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

Abstract

We analyse the early, optically thick, ultraviolet spectra of Nova OS And 1986 using a grid of spherically symmetric, non-LTE, line-blanketed, expanding model atmospheres and synthetic spectra with the following set of parameters: 5000 ≤ Tmodel≤60000 K, solar abundances, ρ∝r-3, vmax=2000 km s-1,L = 6 × 104 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 parameters of the model atmospheres - in particular, Tmodel and the abundances - to arrive at the best fits to the optically thick pseudo-continuum and the features found in the IUE spectra. The IUE spectra show two 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 approximately 3 weeks after maximum light, show the simultaneous presence of allowed, semiforbidden and forbidden lines in the observed spectra. Analysis of these phases indicates that OS And 1986 had solar metallicities, 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 1986, and derive a peak bolometric luminosity of ∼5 × 104 L⊙. The computed nova parameters provide insights into the physics of the 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 observations. © 1997 RAS.