2 Results and discussion

For Au I spectral lines Stark broadening parameters, the full semiclassical perturbation formalism (Sahal-Bréchot 1969a,b), has been applied. A summary of the formalism for neutral emitters is given in Dimitrijevic & Sahal-Bréchot (1984), and for ionized in Dimitrijevic et al. (1991) and Dimitrijevic & Sahal-Bréchot (1996). We note here that the inelastic collision contribution is included in the ion-impact line widths.

Energy levels for Au I lines have been taken from Moore (1971). Oscillator strengths have been calculated by using the method of Bates & Damgaard (1949) and the tables of Oertel & Shomo (1968). For higher levels, the method described by Van Regemorter et al. (1979) has been used. In addition to electron-impact full halfwidths and shifts, Stark-broadening parameters due to proton-, and He II- impacts have been calculated. Our results for six Au I lines are shown in Table 1, for perturber densities $10^{15}- 10^{19}~{\rm cm}^{-3}$ and temperatures $T= 2\,500-50\,000$ K. We also specify a parameter C (Dimitrijevic & Sahal-Bréchot 1984), which gives an estimate for the maximum perturber density for which the line may be treated as isolated when it is divided by the corresponding full width at half maximum. For each value given in Table 1, the collision volume (V) multiplied by the perturber density (N) is much less than one and the impact approximation is valid (Sahal-Bréchot 1969a,b). Values for NV>0.5 are not given and values for $0.1< NV \le 0.5$ are denoted by an asterisk.

Figure 2: Same as in Fig. 1, but for DA ( $T_{\rm eff}=10000$ K, $\log g=6$, curves with circles) and DB ( $T_{\rm eff}=15000$ K, $\log g=7$) white dwarfs

Tabulated Stark broadening parameters are linear with perturber density for perturber densities lower than $10^{15}~{\rm cm}^{-3}$. The accuracy of the semiclasical method is $\pm\ 30\%$ (Griem 1974). When the impact approximation is not valid, the ion broadening contribution may be estimated by using quasistatic approach (Sahal-Bréchot 1991 and Griem 1974). In the region between, where neither of these two approximations is valid, a unified type theory should be used. For example in Barnard et al. (1974), a simple analytical formulas for such a case are given. The accuracy of the results obtained decreases when broadening by ion interactions becomes important.

For Au II spectral lines, the electron-impact broadening calculation has been performed within the modified semiempirical approach. Considering the very complex spectrum of Au II, the jj coupling approximation for matrix-element calculation has been used. The needed atomic data for calculation have been taken from Rosberg & Wyart (1996). In Table 2, the electron-impact parameteres for eight Au II transitions as a function of temperature for an electron density of $N_{\rm e}=10^{17}~{\rm cm}^{3}$ are presented. The average accuracy of the MSE method is $\pm\ 50\%$(Dimitrijevic & Konjevic 1980).

In order to see the influence of Stark broadening mechanism for gold spectral lines in stellar plasma, we have calculated the Stark widths for Au II $\lambda=174.0476$ nm throught the different models of stellar atmospheres. In Figs. 1 and 2, the electron-impact and thermal Doppler widths as function of optical depth for Kurucz's (1979) A type star ( $T_{\rm eff}=10000$ K, $\log g=4$) and models of DA ( $T_{\rm eff}=10000$ K, $\log g=6$) and DB ( $T_{\rm eff}=15000$ K, $\log g=7$) white dwarf atmospheres (Wickramasinghe 1972) are presented. As one can see from Fig. 1, for the case of hot A type star, in photospheric layers the line width due to Stark broadening is one order of magnitude larger than the width due to the thermal Doppler mechanism. In higher layers of the stellar atmosphere ( $\tau\approx -4$) however, the thermal Doppler mechanism is more important. In the case of white dwarf atmospheres (see Fig. 2) the Stark broadening mechanism is important in all layers of atmospheres and in deeper atmosphere layers the Stark width is two or three order of magnitude larger than thermal Doppler width. For the three here considered atmosphere models Stark broadening effect should be taken into account in abundance determination and other investigations of stellar plasmas.

Acknowledgements

This work is a part of the project "Astrometrical, Astrodynamical and Astrophysical Investigations", supported by Ministry of Science and Technology of Serbia.