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Counting Brown Dwarfs
Earth Changes TV, August 25, 2000

Astronomers using NASA's Hubble Space Telescope have taken attendance in a class of brown dwarfs and found indications that these odd and elusive objects also tend to be loners. The Hubble census - the most complete to date - provides new and compelling evidence that stars and planets form in different ways. "Because the brown dwarfs bridge the gap between stars and planets, their properties reveal new and unique insights into how stars and planets form," said Joan Najita of the National Optical Astronomy Observatory in Tucson, AZ. The approximate size of a brown dwarf compared to Jupiter and the Sun. Although brown dwarfs are similar in size to Jupiter, they are much more dense and produce their own light whereas Jupiter shines with reflected light from the Sun.

Considered an astronomical oddity only a few years ago, brown dwarfs are intriguing objects that, unlike stars, are too low in mass to burn hydrogen, but are more massive than planets. At 15 to 80 times the mass of Jupiter, the light that they emit is so faint it's hard to tell how many of them are scattered throughout the galaxy, and how they're formed. The Hubble census finds that, like stars, there are more low-mass brown dwarfs than high-mass ones, and this trend continues down to low, nearly "planetary" masses. "In this respect, the isolated, or free-floating, brown dwarfs found by Hubble appear to represent the low-mass counterparts of the more massive stars. This suggests that stars and free-floating brown dwarfs form in the same way," added Najita.

However, the Hubble finding also offers the strongest evidence so far that free-floating brown dwarfs are far different than the recently discovered planets that orbit nearby stars. Najita's team found brown dwarfs more often alone than in orbit around other stars. "This suggests that the extra-solar planets and, by extension, the planets in our own solar system, formed very differently from how the Sun and other stars formed," Najita noted. Only a few years ago, it was commonly believed that brown dwarfs were rare, perhaps because the star-forming process "stops working" at lower masses. "Nature does not discriminate between stars that can shine by fusion and lower-mass objects that are unable to do so," said Najita. "In fact, the universe easily makes brown dwarfs of all masses, from the most massive to the least."

The study also found that brown dwarfs are unlikely to contribute significantly to the mysterious, unseen "dark matter" that dominates the mass of our galaxy and the universe. Although Hubble found that brown dwarfs are abundant, it turns out that they are not common enough to explain the dark matter. Najita and her colleagues conclude that brown dwarfs probably contribute less than 0.1 percent of the mass of our Milky Way's halo. The inventory was carried out using Hubble's infrared vision to measure the brightness and temperature of stars in the cluster IC 348, located in the constellation Perseus. Because the cluster is young, the brown dwarfs in the cluster are intrinsically brighter, which made it easy to detect about 30 brown dwarfs. A critical step in the observation was picking out the brown dwarfs from the clutter of background stars. To tackle this problem, Najita and colleagues developed a new technique using Hubble's NICMOS camera. The procedure measures the strength of an infrared water-absorption band in the atmospheres of stars. The strength of the band is a sensitive measure of each star's temperature. "The ability to measure the temperature of each star solved several problems simultaneously," Najita said. "In addition to helping us distinguish the cluster brown dwarfs from the background stars, we were also able to measure the masses of the brown dwarfs without having to assume their age. This greatly improved our mass estimates." Najita's study with fellow National Optical Astronomy Observatory researcher Glenn Tiede and John Carr of the Naval Research Laboratory, Washington, DC, will appear in the October issue of the Astrophysical Journal.