GD 99: re-investigation of an old ZZ Ceti friend

GD 99: re-investigation of an old ZZ Ceti friend by Zs. Bognár et al. on Tuesday 22 November Context. Thanks to the photometric space missions, we know more and more about the properties of white dwarf stars, especially the pulsating ones. In the case of pulsators, we have the opportunity to get an insight into their otherwise hidden interiors by the means of asteroseismology. Besides the space-based observations, we also have the opportunity to study the pulsations of white dwarf stars from the ground, either as observations complementary to the space-based measurements, or individual, long time-base observing runs on selected targets, respectively. Aims. We aim at the long-term, single-site observations of the bright and not well-studied ZZ Ceti star, GD 99. Our main goals were to determine as many eigenmodes for asteroseismology as it is possible, and then perform the seismic analysis of this target. Methods. We performed Fourier analysis of the light curves obtained in different epochs. After finding the normal modes of the pulsation, we run the 2018 version of the White Dwarf Evolution Code to build model grids for the period fits. We compared the seismic distance of the best-fit model with the geometric value provided by Gaia measurements. Results. We find that GD 99 is rich in pulsation modes in the 200-1100 s period range, as we detected seven new periods. Together with literature data, we were able to use 11 modes for the asteroseismic fits. We accepted an asteroseismic model solution with Teff = 12 600 K and M* = 0.85 Msun as a best fit, however, this suggests a more massive star than we expected based on the spectroscopic values. The difference between the seismic distance derived by this model solution and the Gaia geometric distance is about 4 pc. We also estimated the rotational rate of the star based on TESS observations to be 12.11 h. arXiv: http://arxiv.org/abs/http://arxiv.org/abs/2211.11676v1