White dwarf blinks better than Christmas lights

Scientists have recently studied the white dwarf WD J0135+5722. It is extremely massive. However, that is not what makes it unique. It pulsates as it moves from one mode to another. The total number of rhythms available to it is 19!

White Dwarf WD J0135+5722. Source: olhardigital.com.br

Pulsating white dwarfs

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

White dwarfs are stellar cores that remain after a luminary exhausts its fuel. Due to their high gravity, they have an atmosphere composed of pure hydrogen or pure helium. However, a small fraction of WD contains traces of heavier elements.

In pulsating WDs, the luminosity varies due to non-radial pulsations of gravitational waves within these objects. One subtype of pulsating WDs is known as DAV, or ZZ Ceti stars. These are WDs of spectral type DA, having only hydrogen absorption lines in their spectrum.

Feature of white dwarf WD J0135+5722

Located 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 supermassive white dwarf. 

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

The observations revealed 19 distinct pulsation periods of WD J0135+5722 with durations ranging from 137 to 1345 seconds, which is typical for ZZ Ceti-type stars. Astronomers noted that this discovery makes WD J0135+5722 the sixth pulsating supermassive white dwarf and the richest in terms of the number of frequencies detected — it surpasses the white dwarf BPM 37093 with 8 detected pulsation modes.

Study of pulsation periods

To investigate the periodicity of the light curves of WD J0135+572, astronomers calculated the Fourier Transform (FT) of each light curve. This analysis allowed them not only to determine the frequencies of the pulsation modes, but also their corresponding amplitudes, phases, and errors.

The study also estimated the mass of WD J0135+5722 using various methods. It turns out that a white dwarf has a mass of about 1.12 solar masses if its core is composed of oxygen and neon, or 1.135 solar masses if the star has a carbon-oxygen core.

Summarizing the results, the authors of the paper emphasized: “This discovery is important for future studies because it sheds light on the final stages of development of high-mass stars and/or merger products that could potentially serve as progenitors of supernovae,” the scientists concluded.

According to phys.org

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