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Weekly Science Update

Friday, August 07, 2009
Jets from a Possible Young Brown Dwarf
A brown dwarf, a star whose mass is less than about 8% of the sun's, lacks sufficient gravitational contraction to heat up its interior to the roughly ten million kelvin temperatures needed for hydrogen burning (hydrogen burning fuels the sun). Instead, it burns dimly by the power of deuterium fusion, and is consequently faint and hard to detect. Astronomers studying how stars like the sun formed and evolved are particularly interested in brown dwarfs because they bridge the gap in mass between stars and giant planets.

The young stellar object HH211, like most newly formed stars, is obscured at optical wavelengths by a cloud of natal gas and dust. Astronomers have suspected for some time that it was less massive than the sun based on its modest luminosity. Like most young stars, HH211 emits bipolar jets of material as it evolves; the jets help to reduce the star's spin as it ages and contracts. The jets thereby facilitate further contraction, and probably play a role in the formation of any developing planetary system.

SAO astronomers Paul Ho, Typer Bourke, and Qizhou Zhang, together with four colleagues, studied HH211 with extremely high spatial precision using the Submillimeter Array (SMA). By measuring the velocities of the material in the disk around the star, the scientists were able to infer the mass of the star: currently only about fifty times that of Jupiter, making it a brown dwarf and the smallest known object to emit these collimated jets (although the star is still accreting material and will presumably grow larger). They determined that the jets were launched from a region very close to the star, probably from the inner edge of a disk, and confirm that these jets play a role in reducing the object's spin. In addition, they found a small companion star only 84 astronomical units away from the main object. The results show for the first time that even small, brown dwarf stars can produce collimated jets, and show the importance of these jets in the early life of a star.

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