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Record low for brown dwarf
Posted: 01 February 2010

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A bizarre failed star with a record-breaking low surface temperature has been discovered by the United Kingdom Infrared Telescope (UKIRT) in Hawaii, extending the range of extremes the properties of these kinds of objects can possess.

Discovered by Dr Ben Burningham of the University of Hertfordshire, the sub-stellar object – termed a brown dwarf – is called SDSS 1416+13B. Existing in a binary system with another brown dwarf, the similarly named SDSS 1416+13A, it has some puzzling features. For instance, it is so dim and cool that it can only be seen in infrared light, whereas its larger, warmer companion can be observed in visible light. Furthermore, when the Japanese 8.2-metre Subaru Telescope on Hawaii observed it in near-infrared wavelengths, it appeared bluer than any brown dwarf ever seen before. This has been attributed to copious amounts of methane in its atmosphere, which causes large gaps to appear in its spectrum.

The near-infrared image of the brown dwarfs on the left, in which brown dwarf B appears very blue, and the mid-infrared image on the right, in which brown dwarf B appears very blue. Image: JAC/UKIRT/Spitzer Space Telescope/University of Hertfordshire.

Things got weirder when NASA’s Spitzer Space Telescope observed it at mid-infrared wavelengths, and scientists found that it appeared redder than any other brown dwarf at those wavelengths. Typically, the colour of a brown dwarf at mid-infrared wavelengths is the best indicator of its surface temperature, and its extreme redness led to a temperature measurement of 227 degrees Celsius (500 kelvin) – the lowest temperature ever recorded for a brown dwarf. By comparison, brown dwarf A in the system has a surface temperature of 1,227 degrees Celsius (1,500 kelvin). The unusual colours are being enhanced by the gravity of the brown dwarfs, and from these enhancements their masses can be estimated: brown dwarf B has an estimated mass of about 30 times that of Jupiter (one Jupiter mass is 1.9 x 1027 kilograms), while its partner is 75 times the mass of Jupiter.

Brown dwarfs are a link between stars and planets. The distinction between a gas giant planet and a brown dwarf can be a little blurred, but the difference between stars and brown dwarfs is clear. Whereas stars generate energy from nuclear fusion instigated by the high pressures and temperatures within their cores, brown dwarfs are not massive enough to ignite nuclear fusion. Instead they generate their meagre energy by gradual gravitational contraction.

The near-infrared spectrum of brown dwarf B shows a large gap in the spectrum between 1.7 and 2.5 microns where there is barely any light. This makes the brown dwarf appear blue. Image: Subaru Telescope (NAOJ)/University of Hertfordshire.

This contraction happens quickest in the first billion years of a brown dwarf’s life, after which the brown dwarf begins to cool, and the more massive the brown dwarf is the slower it cools. Brown dwarf B is a moderately massive object (officially the lower limit for a brown dwarf is 13 Jupiter masses, but this is hotly debated, while brown dwarf A is almost, but not quite, massive enough to be a fully-fledged star) so for it to be so cool it must be very old – about ten billion years old. This is backed up by the fact that both objects are lacking in heavy elements, a characteristic of very old stars that date from a time when there wasn’t the same abundance of heavy elements that there is today.

“This is the fourth time in three years that UKIRT has made a record-breaking discovery of the coolest known brown dwarf, with an estimated temperature not far above 200 degrees Celsius,” says Dr Philip Lucas of the University of Hertfordshire. “Its colours are so extreme that this object will keep a lot of physicists busy trying to explain it.”

Indeed, it is quite likely that there are older, even cooler brown dwarfs out there somewhere, so it’s a good bet that there are still some more record-breaking discoveries to be made.

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