Observations shed more light on the atmosphere of the white dwarf GD 424

Astronomers have made spectroscopic observations of a newly detected white dwarf star known as GD 424. The results of the observation campaign provide more insight into the atmosphere of this object. The study was presented in a paper published Dec. 23 on arXiv.org.

White dwarfs are still dense cores of low-mass stars that have exhausted their nuclear fuel. Although their atmosphere is mostly hydrogen or helium, between 25 and 50 percent of all known white dwarfs show traces of metals in their spectra. These metals are believed to come from the growth of tidal-disturbed planetary bodies. Spectroscopic observations of metal-contaminated white dwarfs could be an essential tool to measure the bulk compositions of the parent bodies.

Therefore, a team of astronomers led by Paula Izquierdo from the University of La Laguna, Spain, performed spectroscopic observations of GD 424 – a metal-contaminated white dwarf in the helium atmosphere of spectral type DB with a large amount of trace hydrogen. For this purpose, they used the Intermediate Dispersion Spectrograph and Imaging System (ISIS) mounted on the 4.2m William Herschel Telescope (WHT) and the High-Resolution Echelle Spectrometer (HIRES) of the 10m Keck I- telescope.

“We presented the discovery and analysis of chemical abundances of GD 424, a metal-contaminated DBA white dwarf with one of the largest trace hydrogen amounts measured to date among white dwarfs of comparable temperatures,” the scientists wrote in the paper.

The researchers used a hybrid method to fit synthetic spectra, survey photometry, and data from ESA’s Gaia DR2 parallax to the obtained WHT optical spectrum, allowing them to determine the photospheric parameters of GD 424. It turned out that the white dwarf has an effective temperature. of about 16,560 K, mass of about 0.01 solar masses, radius of about 0.0109 solar rays, and cooling age estimated at about 215 million years.

Analyzing the spectra from WHT and Keck, the team identified 11 metals in GD 424’s atmosphere, namely oxygen, sodium, manganese, chromium, nickel, silicon, iron, magnesium, titanium, calcium and aluminum. The astronomers assumed that the presence of these elements is due to the growth of a planetary body on the white dwarf.

They added that GD 424 most likely accumulates dry, rocky debris in increasing or steady state. The photometric results also allowed the researchers to estimate the composition of the parent body.

“The estimated parental body composition is consistent with both the CI chondrites and the Earth’s mass. (…) The parental body composition did not reveal any excess oxygen. This suggests that the large amount of trace hydrogen is likely the result of the earlier accretion of water-rich planetesimals, ”concluded the paper’s authors.

Further observations of GD 424, aimed at measuring the abundance of volatile elements, are required to better understand the nature of the parent object.

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