A brown dwarf is an object that is between stars and planets and scientists have long wondered how they are born.
Definition of a Brown Dwarf
Brown dwarfs are intermediate between stars and planets. Their masses are generally between 15 Jupiter masses and 75 Jupiter masses, putting them in a range between giant planets and the smallest theoretical stars. While brown dwarfs are sometimes called failed stars, astronomers didn't really have proof that their creation was that of a star that did not have enough mass to begin nuclear fusion.
Planets form by accumulating rocky debris and become massive enough to draw in gas, while stars form from the gravitational collapse of gas clouds. When the gas cloud becomes hot and dense enough, fusion ignites, "turning on" the new star. As the gas creating a star rotates and swirls inward, it speeds up. This is the same as the conservation of angular momentum seen when a twirling ice skater brings in her arms and spins faster. For the newborn star to gather mass, it must somehow shed the angular momentum, which it does by ejecting material in opposite directions in a bipolar outflow. Astronomers have now found an outflow from a brown dwarf, indicating that brown dwarfs are created in the same way that stars are.
The Birth of a Brown Dwarf
Scientists using the Smithsonian's Submillimeter Array have detected molecules of carbon monoxide streaming out from a brown dwarf with an estimated mass of 60 Jupiters. This brown dwarf, named ISO-Oph 102, provides the first strong evidence that brown dwarfs are born in a manner similar to stars.
The outflow from the brown dwarf contains 1000 times less mass than that of a star and the outflow rate is 100 times smaller. This scaled down outflow is what would be expected from an object that does not have enough mass to form a real star.
"These findings suggest that brown dwarfs and stars aren't different because they formed in different ways," explains Paul Ho, an astronomer at the Harvard-Smithsonian Center for Astrophysics. "They share the same formation mechanism. Whether an object ends up as a brown dwarf or star apparently depends only on the amount of available material."
In the first illustration, the brown dwarf ISO-Oph 102 is forming like a star by accumulating material from the surrounding accretion disk, shown in orange. The brown dwarf sheds angular momentum by ejecting material in two oppositely directed jets, shown in red. The blue bow shocks indicate where those jets are interacting with the interstellar medium. The second illustration gives a close-up look at the brown dwarf and its accretion disk.
Source: Harvard-Smithsonian Center for Astrophysics
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