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Asteroseismology study uncovers new pulsation modes in ultra-massive white dwarf

Based on time-series photometry from three different telescopes, an international team of astronomers has performed a detailed asteroseismology study of WD J0049−2525—the most massive pulsating white dwarf. The study, May 22 on the arXiv pre-print server, resulted in the detection of new pulsation modes of this white dwarf.

White dwarfs (WDs) are stellar cores left behind after a star has exhausted its nuclear fuel and represent the final evolutionary stage for the vast majority of stars. Observations show that most WDs have primary spectral classification DA as they exhibit hydrogen-dominated atmospheres. However, a small fraction of WDs showcases traces of heavier elements.

In pulsating WDs, luminosity varies due to non-radial gravity wave pulsations within these objects. One subtype of pulsating WDs is known as DAVs, or ZZ Ceti stars, which have only hydrogen absorption lines in their spectra.

With a mass of about 1.3 solar masses, WD J004917.14–252556.81, or WD J0049−2525 for short, is an ultra-massive ZZ Ceti star—the most massive pulsating WD so far detected. The white dwarf is located at a distance of some 326 light years and its effective temperature is estimated to be approximately 13,000 K. Previous observations have detected two significant pulsation modes of this object.

A group of astronomers led by Ozcan Caliskan of Istanbul University in Turkey employed the New Technology Telescope (NTT), the Apache Point Observatory (APO) and the Gemini South telescope to conduct an asteroseismology investigation of WD J0049−2525. The main aim of this study was to probe the interior of this WD.

"We present a detailed observational and asteroseismic analysis of the most massive pulsating WD currently known, WD J0049−2525, based on time-series photometry from three different telescopes," the researchers wrote in the paper.

The observations allowed Caliskan's team to identify 13 new significant pulsation frequencies of WD J0049−2525, many of which have high signal-to-noise ratios. The several prominent frequencies are concentrated in the range between 3,868 µHz (258 seconds) and 5,861 (170 seconds) µHz.

Moreover, the study detected two potential frequency splittings, indicating a rotation period of either 0.3 or 0.67 days for WD J0049−2525. The astronomers note that both suggested orbital periods are in agreement with expectations for such a massive WD.

Based on the collected data, it was found that WD J0049−2525 has a mass of approximately 1.29 solar masses and its asteroseismic distance is between 304 and 320 light years, which is consistent with previous estimates. The crystallized core mass fraction for this WD was calculated to be higher than 99%.

Summing up the results, the authors of the paper underline that further high-precision observations are necessary to fully understand the detailed structure of WD J0049−2525, especially to constrain its rotation period and to verify the presence of additional subtle pulsation modes.