ZZ Ceti stars are variable DA white dwarfs (WDs) with effective temperatures . Their amplitudes range from to . The pulsations are non-radial g modes with periods of the order of . A well-known property of g modes is that radius variations are negligible because of the high surface gravity ( ) 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 where 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 . 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
across the WD surface. For now, we use for an expression
similar to that of BFW. However, we can adapt our program to
non-linear variations of , 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
, , , and different
parametrizations of the mixing length theory (MLT). The grid was
calculated with a stellar atmospheres code.

Thu Sep 26 10:49:14 MET DST 1996