THE EFFECTS OF FE-II NON-LTE ON NOVA ATMOSPHERES AND SPECTRA

The atmospheres of novae at early times in their outbursts are very ex tended, expanding shells with low densities. Models of these atmospher es show that non-LTE effects are very important and must be included i n realistic calculations. We have, therefore, been improving our atmos pheric studies by increasing the number of ions treated in non-LTE. On e of the most important ions is Fe II, which has a complex structure a nd numerous lines in the observable spectrum. In this paper we investi gate non-LTE effects for Fe II for a wide variety of parameters. We us e a detailed Fe Ir model atom with 617 level and 13,675 primary lines, treated using a rate-operator formalism. We show that the radiative t ransfer equation in nova atmospheres must be treated with sophisticate d numerical methods and that simple approximations, such as the Sobole v method, cannot be used because of the large number of overlapping li nes in the comoving frame.Our results show that the formation of the F e 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 influ enced 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 L TE. Therefore, Fe II non-LTE must be included with sophisticated radia tive transfer in nova atmosphere models in order to analyze observed n ova spectra reliably. Finally, we show that the number of wavelength p oints required for the Fe II non-LTE model atmosphere calculations can be reduced from 90,000 to about 30,000 without changing the results i f we choose a sufficiently dense UV wavelength grid.