An
international team of astronomers
has discovered the coldest brown dwarf star ever observed. This
finding is a new step
toward filling the gap between stars and planets.
An international team [1] led by French and Canadian astronomers has
just discovered the coldest brown dwarf ever observed. Their results
will soon be published in Astronomy & Astrophysics. This new
finding was made possible by the performance of worldwide telescopes
[2]: Canada France Hawaii Telescope (CFHT) and Gemini North Telescope,
both located in Hawaii, and the ESO/NTT located in Chile.
The brown dwarf is named CFBDS J005910.83-011401.3 (it will be called
CFBDS0059 in the following). Its temperature is about 350°C and its
mass about 15-30 times the mass of Jupiter, the largest planet of our
solar system [3]. Located about 40 light-years from our solar system,
it is an isolated object, meaning that it doesn't orbit another star.
Brown dwarfs are intermediate bodies between stars and giant planets
(like Jupiter). The mass of brown dwarfs is usually less than 70
Jupiter masses. Because of their low mass, their central temperature is
not high enough to maintain thermonuclear fusion reactions over a long
time. In contrast to a star like our Sun, which spends most of its
lifetime burning hydrogen hence
keeping a constant internal
temperature, a brown dwarf spends its lifetime getting colder
and
colder after having been formed.
The first brown dwarfs were detected in 1995. Since then, this type of
stellar object has been found to share common properties with giant
planets, while differences remain. For example, clouds of dust and
aerosols, as well as large amounts of methane, were detected in their
atmosphere (for the coldest ones), just as in the atmosphere of Jupiter
and Saturn. However, there were still two major differences. In the
brown dwarf atmospheres, water is always in gaseous state, while it
condenses into water ice in giant planets; and ammonia has never been
detected in the brown dwarf near-infrared spectra, while it is a major
component from Jupiter's atmosphere. CFBDS0059, the newly-discovered
brown dwarf looks much more like a giant planet than the known classes
of brown dwarfs, both because of its low temperature and because of the
presence of ammonia.
To date, two classes of brown dwarfs have been known: the L dwarfs
(temperature of 1200-2000°C), which have clouds of dust and
aerosols in
their high atmosphere, and the T dwarfs (temperature lower than
1200°C), which have a very different spectrum because of methane
forming in their atmosphere. Because it contains ammonia and has a much
lower temperature than do L and T dwarfs, CFBDS0059 might be the
protoype of a new class of brown dwarfs to be called the Y dwarfs. This
new class would become the coldest stellar objects, hence the missing
link toward giant planets. Astronomers could then fill in the domain
from the hottest stars to the giant planets of less than -100°C.
This discovery also has important implications in the study of
extrasolar planets. The atmosphere of brown dwarfs looks very much like
that of giant planets, therefore the same models are used to reproduce
their physical conditions. Such modeling requires to be constrained
with observations. Observing the atmospheres of extrasolar planets is
indeed very hard because the light from the planets is embedded in the
much stronger light from their parent star. Because brown dwarfs are
isolated bodies, they are much easier to observe. Thus, looking to
brown dwarfs with a temperature close to that of the giant planets will
help in constraining the models of extrasolar planets' atmospheres.
[1] The team of astronomers includes P. Delorme, X. Delfosse
(Observatoire de Grenoble, France), L. Albert (CFHT, Hawaii), E.
Artigau (Gemini Observatory, Chile), T. Forveille (Obs.
Grenoble/France, IfA/ Hawaii), C. Reylé (Observatoire de
Besançon,
France), F. Allard, A. C. Robin (CRAL, Lyon, France), D. Homeier
(Göttingen, Germany), C.J. Willott (University of Ottawa, Canada),
M.
C. Liu, T. J. Dupuy (IfA, Hawaii).
[2] CFBDS0059 was discovered in the framework of the Canada-France
Brown-Dwarfs survey. The object was first identified in pictures from
the wide-field camera MegaCam installed on the CFHT (Canada France
Hawaii Telescope). Infrared pictures were then obtained with the NTT
telescope (La Silla, ESO, Chile) and confirmed the low temperature of
the object. Finally, the spectrum showing the presence of ammonia was
obtained using the Gemini North telescope (Hawaii).
[3] The mass of Jupiter is about 300 times the Earth's mass and about
1/1000e of the Sun's mass. CFBDS J005910.90-011401.3: reaching the T-Y
brown dwarf transition?, by P. Delorme, X. Delfosse, L. Albert, E.
Artigau, T. Forveille, C. Reylé, F. Allard, D. Homeier, A. C.
Robin,
C.J. Willott, M. C. Liu, and T. J. Dupuy. To be published in Astronomy
& Astrophysics, 2008.
