Weekly Science Update

Wednesday, March 28, 2007
Brown Dwarf Twins
The small star with the long name of 2MASS-J1207334-393254 is about 179 light-years from Earth. It has a mass of only about twenty-four Jupiters, too little to be able to ignite hydrogen burning in its core. It is what astronomers call a brown dwarf star, a star that burns dimly by the power of deuterium fusion. Astronomers interested in how stars like the Sun and planets like the Earth formed and evolved are naturally interested in brown dwarfs because these objects bridge the gap in mass between stars and planets. But 2MASS-J1207334-393254 is interesting for another reason as well: it has a companion, an even smaller, eight-Jupiter-mass-size body orbiting it, and furthermore it appears that both objects have protostellar disks surrounding them as they orbit each other.

Scientists over the past few years have concluded that the companion in this binary does not fit the expected behavior of brown dwarf stars. In particular, its inferred temperature is entirely inconsistent with its luminosity - at least as far as the models can determine. SAO astronomers Subhanjoy Mohanty and Eric Mamajek, together with two colleagues, have obtained new data that resolve these and some other discrepancies as well. From sensitive new optical observations and careful modeling, they conclude that the companion is actually surrounded by a disk of material that is seen nearly edge-on; the total mass of that disk is about the same as the mass of our Moon. The primary star also has a disk of material, making this object a binary system with two disks. The astronomers hypothesize that these conclusions, if correct, suggest that about eight million years ago both stars formed together from a large disk of material that was perhaps as much as ten times more massive than it is now. If even brown dwarf stars can have disks this large, then they might also have asteroids or other even smaller bodies in their midst. The new results help to resolve most of the inconsistencies that had been worrying astronomers, and help to illustrate the rich complexity of stellar development in the cosmic cycle of stellar birth.


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