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ZZ Ceti stars are variable DA white dwarfs (WDs) with effective temperatures tex2html_wrap_inline244 . Their amplitudes range from tex2html_wrap_inline246 to tex2html_wrap_inline248 . The pulsations are non-radial g modes with periods of the order of tex2html_wrap_inline250 . A well-known property of g modes is that radius variations are negligible because of the high surface gravity ( tex2html_wrap_inline252 ) of WDs (Robinson et al. 1982; hereafter RKN). Therefore, one can think of these oscillations as temperature waves.

Recently, Brassard et al. (1995; hereafter BFW) calculated spectra semi-analytically for temperature distributions across the surface expressed as tex2html_wrap_inline254 where tex2html_wrap_inline256 is a sum of spherical harmonics. The aim of this analysis was to investigate non-linear effects that occur since the specific intensity I is not a linear function of tex2html_wrap_inline230 . We have developed a program that calculates the time-variable spectrum by integrating I over the visible disk of the WD for a distribution of tex2html_wrap_inline230 across the WD surface. For now, we use for tex2html_wrap_inline230 an expression similar to that of BFW. However, we can adapt our program to non-linear variations of tex2html_wrap_inline230 , which were suggested by Brickhill (1992) and could be caused by the response of the convective flux to linear pressure perturbations in deeper layers of these stars. We interpolate I from a grid containing I as a function of tex2html_wrap_inline274 , tex2html_wrap_inline276 , tex2html_wrap_inline230 , tex2html_wrap_inline252 and different parametrizations of the mixing length theory (MLT). The grid was calculated with a stellar atmospheres code.

Horst Vaeth
Thu Sep 26 10:49:14 MET DST 1996