A failed star is born: the origins of brown dwarfs

In cosmic circles, brown dwarfs are something of a flop. Too
big to be considered true planets, yet not massive enough to be stars, these
free-floating celestial bodies are, in fact, sometimes referred to as failed
stars. But do they really form as stars do—from collapsing clouds of gas—or
are their origins completely different?

A series of publications by University of Michigan astronomer Ray Jayawardhana
and collaborators, including a paper in the Jan. 16 issue of Science, offers
evidence that brown dwarfs and Sun-like stars are born in much the same way.
“They at least have very similar infancies, which may mean that they also
have very similar origins,” said Jayawardhana, assistant professor of
astronomy.

Stars form in cold clouds of gas and dust in interstellar space. Dense clumps
within these clouds contract under their own gravity, spinning up in the
process and gathering material from the surroundings into a disk. Eventually,
if a growing protostar accumulates enough mass, its core becomes hot and dense
enough for nuclear fusion to occur, and the new star begins to shine. Some
scientists have suggested that brown dwarfs form the same way but simply
don’t accumulate enough mass to ignite hydrogen fusion, and calculations
show that it’s at least theoretically possible for objects with masses as
low as those of brown dwarfs to be born this way.

But other scientists have proposed that brown dwarfs are runts kicked out of
stellar litters. In this scenario, brown dwarfs are born in multiple star
systems and compete with their siblings for matter from the natal cloud. In
such systems, the slowest-growing object may be ejected before it gathers
enough material to become a star, computer simulations suggest.
One way to distinguish between the two possibilities is by studying disks of
dust and gas around young brown dwarfs. If brown dwarfs form as stars do, they
should have large, long-lived accretion disks like those found around young
stars. But if they have been ejected from multiple star systems, their disks
should be shaved down by the gravitational interactions that lead to ejection.

Jayawardhana and colleagues searched for dusty disks around young brown dwarfs
by observing their infrared emission with the 8-meter Very Large Telescope
(VLT) of the European Southern Observatory in Chile and the 10-meter Keck I
telescope in Hawaii. Because dust particles in a disk absorb light and
re-radiate the energy at infrared wavelengths, a brown dwarf with a disk will
emit more infrared light than one without a disk.

“We found that the majority of brown dwarfs are surrounded by dusty disks
at an age of a million years or so,” said Jayawardhana. “That’s similar to
young stars at the same age.” Although it’s not possible to directly
determine the disks’ sizes, their presence around some brown dwarfs as old
as 10 million years suggests that they aren’t pared away in early life.

Other spectroscopic observations, using the twin 6.5-meter Magellan telescopes
in Chile (in which the University of Michigan is a partner institution) and the
Keck I telescope, showed that brown dwarfs also accrete material from
surrounding disks the same way as stars do—although at a slower pace.

“We detect telltale signs of gas flowing from the inner edge of the disk
onto the brown dwarf at velocities of over a hundred kilometers per
second,” said Jayawardhana. In one intriguing case, astronomers have also
found evidence of material spewing out from the poles of a brown dwarf.
Such jets have been seen in young stars of the same age, but not until now
in brown dwarfs. “If confirmed, the presence of jets would further
strengthen the case for remarkably similar infancies for brown dwarfs and
Sun-like stars,” said Jayawardhana, whose collaborators include Subhanjoy
Mohanty (Harvard-Smithsonian Center for Astrophysics), Gibor Basri
(University of California, Berkeley), David Barrado y Navascues
(Laboratory of Space Astrophysics and Fundamental Physics in Madrid,
Spain), David Ardila (Johns Hopkins University), Beate Stelzer
(Astronomical Observatory of Palermo in Italy), and Karl Haisch, Jr. and
Diane Paulson (both at the University of Michigan).

“I wouldn’t say that the story is signed, sealed and delivered,”
Jayawardhana said, “but the preponderance of evidence is very much leaning
in the direction of these things forming the same way as stars.” And the
evidence uncovered so far leads to even more tantalizing prospects. “Now
that we know many young brown dwarfs are surrounded by disks,” he said, “I
can’t help but wonder if comets and asteroids—if not small
planets—could form in these disks.”

This research was supported primarily by a grant from the National Science
Foundation.

For more information:

Ray Jayawardhana — http://www.astro.lsa.umich.edu/~rayjay/

Telephone: 734 223 8338 (mobile) / 734 764 3408 (office)

Email: rayjay@umich.edu

“Unraveling Brown Dwarf Origins” by Ray Jayawardhana

Science, vol. 303, p. 322 (2004 January 16 issue)

Magellan Telescopes — http://www.ociw.edu/lco/magellan/

Keck Observatory — http://www2.keck.hawaii.edu/

Very Large Telescope — http://www.eso.org/paranal/