NON-LTE MODEL ATMOSPHERE ANALYSIS OF THE EARLY ULTRAVIOLET-SPECTRA OFNOVA OS-ANDROMEDAE-1986

We analyse the early, optically thick, ultraviolet spectra of Nova OS And 1986 using a grid of spherically symmetric, non-LTE, line-blankete d, expanding model atmospheres and synthetic spectra with the followin g set of parameters: 5000 less than or equal to T-model less than or e qual to 60 000 K, solar abundances, rho proportional to r(-3), v(max) = 2000 km s(-1), L = 6 x 10(4) L., and a statistical or microturbulent velocity of 50 km s(-1). We use the synthetic spectra to estimate the model parameters corresponding to the observed IUE spectra. The fits to the observations are then iteratively improved by changing the para meters of the model atmospheres - in particular, T-model and the abund ances - to arrive at the best fits to the optically thick pseudo-conti nuum and the features found in the IUE spectra. The IUE spectra show t wo different optically thick subphases. The earliest spectra, taken a few days after maximum optical light, show a pseudo-continuum created by overlapping absorption lines. The later observations, taken approxi mately 3 weeks after maximum light, show the simultaneous presence of allowed, semiforbidden and forbidden lines in the observed spectra. An alysis of these phases indicates that OS And 1986 had solar metallicit ies, except for Mg which showed evidence of being underabundant by as much as a factor of 10. We determine a distance of 5.1 kpc to OS And 1 986, and derive a peak bolometric luminosity of similar to 5 x 10(4) L .. The computed nova parameters provide insights into the physics of t he early outburst and explain the spectra seen by IUE. Lastly, we find evidence in the later observations for large non-LTE effects of Fe II which, when included, lead to much better agreement with the observat ions.