The early spectral evolution of Nova Cassiopeiae 1993

Abstract

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 Å and high-resolution 2400-3300 Å) spectra. Our fits show that the effective temperature of the ejecta increased from ∼8000 to about ∼16 000 K between 1993 Dec. 12 and 1993 Dec. 26. The temperature then increased more slowly 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.