The effects of Fe II non-LTE 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 non-LTE 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 non-LTE. 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 non-LTE 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 rate-operator 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 non-LTE effects. For low effective temperatures, T_eff < 20,000 K, the optical Fe II lines are most influenced by non-LTE effects, while for higher T_eff the UV lines of Fe II are very strongly affected by non-LTE. The departure coefficients are such that Fe II tends to be overionized in non-LTE when compared to LTE. Therefore, Fe II non-LTE 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 non-LTE 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.