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Brown Dwarf Seen
Around Sun-like Star
Astronomers have
spotted a brown dwarf in images of a nearby sun-like
star.
by Vanessa Thomas
A brown dwarf
about 65 times the mass of Jupiter orbits a
sun-like star 57.7 light-years from
Earth.
Gemini
Observatory
| | Less than 58 light-years from
Earth, a brown
dwarf orbits a star
much like our own sun. A team of astronomers
presented images of this newfound substellar
object at the American Astronomical Society's
annual meeting in Washington, D.C., today.
While astronomers have imaged nearby brown
dwarfs before, none of the photogenic "failed
stars" orbit as
close to a main-sequence star as this one. (In
fact, some brown dwarfs don't orbit stars at all.)
If the newly imaged brown dwarf was part of our solar
system, it would orbit between Saturn and
Uranus. It orbits approximately 14 astronomical
units from a star called HR 7672, located just
outside of the Summer
Triangle in the small constellation
Sagitta the Arrow. Commonly known as 15 Sge, the
parent star is a G-type star like our sun. Between
one and three billion
years old, 15 Sge is a bit younger than our star.
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The star 15
Sge appears in the sky near Altair in the
constellation Sagitta.
Gemini
Observatory
| | "This discovery implies that
brown dwarf companions to average, sun-like stars
exist at a separation comparable to the distance
between the sun and the outer planets in our solar
system," said Michael Liu of the University of
Hawaii's Institute for Astronomy.
Liu and colleagues from the University of
California at Berkeley and the Carnegie
Institution estimate that the brown dwarf circling
15 Sge has between 55 and 78 times the mass of
Jupiter.
"This companion is probably too
massive to have formed the way we believe planets
do," Liu said. "This finding suggests that a
diversity of processes act to populate the outer
regions of other solar systems. The parent star is
very similar to our sun, yet it has a brown dwarf
companion whose mass is dozens of times the
combined mass of all the planets in our solar
system."
The first clues suggesting 15 Sge
might have a substellar companion came from Lick
Observatory data collected about ten years ago,
when Paul Butler of the Carnegie Institution and
Berkeley astronomers Geoff Marcy and Debra Fischer
obtained radial-velocity measurements of 15 Sge.
The method, used to detect extrasolar
planets, suggested that there was an unseen
companion too large to be considered a planet.
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The faint
brown dwarf is seen at about the 7 o'clock
position below the star 15
Sge.
Gemini Observatory / University
of Hawaii Institute for Astronomy / Michael Liu
/
NSF
| | When Liu imaged 15 Sge with the
8.1-meter Gemini North telescope
and its adaptive
optics system in the summer of 2001, he
spotted a faint object near the star. To make sure
the unidentified object wasn't just a background
star, Liu used the 10-meter Keck II and its
adaptive optics
system to monitor 15 Sge over the next six months.
The fainter object moved with 15 Sge, proving that
it orbited the star. The companion's spectrum
confirmed that it had a very cool temperature,
typical of brown dwarfs.
"Only by using
adaptive optics to produce very sharp images could
we have found this companion," Liu states. "It is
too faint and too close to its parent star to be
seen otherwise."
Liu's team is now trying
to find substellar objects around other stars.
"Now that we know brown dwarfs exist in the region
of giant planet formation, we would like to
understand how often these oddball pairings occur
in the universe,
and what that can tell us about the alternate and
divergent ways in which solar systems form around
sun-like stars," Liu says.
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01/07/2002 | |
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