GD 99: Re-investigation of an old ZZ Ceti companion

TL;DR

This study conducts a detailed asteroseismic analysis of the ZZ Ceti star GD 99, identifying multiple pulsation modes, modeling its internal structure, and comparing seismic and Gaia distance measurements to better understand its properties.

Contribution

It identifies new pulsation modes in GD 99 and provides a comprehensive seismic model, enhancing understanding of its internal structure and evolutionary status.

Findings

Detected seven new pulsation periods in GD 99.

Best-fit seismic model suggests a hotter, more massive star than spectroscopic data indicate.

Estimated the star's rotational period to be approximately 13.17 hours.

Abstract

Context. Thanks to photometric space missions, we have access to more and more information on the properties of white dwarf stars, especially 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. In addition to space-based observations, we also take advantage of the opportunity to study the pulsations of white dwarf stars from the ground, either as observations that are complementary to space-based measurements or individual observing runs on selected targets across long timescales. Aims. We aim to investigate long-term, single-site observations of the bright, yet scarcely studied ZZ Ceti star, GD 99. Our main goals are to determine as many eigenmodes for asteroseismology as possible and then to carry out a seismic analysis of this target. Methods. We performed a Fourier analysis of the light curves obtained in different epochs. After finding the normal modes of the pulsation, we ran 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 the literature data, we were able to use 11 modes for the asteroseismic fits. We accepted an asteroseismic model solution with Teff = 13 500 K and M* = 0.80 Msun as a best fit, however, this suggests a hotter and more massive star than we might have expected based on the spectroscopic values. We also estimated the rotational rate of the star to be 13.17 h, based on TESS observations.