superoutburst than the normal outburst. The thermal response of the OY Car white dwarf to outburst heating is compared with WZ Sagittae, VW Hydri (the most similar dwarf nova to OY Car), and the cooling timescales of other dwarf novae after superoutburst. The measured cooling timescales of the five systems with superoutbursts appear to be shorter the longer the orbital period (accretion rate). Possible implications are discussed. There is evidence of a disk flux variation, independent of the effect of white dwarf cooling, which suggests a possible contradiction of the disk instability model. To establish this, however, data are required throughout a quiescent cycle.