New Insight into the Spatial Distribution of Novae in M31
We use a Monte Carlo technique together with a simple model for the distribution of dust in M31 to investigate the observability and spatial distribution of classical novae in M31.
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arXiv subjects
Sumner Starrfield
Publications and source records attributed to Sumner Starrfield.
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 And 1986 using a grid of spherically symmetric, non-LTE, line-blanketed, expanding model atmospheres and synthetic spectra with the following set of parameters: $5,000\le$ T$_{model}$ $\le 60,000$K, solar abundances, $ρ\propto r^{-3}$, $\v_{max} = 2000\kms$, $L=6 \times 10^{4}\Lsun$, and a statistical or microturbulent velocity of 50 $\kms$. We used the synthetic spectra to estimate the model parameters corresponding to the observed {\it IUE} spectra. The fits to the observations were then iteratively improved by changing the parameters of the model atmospheres, in particular T$_{model}$ and the abundances, to arrive at the best fits to the optically thick pseudo-continuum and the features found in the {\it IUE} spectra. The {\it 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, semi-forbidden, and forbidden lines in the observed spectra. Analysis of these phases indicate that OS And 86 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 86 and derive a peak bolometric luminosity of $\sim$ 5 $\times$ 10$^4$ L$_{\odot}$. The computed nova parameters provide insights into the physics 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 of Fe{\sc ii} which, when included, lead to much better agreement with the observations.
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The Effects of Fe II NLTE on Nova Atmospheres and Spectra
The atmospheres of novae at early times in their outbursts are very extended, expanding shells with low densities. Models of these atmospheres show that NLTE effects are very important and must be included in realistic calculations. We have, therefore, been improving our atmospheric studies by increasing the number of ions treated in NLTE. One of the most important ions is Fe II which has a complex structure and numerous lines in the observable spectrum. In this paper we investigate NLTE effects for Fe II for a wide variety of parameters. We use a detailed Fe II model atom with 617 level and 13675 primary lines, treated using a rate-operator formalism. We show that the radiative transfer equation in nova atmospheres {\em must} be treated with sophisticated numerical methods and that simple approximations, such as the Sobolev method, {\em cannot} be used because of the large number of overlapping lines in the co-moving frame. Our results show that the formation of the Fe II lines is strongly affected by NLTE effects. For low effective temperatures, $\Teff < 20,000\,$K, the optical Fe II lines are most influenced by NLTE effects, while for higher $\Teff$ the UV lines of Fe II are very strongly affected by NLTE. The departure coefficients are such that Fe II tends to be overionized in NLTE when compared to LTE. Therefore, Fe II-NLTE must be included with sophisticated radiative transfer in nova atmosphere models in order to reliably analyze observed nova spectra.
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