Elusive brown dwarfs, the missing link between gas giant planets like Jupiter
and small, low-mass stars, have now been "fingerprinted" by UCLA astronomy
professor Ian S. McLean and colleagues, using the Keck II Telescope at the W.M.
Keck Observatory in Hawaii.
|Brown dwarfs, such as Gliese 229B, lack sufficient mass (at
least 75-80 Jupiters) to ignite core hydrogen fusion.
Credit: American Scientist/Linda Huff
McLean and his research team will publish the most systematic and comprehensive
near-infrared spectral analysis of more than 50 brown dwarfs in the Oct. 10 issue
of the Astrophysical Journal, the premier journal in astronomy, published by
the American Astronomical Society.
"The infrared spectra of brown dwarfs reveal their atomic and molecular fingerprints," said
McLean. "Each class of brown dwarfs has a unique fingerprint. We have taken
the spectra of more than 50 of them, which reveal their physical and chemical
Brown dwarfs are failed stars about the size of Jupiter, with a much larger mass
but not quite large enough to become stars. Like the sun and Jupiter, they are
composed mainly of hydrogen gas, perhaps with swirling cloud belts. Unlike the
sun, they have no internal energy source and emit almost no visible light. Brown
dwarfs are formed along with stars by the contraction of gases and dust in the
interstellar medium, McLean said. The first brown dwarf was not discovered until
1995, yet McLean suspects the galaxy is teeming with them.
|Gas giant planets like Jupiter were formed
from gaseous material surrounding the young star.
"Brown dwarfs are so elusive, so hard to find," McLean said. "They can
be detected best in the infrared, and even within the infrared, they are very
difficult to detect. We detect the heat glow from these faint objects in the
infrared. Typically, they have to be relatively close by, within 100 light years,
for us to even detect the heat signature."
McLean and his colleagues do so using a sophisticated instrument that McLean
designed and built at UCLA with other astronomers from UCLA and the University
of California, Berkeley. The instrument, attached to the W.M. Keck Observatory's
10-meter Keck II Telescope atop Mauna Kea in Hawaii -- the world's largest optical
and infrared telescope -- is called NIRSPEC. It is six feet high, weighs one
ton and contains the most powerful infrared spectrometer in the world.
"This is the first time a large quantity of high quality spectral data are presented
systematically in the infrared, where brown dwarfs emit most of their light," said
Davy Kirkpatrick, staff scientist at the California Institute of Technology's
NASA-funded Infrared Processing and Analysis Center. "Approximately 2 percent
of brown dwarfs near the sun are oddballs, and we are starting to be able to
identify them and understand what makes them different. In addition, many brown
dwarfs have been reported in different ways, and we now present them in a consistent
manner that will become a standard reference for the future."
McLean built the world's first infrared camera for wide use by astronomers in
1986, and he has built six increasingly sophisticated infrared cameras and spectrometers
since then. (A spectrometer splits light into its component colors.)
The quality of infrared spectra has improved drastically over the last decade," he
Credit: University of Hawaii
The detectors in McLean's spectrometers, such as NIRSPEC, have more than 250
times as many picture elements as in the 1980s.
"The spectrum reveals what's present and what's missing," McLean said. "What's
missing in the light tells us something in the atmosphere of the brown dwarf
has absorbed the light.
"When we first studied the brown dwarf spectra, they were peculiar like no star
we had ever seen before. The reason we saw missing light in the spectra of the
coolest brown dwarfs is the presence of methane in the atmosphere, which we also
see in the outer gas giants of the solar system: Jupiter, Saturn, Uranus, Neptune.
"We also see evidence of water in the form of superheated steam, which we don't
see in any normal star like the sun. The sun is much too hot to have water molecules.
Methane and water sculpt the infrared spectrum in a very distinctive way. The
spectra of brown dwarfs show a gradual change from that of a star to that of
"Brown dwarfs are the missing link between gas giant planets like Jupiter and
small stars like red dwarfs," he said.
If large numbers of brown dwarfs exist, they "could make a small, but significant
contribution to dark matter," the so-called "missing mass" in
the universe, McLean said.
|The brown dwarf LP 944-20 (Digital Sky Survey).
Credit: European Southern Observatory
"Brown dwarfs won't account for all of the so-called dark matter," he said. "There
is mass in the form of ordinary matter that is unaccounted for because we don't
yet have the technology to find it. There are brown dwarfs, and maybe small black
holes, and faint white dwarfs regular stars that lost their outer gaseous envelopes
leaving the burned-out core of old stars. White dwarfs, brown dwarfs, black holes
and gas account for some of the dark matter. The rest is presumably a new form
McLean and his colleagues Kirkpatrick; Adam Burgasser, a UCLA postdoctoral scholar
in McLean's group and recipient of a NASA-funded Hubble fellowship; UCLA graduate
student Mark McGovern and postdoctoral scholar Lisa Prato, who both work in McLean's
group; and former UCLA postdoctoral scholar Sungsoo Kim will publish an atlas
and analysis of the brown dwarf infrared spectra in the Oct. 10 Astrophysical
Journal. Kirkpatrick and Burgasser were responsible for most of the initial brown
dwarf identifications using an infrared all-sky survey called 2MASS. McLean praised
the members of his research team as "instrumental to the success" of
"After four years of data gathering from NIRSPEC, we have obtained and studied
spectra from more than 50 brown dwarfs, and analyzed the variations," McLean
said. "Astronomers in the future will be able to obtain the infrared spectrum
of a newly discovered brown dwarf and compare the spectrum with those we have
published and instantly identify what kind of brown dwarf they have found. Probing
more distant regions of the galaxy to study the youngest, recently-formed brown
dwarfs is the next step."
His research, including NIRSPEC, was funded by the California Association for
Research in Astronomy, the entity that operates the W.M. Keck Observatory.
To do his research, McLean designs and builds spectrometers and analyzes astrophysics
"The astronomy motivates me, but I have to be a bit of an engineer and experimental
physicist as well," he said, "because technology is the key to new
Since the discovery of brown dwarfs less than a decade ago, astronomers have
come to think that they might be more numerous than the visible stars in the
sky. In broad terms, 80 percent of the nearby stars are red
dwarfs, 10 percent are solar-type stars, and 10 percent are more massive.
There are probably around the same number of brown dwarfs as stars within the
immediate solar neighborhood.
The true abundance of brown dwarfs, sub-brown dwarfs and extrasolar planets is
not known, and large areas of the sky still need to be explored. Most of the
brown dwarfs have been located by the Two Micron All Sky Survey (2MASS), although
the Sloan Digital Sky Survey (SDSS) and the planet-finding Doppler
technique also have been used to find brown dwarfs. Reid is completing a
census of low-mass stars and brown dwarfs in the immediate solar neighborhood.
From Astrobiology Magazine
Based on a UCLA report