HL CANIS MAJORIS IN PREOUTBURST AND SS CYGNI -- THE INTEROUTBURST DISK INSTABILITY

Abstract

ABSTRACT SS Cygni and HL Canis Majoris were observed by IUE for three consecutive nights in November of 1992. During the first two nights, simultaneous photometric ground-based observations of SS Cyg were made at the Ball State University Observatory. Observations of SS Cyg and HL CMa were also obtained simultaneously with the 90-inch telescope at the Steward Observatory on the last two nights of the IUE run. These spectroscopic observations covered the wavelength range from 4100 A to 5000 A, while the spectra taken with the short wavelength camera on IUE resulted in wavelength coverage from 1150 A to 1980 A. SS Cyg is a U Gem type dwarf nova with an orbital period of 6.6 hours. Good simultaneous UV and optical orbital coverage was obtained for this system. HL CMa is a Z Cam type dwarf nova with a mean outburst interval of 15 days. The AAVSO reports that this system was in outburst 4 days after the observing run. Therefore, HL CMa may have been in a preoutburst state during these observations. Optical spectra of HL CMa indicate a warm front passed through the outer disk four days before outburst, but no changes were seen in the UV spectra. Signs of a preoutburst state were observed to develop in SS Cyg, but no outburst occurred for another 30 days.