Another helium white dwarf binary does its stuff

You wait ages for an outburst from a white dwarf binary system and two pop along at once. Following the discovery of an eclipsing white dwarf binary system, which I blogged about in an earlier post, another (not eclipsing) one has reared its head.

The new one is KL Draconis, in the constellation of Draco. As with the earlier white dwarf binary, the two stars have masses of around 0.6 and 0.1 times that of our Sun. At least one of the dwarfs is a helium dwarf too. Most white dwarfs are the carbon rich cores of low mass stars that have reached the end of their lives and have blown away their outer layers. However, binary systems can be a little different, with material whipped away from one star by another. This leaves the donor star stripped down to its core before its completed its life cycle and so with a helium rich core. Unlike the previous mentioned system, this one sees the two dwarfs orbitting one another with a period of 25 minutes. In the first system, there are periodic outbursts as the denser dwarf steals material from the more bloated one (even after the death of the stars) and gorges itself until there is sufficient material built up on the surface to temporarily ignite fusion that then creates sufficient heat to blow the material off once more. In this new system, the outbursts are more regular, once every two months, and rather than the surface of the dwarf detonating, the explosion occurs in the disc of material falling down onto the dwarf. How this happens has so far defied explanation despite the attentions of both the Liverpool telescope and the Swift satellite. More detail in Astronomy Now’s report.

Credit: Liverpool Telescope/Gavin Ramsay; Swift Satellite–UVOT/Gavin Ramsay

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