German astronomers discover three new hydrogen-deficient pre-white dwarfs

Using the X-shooter instrument at ESO’s Very Large Telescope (VLT), German astronomers have detected three new pre-white dwarfs, which turned out to be strongly hydrogen-deficient. The finding was reported in a research paper published December 20 on the pre-print server arXiv.

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.

Although WDs have a relatively small size, comparable to that of the Earth, they are a few million times more massive than our planet. Pre-white dwarfs (PWDs) are a few times larger and slated to shrink in size, eventually becoming WDs within about a few thousand years.

Now, a team of astronomers led by Klaus Werner of the University of Tübingen in Germany, reports the finding of three new PWDs with extremely low hydrogen mass fraction.

“We have detected three new hydrogen-deficient (H < 0.001 mass fraction) pre-white dwarfs with helium-dominated atmospheres,” the researchers wrote in the paper.

The largest of the three newfound PWDs, designated GSC 08265, is about half the size of the sun. The star, which is located some 5,000 light years away, has an effective temperature of about 72,000 K and its mass is estimated to be 0.53 solar masses.

The astronomers note that GSC 08265 is a PG1159 star in the earliest pre-white dwarf phase. The so-called PG 1159 stars are precursors of the DO white dwarfs or DA white dwarfs, retaining some hydrogen in their envelope, named after their prototype.

One of the stars reported in the study, named Gaia DR3 52, turned out to be a hot subdwarf of spectral type O(He). It has a radius of about 0.23 solar radii, while its mass is around 0.52 solar masses. The star, located approximately 7,400 light years away, has an effective temperature of about 90,000 K.

The third newfound PWD is UCAC4 108—at a distance of some 5,700 light years. The star is about three times smaller than the sun and its mass is estimated to be most likely about 0.8 solar masses. Werner’s team has classified UCAC4 108 as a hot subdwarf of spectral type CO-sdO, with an effective temperature of 50,000 K.

Summing up the results, the authors of the paper underline that all the three PWDs are strongly hydrogen-deficient and have nitrogen abundance at a level of six times greater than that of the sun.

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