Astronomers weigh ultra-cool brown dwarf

An international team of astronomers have been able to measure the mass of an ultra-cool brown dwarf "star", and its binary companion, directly, for the first time.

Just over 60 times the mass of Jupiter, the brown dwarf was first detected in 2000 when Hubble spotted it orbiting the poetically named 2MASSW J0746425+2000321.

Because the two "stars" orbit each other so closely - separated by a mere 0.18 arc seconds - the researchers needed incredibly high resolution images to follow the intricacies of their interaction. They made observations with the ground-based Keck, Gemini and VLT, as well as Hubble. Keck, Gemini and the VLT all have powerful adaptive optics systems that correct for atmospheric turbulence, not a factor for the space-based Hubble.

The astronomers, led by Hervé Bouy from the Max Planck Institute in Germany, tracked the binary system for four years. Using the data they collected, they were able to reconstruct the whole ten-year orbit of the pair. Once this was known, they could calculate the masses using Kepler's laws.

The more massive component of the system is just 8.5 per cent of our sun's mass, just over the threshold for nuclear ignition. Its companion, meanwhile has only 6.6 per cent of the mass of the sun. This means it is not really a star at all, but a "sub-stellar" object or brown dwarf.

A brown dwarf is an object that exists somewhere on the continuum between the lightest stars and the heaviest planets. It is not massive enough for nuclear fusion to have started in the core. For fusion to start, a star must be at least 7.5 per cent of the mass of the sun, or 75 times the mass of Jupiter.

Brown dwarves have been theoretically predicted for a long time, but were only discovered in 1995. Candidates were identified by their colour and brightness, but the only certain way to classify an object as a brown dwarf is by directly measuring its mass.

The challenge is that there is no way to measure a star's mass unless it is in a binary system, and brown dwarf binaries are particularly hard to resolve, because they tend to be very close to one another, and not especially bright.

In a statement, the Journal of Astronomy and Astrophysics said: "The mass measurement performed by Hervé Bouy and his colleagues is thus a major step toward our understanding of these sub-stellar objects that occupy the gap between stars and planets". ®

Bootnote

In a fabulous twist of fate, one of the researchers on the team is actually called Henri Boffin. Marvellous stuff.

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