Ultraviolet and Optical Spectroscopy Of AL Comae 1 Year After Superoutburst

Abstract

We present Hubble Space Telescope (HST) ultraviolet spectra of AL Com obtained with the Faint Object Spectrograph (FOS) during four epochs 10-14 months following its 1995 April superoutburst. During our 1996 observations, the continuum fluxes underwent a slow decline of 20%, while the spectrum showed Lyα and Si II absorption features indicative of a 20,000 K white dwarf. The cooling sequence in AL Com is found to be about half as long as that of WZ Sge. This timescale is what is predicted for an outburst that was half the duration of WZ Sge, if shear mixing and angular momentum result in nonspherical accretion into an equatorial belt. Our longest data set (1996 June) showed spectral line variations on the orbital period with C IV in emission for about one-third of the orbit but revealed no orbital variability in the continuum. Ground-based optical spectra obtained close in time to our February HST data also show stronger line emission for one-third of an orbit and highly variable broad absorption from the secondary star. The spectral variations may be due to the changing visibility of a nonuniform disk or of a secondary star irradiated by the hot white dwarf.