Astronomers using the Large Binocular Telescope have discovered two unusual hot subdwarf stars: PG 1654+322 and PG 1528+025. While normal stars have surfaces composed of hydrogen and helium, these two subdwarfs are covered with helium-burning ash and can be identified as the remnants of a helium-core white dwarf that accreted matter of a low-mass carbon-oxygen white dwarf.
PG 1654+322 and PG 1528+025 were found as part of a large-scale search program in which researchers are tracking down short-lived, hot stars to better understand the final stages of stellar evolution.
“We normally expect the chemical surface composition of the stars discovered to have completed the helium fusion in their centers and to be in the final stages of becoming white dwarfs,” said Dr. Klaus Werner, an astronomer with the Institut für Astronomie und Astrophysik in the Kepler Center for Astro and Particle Physics at the Universität Tübingen, and lead author of a paper published in the Monthly Notices of the Royal Astronomical Society: Letters.
“It is known that there are objects covered with carbon and oxygen instead of hydrogen.”
“The cause is thought to be an explosive resumption of helium fusion, which then brings the burning ash — carbon and oxygen — to the surface.”
“However, this event cannot explain the two newly-discovered subdwarf stars, PG 1654+322 and PG 1528+025,” he added.
“They have larger radii and carry out helium fusion peacefully at their centers.”
“We believe that these stars were formed by a very rare type of merger of two white dwarfs,” added Dr. Miller Bertolami, an astronomer with the Max-Planck-Institut für Astrophysics and UNLP-CONICET and lead author of a companion paper in the Monthly Notices of the Royal Astronomical Society: Letters.
Stellar mergers are known to happen between white dwarfs in close binary systems due to the shrinking of the orbit caused by the emission of gravitational waves.
“This does not normally lead to the formation of a star enriched in carbon and oxygen,” said Dr. Nicole Reindl, an astronomer with the Institut für Astronomie und Astrophysik in the Kepler Center for Astro and Particle Physics at the Universität Tübingen, and co-author of the main paper.
“However, we believe that in binary systems with very specific stellar masses, a white dwarf with a carbon-oxygen core can be ripped apart by tidal forces.”
“Its material is then dumped on the surface of its white dwarf companion, leading to the formation of these exotic stars.”
“However, more detailed evolutionary models are needed to fully explain the phenomenon.”
Klaus Werner et al. 2022. Discovery of hot subdwarfs covered with helium-burning ash. MNRASL 511 (1): L66-L71; doi: 10.1093/mnrasl/slac005
M.M. Miller Bertolami et al. 2022. An evolutionary channel for CO-rich and pulsating He-rich subdwarfs. MNRASL 511 (1): L60-L65; doi: 10.1093/mnrasl/slab134
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