Brown Dwarf Found around Nearby, Sun-Like Star
by Michael Liu
M. Liu, University of Hawaii
Adaptive optics at the Keck telescope was used to
produce this infrared image of the 15 Sge system. The arrow points to the brown
dwarf, 15 Sge B.
Stars derive their energy from
nuclear fusion in their cores, stably burning lighter elements into heavier
ones. In the process, energy is released and emitted as radiation, that is,
light. However, below a minimum mass, about 8% of the Sun's mass, the center
of an object does not become hot enough to ignite this process. Such very low-mass
objects are known as brown dwarfs. Without a stable internal energy source,
brown dwarfs contain only the energy stored from their formation, presumably
as collapsing masses of interstellar gas. This energy supply is steadily released
as heat, and in the process these "failed stars" grow ever cooler and fainter.
With masses from 15 to 80 times that of the planet Jupiter, brown dwarfs are
objects intermediate between stars and planets.
were confined to the realm of theoretical speculation until their discovery
in 1995. Now, many have been found as isolated objects, free-floating in interstellar
space. But very few are found orbiting other stars. 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 dwarf companions. However, such studies probe only the inner
regions around other stars, inside of 4 AU (1 AU = astronomical unit = distance
from the Earth to the Sun = 93 million miles). Little is known about the region
outside of 4 AU, the domain of giant planets in our own solar system.
my colleagues and I discovered a brown dwarf companion orbiting a nearby star
similar to the Sun. We used two of the largest telescopes on Mauna Kea to take
pictures of this very faint companion. Separated from its parent star by less
than the distance between the Sun and the planet Uranus, it is the smallest
separation brown dwarf companion seen with direct imaging. This work shows that
brown dwarf companions to typical, Sun-like stars exist at a separation comparable
to the distance between the Sun and the outer planets in our solar system. 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.
Art by Jon Lomberg, Gemini Observatory
The newly found brown dwarf dubbed "15 Sge B," is
located in the summer constellation Sagitta (The Arrow), around a star commonly
known as 15 Sge. This star is very similar to our Sun (both are G-type stars)
and is about 58 light-years from Earth.
found companion, dubbed "15 Sge B," is located in the summer constellation Sagitta
(The Arrow), around a star commonly known as 15 Sge. This star is very similar
to our Sun (both are G-type stars) and is about 58 light-years from Earth. The
projected physical separation of the brown dwarf from its parent star is only
14 AU. For comparison, Saturn orbits the Sun at 10 AU, with Uranus the next
planet outward at 19 AU. We estimate the companion's mass at 55 to 78 times
the mass of Jupiter. It 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 parent
star was young. This finding suggests that a diversity of processes acts 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.
in summer 2001, we took very high-resolution pictures of 15 Sge. We first used
the 8.1-meter Gemini North Telescope, equipped with a state-of-the-art adaptive
optics system designed and built by IfA astronomers. Astronomical images taken
from the ground are blurred due to atmospheric turbulence, limiting their resolution.
Adaptive optics technology corrects for this blurring in real time. This results
in a factor of 10 times improvement in resolution, producing images sharper
than even the Hubble Space Telescope. We spotted 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.
course of six months, we continued to monitor the star with one of the Keck
10-meter telescopes 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. It is too faint and too close to its parent star
to be seen otherwise.
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, and what that can tell us about the alternate
and divergent ways in which solar systems form around Sun-like stars.
of the 15 Sge system are available on the Web at http://www.ifa.hawaii.edu/~mliu/aas2002.