Astronomers have found two unexpectedly hot stars covered in ash

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

By Leah Crane

Two strange stars that researchers describe as “freaks” have been spotted in our galaxy. Both of these stars 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 these two stars, 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 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.

“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 star 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 their explanation, the core of the larger of two white dwarfs must be made mostly of helium, and the smaller one mostly of carbon and oxygen. Then, when the helium star eats the other, the resulting object will have carbon and oxygen on its surface. If this happens, it’s expected to be rare – usually, helium white dwarfs are smaller than carbon-oxygen ones, Reindl says. 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