Vega ($\alpha$ Lyr)

The detailed analysis of the spectrum of Vega with ATLAS9 and ATLAS12 models has been discussed by [Castelli & Kurucz(1994)Castelli and Kurucz]. Here, we compare PHOENIX LTE and NLTE models computed with the parameters found for Vega by [Castelli & Kurucz(1994)Castelli and Kurucz] to the observed Vega spectrum to show the effects of NLTE on the spectral energy distribution for stars with Vega's effective temperature. We have computed an NLTE model with $\hbox{$\,T_{\rm eff}$}=9550\,{\rm K}$, $\log(g)=3.95$ and the ``Vega abundances'' given in table 4 of [Castelli & Kurucz(1994)Castelli and Kurucz]. We have used the following species in NLTE: H I, He I-II, Ne I, Mg II, Ca II, Ti I-II, S II-III, Si II-III, C I-II, N I-II, O I-II, and Fe I-III, which includes the most important species for the conditions in Vega. This resulted in a total of 3,787 NLTE levels and 47,593 primary NLTE transitions. 25,449 of the primary NLTE lines were treated with detailed Voigt profiles, the remaining 22,144 weak primary NLTE lines were treated with Gaussian profiles to save CPU time. In addition, the NLTE models include 223,563 secondary NLTE lines as well 219,774 LTE background lines, these lines were selected using the same criterion that we use for the LTE models. The calculation was performed with a variable resolution wavelength grid with 204,434 points, including extra points that are inserted to resolve NLTE lines.

We show a comparison between the NLTE model and Vega observational data in Fig. 9. In general, the fit is good, no attempts have been made to fine-tune some of the abundances (e.g., to reduce the too strong absorption feature just shortward of $1600\hbox{\AA}$). A more detailed analysis of the Vega data will be presented elsewhere (Aufdenberg et al, in preparation).

We compare the NLTE and LTE structures for a Vega model with $\hbox{$\,T_{\rm eff}$}=9500\k$ in Fig. 11 and the departure coefficients of the ground states of some selected ions are shown in Fig. 10. The departures from LTE are larger for the Vega model than for the solar model shown above. The changes in the temperature structure are not very large in the deeper layers, but can reach nearly $1000\,{\rm K}$ in the outermost, optically thin, zones of the model. In the line forming regions, NLTE effects are generally small so that the spectra of the NLTE and the LTE models are not very different. Since the departures from LTE are larger for Vega than for the solar model, the validity of the assumption of LTE is weaker for Vega than for the solar model. This is the main reason why the LTE models that we present in this paper essentially stop at $\hbox{$\,T_{\rm eff}$}=10000\k$, for higher effective temperatures NLTE effects will rapidly become important.