Astronomers
using adaptive optics on the Gemini North and Keck telescopes have taken
an image of a brown dwarf orbiting a nearby star similar to the Sun. The
faint companion is separated from its parent star by less than the distance
between the Sun and the planet Uranus and is the smallest separation brown
dwarf companion seen with direct imaging.
The
research team estimates the mass of the brown dwarf at 55 to 78 times the
mass of planet Jupiter. The discovery
raises puzzling questions about how the brown dwarf formed, and it adds
to the surprising diversity of extrasolar planetary systems being found
with cutting-edge observational techniques.
“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, the Beatrice Parrent
fellow at the University of Hawaii’s Institute for Astronomy.
Liu
is lead author of a paper presented today at a press conference in Washington,
DC, at the 199th meeting of the American Astronomical Society.
Found
with adaptive optics technology at the Gemini North and Keck Telescopes
on Mauna Kea, Hawaii, the brown dwarf is located in the constellation Sagitta
(The Arrow) around a star commonly known as 15 Sge. The
star, a G-star formally identified as HR 7672, is one to three billion
years old, making it slightly younger than the Sun. It
is located approximately 58 light-years from Earth.
“This
companion is probably too massive to have formed the way we believe that
planets do, namely from a circumstellar disk of gas and dust when the star
was young,” Liu added. “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.”
Co-authors
on the paper presented today are Debra Fischer, James Graham, James Lloyd
and Geoff Marcy of the University of California at Berkeley, and Paul Butler
of the Carnegie Institution’s Department of Terrestrial Magnetism in Washington,
DC.
The
brown dwarf lies at a separation of 14 Astronomical Units (AU), 14 times
the distance between Earth and the Sun (1 AU = 150 million kilometers or
93 million miles). This makes it
the closest substellar object yet seen by direct imaging around a main
sequence (stable, hydrogen-burning) star. For
comparison, Saturn orbits the Sun at 10 AU, with Uranus the next planet
outward at 19 AU. Astronomers
believe that brown dwarfs are intermediate objects between planets and
stars.
Often described as ‘failed
stars’, they are more massive than Jupiter, the largest planet in our Solar
System. However, they fall short
of the minimum mass need to sustain nuclear fusion, estimated at 8 percent
of the Sun’s mass. After a modest
initial outburst of higher temperatures at birth, brown dwarfs cool off
and steadily grow fainter.
While
many planets have been found around other stars by radial velocity studies
(which search for the very weak wobbling of stars due to an unseen planet),
the same studies find almost no brown dwarfs, a phenomenon known as the
“brown dwarf desert.” However, such
work only probes the inner four AU around other stars.
Very
little is known about region outside of four AU, the domain of giant planets
in our own solar system.
Hints
of an interesting object around 15 Sge first arose in data gathered ten
years ago from Lick Observatory on Mount Hamilton, CA.
Marcy,
Butler, and Fischer obtained high-precision radial velocity measurements
of this star as part of their effort to find planets. While
they did not find any planets, they did notice clues of a more massive,
distant companion.
In
the summer of 2001, Liu took high-resolution pictures of 15 Sge using the
University of Hawaii’s QUIRC camera and Hoku’pa’a adaptive optics (AO)
system on the 8.1-meter Gemini North telescope. Second
and third epoch imaging were obtained using the Keck AO system in August
and December 2001.
Ground-based
astronomical images normally are blurred due to the turbulence of the Earth’s
atmosphere. AO is an exciting technology
that compensates for this effect in real time, correcting the blurring
and making some images sharper than even those produced by the Hubble Space
Telescope.
Liu
noticed a very faint object next to 15 Sge, akin to distinguishing a firefly
next to a bright searchlight. However,
the object could have been a distant star in the background, merely appearing
to be close to 15 Sge when projected on the sky.
Over
the course of six months, Liu and his collaborators monitored the star
with Keck AO and NIRSPEC on the 10-meter Keck II telescope and found that
the faint object moved on the sky along with the primary star, proving
the two objects were physically associated.
A
spectrum of the companion indicated a very cool temperature, characteristic
of brown dwarfs.
“Only
by using adaptive optics to produce very sharp images could we have found
this companion,” Liu explains. “It is too faint and too close to its parent
star to be seen otherwise.”
Liu
and his collaborators are continuing to search for such objects. “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,” he says.
Full resolution images of the 15 Sge system and
a conceptualization from artist Jon Lomberg are available on the web at:
http://www.ifa.hawaii.edu/~mliu/aas2002
http://www.gemini.edu/media/BDImages.html
 Gemini-Northadaptive optics image of 15 Sge and its newly found companion (15 Sge B). The data were obtained in the near-infrared, at a wavelength of 2.2 microns. The image has been computer processed to subtract the light from the much brighter primary star in the vicinity of companion. |
Keck |
