White dwarf embryos are remnant stars that have just passed the red giant stage and shed their outer layers, but have not yet had time to shrink to the size of Earth. A study of three such sites showed a deficiency of hydrogen at them.
Astronomers’ new discovery
Using the X-shooter instrument on ESO’s Very Large Telescope (VLT), German astronomers have discovered three new white dwarfs that appear to be severely hydrogen-deficient. This is reported in a research paper published on December 20 on the arXiv preprint server.
White dwarfs are stellar cores left after a luminary has exhausted its nuclear fuel and passed the red giant stage. Due to their high gravity, they are known to have an atmosphere consisting of either pure hydrogen or pure helium. However, a small fraction of white dwarfs have traces of heavier elements.
Although white dwarfs are relatively small, comparable to the size of Earth, they are several million times more massive than our planet. But they do not form in this form immediately after the red giant sheds its outer shells. Their embryos, or as they are also called, pre-white dwarfs (PWD) are several times larger and gradually decrease in size, eventually becoming WDs over several thousand years.
Hydrogen-deficient white dwarfs
Now a team of astronomers led by Klaus Werner at the University of Tübingen in Germany reports the discovery of three new PWDs with extremely low hydrogen mass fractions.
“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 newly found PWDs, named GSC 08265, is about half the size of the Sun. The star, which is about 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.
Features of discovered stars
Astronomers note that GSC 08265 is star PG1159, which is in the earliest stage of a white dwarf. The so-called PG 1159 stars are precursors of DO white dwarfs or DA white dwarfs that retain some hydrogen in their shells, 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). Its radius is about 0.23 of the Sun’s radius and its mass is about 0.52 of the Sun’s mass. The star, located about 7,400 light-years away, has an effective temperature of about 90,000 K.
The third newly discovered PWD, UCAC4 108, is about 5,700 light-years away. The star is about three times smaller than the Sun, and its mass is likely about 0.8 solar masses. Werner’s team classified UCAC4 108 as a hot CO-sdO spectral subdwarf with an effective temperature of 50,000 K.
Summarizing the results, the authors of the paper emphasize that all three PWDs are highly hydrogen-deficient and have nitrogen contents at levels six times higher than those of the Sun.
According to phys.org
