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Using the Gran Telescopio Canarias (GTC) and the Apache Point Observatory (APO), an international team of astronomers has detected 19 pulsation modes in an ultra-massive white dwarf known as WD J0135+5722. The discovery, on the arXiv preprint server, makes WD J0135+5722 the richest pulsating ultra-massive white dwarf known to date.

White dwarfs (WDs) are stellar cores left behind after a star has exhausted its nuclear fuel. Due to their high gravity, they are known to have atmospheres of either pure hydrogen or pure helium. However, a small fraction of WDs shows 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—these are WDs of spectral type DA, having only hydrogen absorption lines in their spectra.

Located some 165.5 light years away, WD J0135+5722 is a white dwarf with a mass of about 1.11 solar masses and an effective temperature of 12,415 K. Previous studies of WD J0135+5722 have classified it as a potential pulsating ultra-massive white dwarf.

Therefore, WD J0135+5722 was selected as one of the targets to search for pulsations by a group of astronomers led by Francisco C. De Gerónimo of the La Plata National University in Buenos Aires, Argentina.

"We have begun a program aimed at increasing the number of detected UM [ultra-massive] DAVs and the pulsation modes in already known DAVs with the Gran Telescope Canarias (GTC) and Apache Point Observatory (APO)," the researchers explained.

The observations detected 19 distinct pulsation periods of WD J0135+5722, ranging from about 137 to 1,345 seconds, typical for ZZ Ceti stars. The astronomers noted that this discovery makes WD J0135+5722 the sixth pulsating ultra-massive white dwarf and the richest one in terms of the number of detected frequencies—surpassing white dwarf BPM 37093 with 8 identified pulsation modes.

In order to investigate the periodicities in the light curves of WD J0135+572, the astronomers computed the Fourier Transforms (FT) of each light curve. This analysis allowed them not only to identify the frequencies of the pulsation modes, but also their respective amplitudes, phases and errors.

The study also estimated the mass of WD J0135+5722 using different methods. It was found that the white dwarf's mass is approximately 1.12 solar masses if its core is made of oxygen and neon, or 1.135 solar masses if the star has a carbon-oxygen core.

Summing up the results, the authors of the paper underlined the importance of their discovery. "This discovery is significant for future investigations, as it sheds light on the final stages of high-mass stars and/or merger products, potentially serving as progenitors for supernovae," the scientists concluded.

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