White dwarf stars: Astronomers find unusually hot stars covered in ash

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An unusual pair of stars may have been formed in a rare type of stellar merger when a helium star eats another one made of carbon and oxygen



Space



15 February 2022

Artist's impression of a rare kind of stellar merger event between two white dwarf stars.

Artist’s impression of a merger between two white dwarf stars

Nicole Reindl, University of Potsdam

Two strange stars that researchers describe as “freaks” have been spotted in our galaxy. Both are far hotter than stars that are otherwise similar to them, and they seem to be covered in a layer of carbon and oxygen.

Nicole Reindl at the University of Potsdam in Germany and her colleagues found these two stars, called PG1654+322 and PG1528+025, using the Large Binocular Telescope in Arizona. When the researchers analysed them, they found two unexpected properties.

First, stars of this size, with masses around half that of the sun, are generally relatively cool, but these stars are far hotter than others of their size. Second, stars of this size and temperature are generally made almost entirely of helium with only small traces of heavier elements, but both of these stars have surface compositions that are about 20 per cent carbon and 20 per cent oxygen – the ashes left behind from burning helium.

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“This is a very exotic type of star which has never been seen before, and we have two of them,” says Reindl. “There have been other stars that have these high temperatures, but they have different surface compositions.”

Those similar stars are formed when one white dwarf slowly devours another, smaller white dwarf. A second group of researchers led by Marcelo M. Miller Bertolami at the Institute of Astrophysics La Plata in Argentina came up with a variation of this process that could form this new type of star.

In that explanation, the core of the larger of the two white dwarfs must be made mostly of helium, and the smaller one mostly of carbon and oxygen. Then, when the helium white dwarf eats the other one, the resulting object will have carbon and oxygen on its surface.

If this happens, it is expected to be rare – usually, helium white dwarfs are smaller than carbon-oxygen ones, says Reindl. To confirm this hypothesis about how these strange stars are born, astronomers will need to look for those progenitor systems, she says.

Journal references: Monthly Notices of the Royal Astronomical Society Letters, DOI:10.1093/mnrasl/slac005, DOI:10.1093/mnrasl/slab134

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