THE EARLY SPECTRAL EVOLUTION OF NOVA CASSIOPEIAE 1993

In this paper we describe the evolution of Nova Cas 1993 over the first two months of its outburst. We present an ultraviolet light curve that covers the period from announcement to just after dust began forming in the ejecta (1994 Feb. 15) and IUE spacecraft constraints forced us to halt our observations. We have used spherical, expanding, NLTE stellar atmospheres to compute synthetic spectra and have compared the results to combined ultraviolet (low-resolution 1200-3400 Angstrom A and high-resolution 2400-3300 Angstrom A) spectra. Our fits show that the effective temperature of the ejecta increased from similar to 8000 to about similar to 16 000 K between 1993 Dec. 12 and 1993 Dec. 26. The temperature then increased more slowly to similar to 24 000 on 1994 Jan. 28. A preliminary abundance analysis shows evidence for hydrogen depletion, as we also found for Nova V1974 Cygni; however we find a larger enhancement of carbon, nitrogen, and oxygen. We also show that the principal mechanism for mass ejection in this nova is a radiation pressure driven wind and that mechanical driving is not necessary.