THE EARLY SPECTRAL EVOLUTION OF NOVA CASSIOPEIAE 1993

In this paper we describe the evolution of Nova Cas 1993 over the firs t two months of its outburst. We present an ultraviolet light curve th at covers the period from announcement to just after dust began formin g in the ejecta (1994 Feb. 15) and IUE spacecraft constraints forced u s to halt our observations. We have used spherical, expanding, NLTE st ellar atmospheres to compute synthetic spectra and have compared the r esults to combined ultraviolet (low-resolution 1200-3400 Angstrom A an d high-resolution 2400-3300 Angstrom A) spectra. Our fits show that th e effective temperature of the ejecta increased from similar to 8000 t o 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 J an. 28. A preliminary abundance analysis shows evidence for hydrogen d epletion, as we also found for Nova V1974 Cygni; however we find a lar ger enhancement of carbon, nitrogen, and oxygen. We also show that the principal mechanism for mass ejection in this nova is a radiation pre ssure driven wind and that mechanical driving is not necessary.