The Effects of Fe II NonLTE on Nova Atmospheres and Spectra
Abstract
The atmospheres of novae at early times in their outbursts are very extended, expanding shells with low densities. Models of these atmospheres show that nonLTE 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 nonLTE. 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 nonLTE effects for Fe II for a wide variety of parameters. We use a detailed Fe II model atom with 617 level and 13,675 primary lines, treated using a rateoperator formalism. We show that the radiative transfer equation in nova atmospheres must be treated with sophisticated numerical methods and that simple approximations, such as the Sobolev method, cannot be used because of the large number of overlapping lines in the comoving frame.
Our results show that the formation of the Fe II lines is strongly affected by nonLTE effects. For low effective temperatures, T_{eff} < 20,000 K, the optical Fe II lines are most influenced by nonLTE effects, while for higher T_{eff} the UV lines of Fe II are very strongly affected by nonLTE. The departure coefficients are such that Fe II tends to be overionized in nonLTE when compared to LTE. Therefore, Fe II nonLTE must be included with sophisticated radiative transfer in nova atmosphere models in order to analyze observed nova spectra reliably. Finally, we show that the number of wavelength points required for the Fe II nonLTE model atmosphere calculations can be reduced from 90,000 to about 30,000 without changing the results if we choose a sufficiently dense UV wavelength grid.
 Publication:

The Astrophysical Journal
 Pub Date:
 May 1996
 DOI:
 10.1086/177160
 arXiv:
 arXiv:astroph/9601149
 Bibcode:
 1996ApJ...462..386H
 Keywords:

 STARS: NOVAE;
 CATACLYSMIC VARIABLES;
 RADIATIVE TRANSFER;
 STARS: ATMOSPHERES;
 Astrophysics
 EPrint:
 25 pages, latex, AASTEX, figures not included, full text plus figures available via http://brian.la.asu.edu/ or at ftp://brian.la.asu.edu/FeIInova.tgz, to appear in ApJ