The Starbase One Astronomy & Space Collection/STARBASE_ONE.ISO/a96/disk12/95_48.txt

Text File | 1996-01-19 | 8.6 KB | 172 lines

EMBARGOED UNTIL: 3:00 PM (EST) November 29, 1995

CONTACT:  Don Savage
          NASA Headquarters, Washington, DC
          (Phone: [phone])

          Jim Sahli
          Goddard Space Flight Center, Greenbelt, MD
          (Phone: [phone]

          Ray Villard
          Space Telescope Science Institute, Baltimore, MD
          (Phone: [phone])

PRESS RELEASE NO.: STScI-PR95-48


ASTRONOMERS ANNOUNCE FIRST CLEAR EVIDENCE
OF A BROWN DWARF

Astronomers have made the first unambiguous detection and image of an
elusive type of object known as a brown dwarf.

The evidence consists of an image from the 60-inch observatory on Mt.
Palomar, a spectrum from the 200-inch Hale telescope on Mt. Palomar and
a confirmatory image from NASA's Hubble Space Telescope.  The
collaborative effort involved astronomers at the California Institute
of Technology, Pasadena, CA, and the Johns Hopkins University,
Baltimore, MD.

The brown dwarf, called Gliese 229B (GL229B), is a small companion to
the cool red star Gliese 229, located 19 light-years from Earth in the
constellation Lepus.  Estimated to be 20 to 50 times the mass of
Jupiter, GL229B is too massive and hot to be classified as a planet as
we know it, but too small and cool to shine like a star.  At least
100,000 times dimmer than Earth's Sun, the brown dwarf is the faintest
object ever seen orbiting another star.

"This is the first time we have ever observed an object beyond our
solar system which possesses a spectrum that is astonishingly just like
that of a gas giant planet," said Shrinivas Kulkarni, a member of the
team from Caltech.

Kulkarni added, however, that "it looks like Jupiter, but that's what
you'd expect for a brown dwarf."  The infrared spectroscopic
observations of GL229B, made with the 200-inch Hale telescope at
Palomar, show that the dwarf has the spectral fingerprint of the planet
Jupiter -- an abundance of methane.  Methane is not seen in ordinary
stars, but it is present in Jupiter and other giant gaseous planets in
our solar system.

The Hubble data obtained and analyzed so far already show the object is
far dimmer, cooler (no more than 1,300 degrees Fahrenheit) and less
massive than previously reported brown dwarf candidates, which are all
near the theoretical limit (eight percent the mass of our Sun) where a
star has enough mass to sustain nuclear fusion.

Brown dwarfs are a mysterious class of long-sought object that forms
the same way stars do, that is, by condensing out of a cloud of
hydrogen gas.  However, they do not accumulate enough mass to generate
the high temperatures needed to sustain nuclear fusion at their core,
which is the mechanism that makes stars shine.  Instead brown dwarfs
shine the same way that gas giant planets like Jupiter radiate energy,
that is, through gravitational contraction.  In fact, the chemical
composition of GL229B's atmosphere looks remarkably like that of
Jupiter.

The discovery is an important first step in the search for planetary
systems beyond the Solar System because it will help astronomers
distinguish between massive Jupiter-like planets and brown dwarfs
orbiting other stars.  Advances in ground- and space-based astronomy
are allowing astronomers to further probe the "twilight zone" between
larger planets and small stars as they search for substellar objects,
and eventually, planetary systems.

Caltech astronomers Kulkarni, Tadashi Nakajima, Keith Matthews, and Ben
Oppenheimer, and Johns Hopkins scientists Sam Durrance and David
Golimowski first discovered the object in October 1994.  Follow-up
observations a year later were needed to confirm it is actually a
companion to Gliese 229.  The discovery was made with a 60-inch
reflecting telescope at Palomar Observatory in southern California,
using an image-sharpening device called the Adaptive Optics
Coronagraph, designed and built at the Johns Hopkins University.

The same scientists teamed up with Chris Burrows of the Space Telescope
Science Institute to use Hubble's Wide Field Planetary Camera-2 for
follow-up observations on November 17.  Another Hubble observation six
months from now will yield an exact distance to GL229B.

The astronomers suspect that the brown dwarf developed during the
normal star-formation process as one of two members of a binary
system.  "All our observations are consistent with brown dwarf theory,"
Durrance said.  However, the astronomers say they cannot yet fully rule
out the possibility that the object formed out of dust and gas in a
circumstellar disk as a "super-planet."

Astronomers say the difference between planets and brown dwarfs is
based on how they formed.  Planets in the Solar System are believed to
have formed out of a primeval disk of dust around the newborn Sun
because all the planets' orbits are nearly circular and lie almost in
the same plane.  Brown dwarfs, like full-fledged stars, would have
fragmented and gravitationally collapsed out of a large cloud of
hydrogen but were not massive enough to sustain fusion reactions at
their cores.

The orbit of GL229B could eventually provide clues to its origin.  If
the orbit is nearly circular then it may have formed out of a dust
disk, where viscous forces in the dense disk would keep objects at
about the same distance from their parent star.  If the dwarf formed as
a binary companion, its orbit probably would be far more elliptical, as
seen on most binary stars.  The initial Hubble observations will begin
providing valuable data for eventually calculating the brown dwarf's
orbit.  However, the orbital motion is so slow, it will take many
decades of telescopic observations before a true orbit can be
calculated.  GL229B is at least four billion miles from its companion
star, which is roughly the separation between the planet Pluto and our
Sun.

Astronomers have been trying to detect brown dwarfs for three decades.
Their lack of success is partly due to the fact that as brown dwarfs
age they become cooler, fainter, and more difficult to see.  An
important strategy used by the researchers to search for brown dwarfs
was to view stars no older than a billion years.  Caltech's Nakajima
reasoned that, although brown dwarfs of that age would be much fainter
than any known star, they would still be bright enough to be spotted.

"Another reason brown dwarfs were not detected years ago is that
imaging technology really wasn't up to the task," Golimowski said.
With the advent of sophisticated light sensors and adaptive optics,
astronomers now have the powerful tools they need to resolve smaller
and dimmer objects near stars.

Hubble was used to look for the presence of other companion objects as
bright as the brown dwarf which might be as close to the star as one
billion miles.  No additional objects were found, though it doesn't
rule out the possibility of Jupiter-sized or smaller planets around the
star, said the researchers.

The Palomar results will also appear in the November 30 issue of the
journal Nature and the December 1 issue of the journal Science.

                              * * * *

The Space Telescope Science Institute is operated by the Association of
Universities for Research in Astronomy, Inc. (AURA), for NASA, under
contract with the Goddard Space Flight Center, Greenbelt, MD.  The
Hubble Space Telescope is a project of international cooperation
between NASA and the European Space Agency (ESA).

Image files in GIF and JPEG format, captions, and press release text
may be accessed on Internet via anonymous ftp from ftp.stsci.edu in
/pubinfo:
                                    GIF            JPEG
PRC95-48  Brown Dwarf Gl229B  gif/Gl229B.gif    jpeg/Gl229B.jpg

Higher resolution digital versions (300dpi JPEG) of the release
photograph will be available temporarily in /pubinfo/hrtemp:  95-48.jpg
(color) and 95-48b.jpg (black & white).

GIF and JPEG images, captions and press release text are available via
World Wide Web at URL http://www.stsci.edu/pubinfo/PR/95/48.html, or
via links in http://www.stsci.edu/pubinfo/Latest.html, and in
http://www.stsci.edu/pubinfo/Pictures.html.

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