11 January 2006
Astronomers Use Spitzer Space Telescope to Challenge Brown Dwarf Formation Models

WASHINGTON, D.C. — A group of astronomers led by Neal Evans of The University of Texas at Austin has used NASA’s Spitzer Space Telescope to show that brown dwarfs form like stars — by pulling in matter from a collapsing gas cloud and forming disks of potentially planet-forming material around themselves — and that such disks are common around young brown dwarfs.

The formation process that creates brown dwarfs has long been a mystery. In terms of their mass, “brown dwarfs are found in the ‘no-man’s land’ between stars and planets,” Evans says. “They have masses too small to be a star, but too large to be a planet.”

Two lines of evidence point to a star-like formation process for brown dwarfs: the presence of disks found around brown dwarfs, and the discovery of extremely dim objects forming inside clouds of gas and dust — objects too dim to be protostars.

Both lines of evidence come from Spitzer observations by Evans’ team, a group of about 60 astronomers from various institutions, called the “Cores to Disks” or “c2d” Spitzer Legacy Project.

“None of these objects could have been found without the unprecedented sensitivity of the instruments on the Spitzer Space Telescope,” Evans says.

First, the new c2d Spitzer observations, combined with supporting observations from a number of ground-based telescopes, show that a substantial number of brown dwarfs are surrounded by disks of dusty material, similar to those found around forming stars.

C2d team members Katelyn Allers, Jacqueline Kessler-Silacci, Daniel Jaffe, and Lucas Cieza of The University of Texas at Austin found about a dozen disk-surrounded brown dwarfs in the southern-hemisphere constellations Chamaeleon, Lupus, and Ophiuchus. Some of the brown dwarfs have a mass of five to 10 Jupiters, and are only a few million years old — young, astronomically speaking.

The disk discoveries were made by comparing observations of these objects at different wavelengths. The brown dwarfs were first studied in near-infrared light using the four-meter Blanco Telescope at the National Science Foundation’s Cerro Tololo Interamerican Observatory in Chile.

Astronomers used the near-infrared information to predict how much mid-infrared light they should give off. The Spitzer observations showed that the objects gave off much more mid-infrared light than expected. This can be explained by the presence of a disk around the brown dwarf. Disks are made up of dust, which absorbs light radiated from the brown dwarf and re-emits it at lower energies — that is, in the particular mid-infrared wavelengths detectable by Spitzer.

The group also found these objects are less massive than the smallest stars. “You can’t weigh these brown dwarfs directly,” Allers says. “We used theoretical models to figure out that they may have masses as low as five to 10 Jupiter masses.”

“The disks around the brown dwarfs are analogous to the disks around very young Sun-like stars,” Allers says, “disks that we believe provide the raw materials for planets.” In fact, Daniel Apai of the University of Arizona and his collaborators announced in October 2005 that they had found evidence that disks around more massive brown dwarfs might form planets.

Allers’ discoveries broaden the original finding of a disk around the more massive brown dwarf OTS 44 by Kevin Luhman of Penn State, announced in February 2005, and his more recent discovery of a less massive disk-surrounded brown dwarf. Today’s c2d announcement shows these are part of a wide-spread phenomenon — not oddballs, but the norm.

The presence of these disks around brown dwarfs challenges one idea for their formation, namely ejection caused by gravitational interactions inside a region of star formation densely packed with stars. These results conflict with that theory in three ways: First, computer models show that it would be difficult for ejected brown dwarfs to keep their disks. Second, one of Allers’ brown dwarfs is in a wide binary system, which is difficult for the ejection model to produce. Finally, neither Lupus nor Chamaeleon are forming stars in the dense clusters the ejection model requires.

The discovery of a substantial number of disks around even very low mass brown dwarfs increases the likelihood that the alternative formation scenario applies: that brown dwarfs form more or less like stars do, by accreting matter from a collapsing cloud of gas and dust — or, in the jargon of star-formation researchers, a “core.”

“These results suggest an origin for brown dwarfs similar to that of stars: a collapsing ‘core’ of gas and dust,” Evans says. “If this is right, we should see evidence for very low mass objects in cores.”

Mass is hard to measure in the very early stages of brown-dwarf formation. But astronomers know that forming objects give off light in amounts related to their mass and the rate at which they are accreting new material onto themselves. So a low mass, accreting object would be very faint.

Evidence for such tiny, dim objects exists. The first discovery of a very dim object (called L1014-IRS) forming inside what was previously thought to be a “starless core” in early Spitzer images was made by c2d team member Chadwick Young of UT-Austin (now at Nicholls State University) and collaborators and announced in November 2004. Now, c2d team members have found about a dozen very faint objects that may be brown dwarfs in this earlier disk phase, embedded in cores of gas and dust. Once again, this shows that L1014-IRS, like OTS 44, is not an oddball, but the norm.

The new examples were found by c2d team members Tyler Bourke, Tracy Huard, and Philip Myers of the Harvard-Smithsonian Center for Astrophysics; Michael Dunham of UT-Austin; and Jens Kauffmann of the Max-Planck-Institut für Radioastronomie. These findings suggest a new class of objects is emerging. Dubbed “Very Low Luminosity Objects,” or “VeLLOs,” they have less than one-tenth the Sun’s luminosity.

These are unlikely to be stars in a very early stage of formation. “Accreting protostars are much more luminous than they will be when they become stars,” Evans says. “So finding such a low luminosity in these objects is surprising. It implies that the product of the current mass and the rate at which mass is being added is unusually low.”

These studies show that the VeLLOs embedded in what were thought of as “starless cores” may be earlier stages of the disk-surrounded brown dwarfs found by Katelyn Allers and her c2d collaborators. In fact, further studies by Bourke and Huard show strong evidence for a disk around L1014-IRS, as announced in October 2005.

“Cores to Disks” is one of six Spitzer Legacy Science Projects selected in November 2000 to complete major surveys with Spitzer. The c2d team was awarded 400 hours of Spitzer observations, and produces data freely available to all astronomers.

The Spitzer Space Telescope is managed for NASA by the Jet Propulsion Laboratory, a division of Caltech, in Pasadena, Calif. Science operations are conducted at the Spitzer Science Center at Caltech, also in Pasadena.

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Science Contacts: Neal Evans of UT-Austin may be reached at (512) 471-4396 or nje@astro.as.utexas.edu; Dan Jaffe of UT-Austin may be reached at (512) 471-3425 or dtj@astro.as.utexas.edu, and Philip Myers of the Smithsonian Astrophysical Observatory may be reached at (617) 495-7295 or pmyers@cfa.harvard.edu.