Using the 1m Ritchey–Chrétien–Coudé Telescope at the Konkoly Observatory in Budapest, Hungary, and NASA’s Transiting Exoplanet Survey Satellite (TESS), Hungarian astronomers have observed a variable star known as the name of GD 99. The results of the study, published on November 21 on the preprint server arXiv, shed more light on the properties of this variable.
The detection and study of variable stars could offer important clues to certain aspects of stellar structure and evolution. The study of variables could also be useful for a better understanding of the distance scale of the universe.
ZZ Ceti stars (also called DAV stars) are variables with modest variations in brightness and periods between 30 and 1,200 seconds. They are DA-type pulsating white dwarfs, which have an outer shell composition of pure hydrogen.
GD 99 is one of the brightest ZZ Ceti stars known to date. However, although GD 99 was identified as a variable white dwarf almost half a century ago, its pulsating behavior and asteroseismology have not yet been thoroughly studied. In order to change this, a team of astronomers from the Konkoly Observatory, led by Zsófia Bognár, carried out long-term, single-site observations of GD 99 with the primary goal of performing a seismic analysis of this star. .
“We selected GD 99 for a long-term ground tracking measurement, which resulted in 30 nights of observations, spanning over 119 hours of observation…We conducted observations between 2002 and 2022 with the 1 -m Ritchey– Chretien-Coudé Telescope at the Piszkésteto Mountain Station of the Konkoly Observatory, Hungary writes in the journal.
The observations allowed the team to identify 11 eigenmodes for seismology. GD 99 in general was found to be rich in pulsation modes in the period range of 200 to 1100 s, as astronomers detected seven new periods.
Based on the results, astronomers were able to determine the fundamental parameters of GD 99. The mass of the star was calculated to be around 0.8 solar masses, and its effective temperature was estimated at a level of 13,500 K. It turned out that the is located about 110 light years away and its rotation period is about 13.17 hours.
Comparing the results with previous studies, GD 99 is hotter, more massive, and located closer than previously thought. The paper’s authors speculate that the star may lie at the blue edge of the ZZ Ceti empirical instability band – a region of the Hertzsprung-Russell diagram largely occupied by several related classes of pulsating variables.
Overall, the researchers pointed out that GD 99 joins the currently known small group of mode-rich white dwarf pulsators.
Zs. Bognár et al, GD 99: Reinvestigation of a former companion of ZZ Ceti, arXiv (2022). DOI: 10.48550/arxiv.2211.11676
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