Posted: 05 December, 2008
Astronomers have uncovered strong evidence that brown dwarfs – the dividing line between stars and planets – form like stars.
Brown dwarfs sit on the fence between planets and stars, and generally have masses between 15 and 75 Jupiters. According to theory, a star needs to have a mass of at least 75 times that of Jupiter in order to be able to sustain nuclear fusion. As a result, brown dwarfs are sometimes called failed stars. However, the jury has been out as to whether they still form like ‘real’ stars from the gravitational collapse of gas clouds, or if they grow like planets, accumulating rocky material until they reach a critical mass enabling them to draw in nearby gas.
This artist's conception shows the brown dwarf ISO-Oph 102. Observations by the Submillimeter Array suggest that it is forming like a star, by accumulating material from the surrounding accretion disc (orange) shown here. The brown dwarf sheds angular momentum by ejecting material in two oppositely directed jets (red). Blue bow shocks indicate where those jets are interacting with the interstellar medium. Image: ASIAA.
When a star forms from a cloud of interstellar gas it draws itself together through gravity, growing denser and hotter until fusion is initiated. If the initial gas cloud is rotating, that rotation will speed up as it collapses inward, but in order to gather mass, the young protostar must somehow shed that angular momentum. It does this by spewing material in opposite directions as a bipolar outflow. Because brown dwarfs are less massive than a star, there is less gravity available to pull it together. As a result, astronomers debated whether a brown dwarf could form the same way as a star.
Previous observations provided hints that they could, and now, astronomers using the Smithsonian's Submillimeter Array (SMA) made the serendipitous discovery of a bipolar molecular outflow at a 60 Jupiter mass brown dwarf known as ISO-Oph 102, providing the first strong evidence in favour of brown dwarf formation through gravitational collapse. The clue came in the form of molecules of carbon monoxide shooting outward from ISO-Oph 102 in a fashion typically seen from young stars or protostars.
"We thought that any such outflow would be too weak to detect with current facilities and would have to wait until a next-generation instrument like ALMA (the Atacama Large Millimeter Array)," says Ngoc Phan-Bao of the Academia Sinica Institute of Astronomy and Astrophysics (ASIAA), lead author on the paper announcing the find. "This was a big surprise. Finding the molecular outflow with the SMA shows the extraordinary capabilities of the array."
This artist's conception zooms in on the brown dwarf and its accretion disc. The discovery of a bipolar molecular outflow at ISO-Oph 102 offers the first strong evidence in favour of brown dwarf formation through gravitational collapse. Image: David A. Aguilar (CfA).
As might be expected for a brown dwarf, the outflow contains much less mass than the outflow from a typical star – around 1,000 times less in this case. Similarly, the outflow rate is also smaller by a factor of 100. In all respects, the molecular outflow of ISO-Oph 102 is a scaled-down version of the outflow process seen in young stars.
"These findings suggest that brown dwarfs and stars aren't different because they formed in different ways," says Paul Ho, an astronomer at the Harvard-Smithsonian Center for Astrophysics and director of ASIAA. "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."
The paper on ISO-Oph 102 will be published in the 20 December issue of the Astrophysical Journal Letters.