HD 49798 is a

binary star A binary star or binary star system is a system of two stars that are gravitationally bound to and in orbit around each other. Binary stars in the night sky that are seen as a single object to the naked eye are often resolved as separate stars us ...

in the constellation

Puppis Puppis ("poop deck, stern") is a constellation in the southern sky. It was originally part of the Former constellations, traditional constellation of Argo Navis (the ship of Jason and the Argonauts), which was divided into three parts, the other ...

about from Earth. It has an apparent magnitude of 8.3, making it one of the brightest known O class subdwarf stars. HD 49798 was discovered in 1964 to be a rare hydrogen-deficient O class subdwarf, and was the brightest known at the time. This was identified as a binary star, but the companion could not be detected visually or spectroscopically. The X-ray source RX J0648.0-4418 was discovered close to HD 49798's location in the sky. Only the space telescope

XMM-Newton ''XMM-Newton'', also known as the High Throughput X-ray Spectroscopy Mission and the X-ray Multi-Mirror Mission, is an X-ray space observatory launched by the European Space Agency in December 1999 on an Ariane 5 rocket. It is the second corners ...

was able to identify the source. It is a

white dwarf A white dwarf is a Compact star, stellar core remnant composed mostly of electron-degenerate matter. A white dwarf is very density, dense: in an Earth sized volume, it packs a mass that is comparable to the Sun. No nuclear fusion takes place i ...

with about 1.3 solar masses, in orbit about HD 49798 and rotating once every 13 seconds; this rotation is speeding up by per year. This is detected from the 13-second X-ray pulse, which results from the

stellar wind A stellar wind is a flow of gas ejected from the stellar atmosphere, upper atmosphere of a star. It is distinguished from the bipolar outflows characteristic of young stars by being less collimated, although stellar winds are not generally spheri ...

accreting onto the compact object. It has been proposed that the white dwarf is surrounded by a

debris disk A debris disk (American English), or debris disc ( Commonwealth English), is a circumstellar disk of dust and debris in orbit around a star. Sometimes these disks contain prominent rings, as seen in the image of Fomalhaut on the right. Debris ...

. In this model, the material of the disk would be funneled onto the poles of the dwarf via the magnetic field, which would explain the observed X-ray pulses. This system is considered a likely candidate to explode as a

type Ia supernova A Type Ia supernova (read: "type one-A") is a type of supernova that occurs in binary systems (two stars orbiting one another) in which one of the stars is a white dwarf. The other star can be anything from a giant star to an even smaller white ...

within a few tens of thousands of years.

References

{{DEFAULTSORT:HD 49798 Puppis O-type subdwarfs 032602 049798 CD-44 02920 White dwarfs X-ray binaries