Non-LTE model atmosphere analysis of the early ultraviolet spectra of Nova OS Andromedae 1986

We have analyzed the early optically thick ultraviolet spectra of Nova OS And1986 using a grid of spherically symmetric, non-LTE, line-blanketed, expandingmodel atmospheres and synthetic spectra with the following set of parameters:$5,000\le$ T$_{model}$ $\le 60,000$K, solar abundances, $\rho \propto r^{-3}$,$\v_{max} = 2000\kms$, $L=6 \times 10^{4}\Lsun$, and a statistical ormicroturbulent velocity of 50 $\kms$. We used the synthetic spectra to estimatethe model parameters corresponding to the observed {\it IUE} spectra. The fitsto the observations were then iteratively improved by changing the parametersof the model atmospheres, in particular T$_{model}$ and the abundances, toarrive at the best fits to the optically thick pseudo-continuum and thefeatures found in the {\it IUE} spectra. The {\it IUE} spectra show twodifferent optically thick subphases. The earliest spectra, taken a few daysafter maximum optical light, show a pseudo-continuum created by overlappingabsorption lines. The later observations, taken approximately 3 weeks aftermaximum light, show the simultaneous presence of allowed, semi-forbidden, andforbidden lines in the observed spectra. Analysis of these phases indicate thatOS And 86 had solar metallicities except for Mg which showed evidence of beingunderabundant by as much as a factor of 10. We determine a distance of 5.1 kpcto OS And 86 and derive a peak bolometric luminosity of $\sim$ 5 $\times$10$^4$ L$_{\odot}$. The computed nova parameters provide insights into thephysics of the early outburst and explain the spectra seen by {\it IUE}.Lastly, we find evidence in the later observations for large non-LTE effects ofFe{\sc ii} which, when included, lead to much better agreement with theobservations.