B Hot white dwarfs and pre-white dwarfs discovered with SALT, MNRAS

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An international team of astronomers has discovered eight of the hottest stars, all exceeding 100,000 degrees Celsius, using the Southern African Large Telescope (SALT). The findings, published in the Monthly Notices of the Royal Astronomical Society, highlight a survey of helium-rich subdwarf stars that led to the identification of these hot white dwarf and pre-white dwarf stars, with the hottest reaching 180,000 degrees Celsius. Notably, one of the stars is the central star of a newly discovered planetary nebula, while two are classified as variable stars. These stars are nearing the end of their life cycles and are over one hundred times brighter than the sun, which is atypical for white dwarfs. The extreme temperatures may be due to their recent liberation from AGB stars or potential mergers with other white dwarfs.
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An international team of astronomers has discovered eight of the hottest stars in the universe, all with surfaces hotter than 100,000 degrees Celsius. The work was published in Monthly Notices of the Royal Astronomical Society.

The paper is based on data gathered using the Southern African Large Telescope (SALT), the largest single optical telescope in the southern hemisphere, with a 10m x 11m mirror. The study describes how a survey of helium-rich subdwarf stars led to the discovery of several very hot white dwarf and pre-white dwarf stars, the hottest of which has a surface temperature of 180,000 degrees Celsius. For comparison, the sun's surface is a mere 5,800 degrees.
https://phys.org/news/2023-01-astronomers-super-hot-stars.html

One of the stars identified is the central star of a newly discovered planetary nebula, which is one light year in diameter. Two of the others are pulsating, or "variable" stars. All of these stars are at an advanced stage of their life cycle and are approaching the end of their lives as white dwarfs. Due to their extremely high temperatures, each of these new discoveries is more than one hundred times brighter than the sun, which is considered unusual for white dwarf stars.

https://academic.oup.com/mnras/article/519/2/2321/6967306?login=false
 
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White dwarfs are very hot when first liberated from the AGB star that used to surround them, since they are what is left over from the hot stellar core. So one presumes these are recently liberated and have not yet had much time to cool. If they are older, it might be that they recently merged with another white dwarf, which could heat them up again.
 
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