First Directly
Imaged Brown Dwarf Companion to an Exoplanet Host Star
(19 October 2006) Astronomers have
detected a new faint companion to the star HD 3651, already known to host a
planet.
This companion, a brown dwarf, is the faintest known
companion of an exoplanet host star imaged directly and one of the faintest T
dwarfs detected in the Solar neighbourhood so far. The detection yields
important information on the conditions under which planets form.
The SofI small field image of the planet host star HD 3651, taken in June 2006 in the H-band. The co-moving companion HD 3651B is indicated with a black arrow. (courtesy ESO)
"Such a system is an interesting example
that might prove that planet and brown dwarf can form around the same star",
said Markus Mugrauer, lead author of the paper presenting the
discovery.
HD 3651 is a star slightly less massive than the Sun, located
36 light-years away in the constellation Pisces (the "Fish"). For several
years, it has been known to harbour a planet less massive than Saturn, sitting
closer to its parent star than Mercury is from the Sun: the planet accomplishes
a full orbit in 62 days.
Mugrauer and his colleagues first spotted the
faint companion in 2003 on images from the 3.8-m United Kingdom Infrared
Telescope (UKIRT) in Hawaii. Observations in 2004 and 2006 using ESO's 3.6 m
New Technology Telescope (NTT) at La Silla provided the crucial confirmation
that the speck of light is not a spurious background star, but indeed a true
companion. The newly found companion, HD 3651B, is 16 times further away from
HD 3651 than Neptune is from the Sun.
HD 3651B is the dimmest directly
imaged companion of an exoplanet host star. Furthermore, as it is not detected
on the photographic plates of the Palomar All Sky Survey, the companion must be
even fainter in the visible spectral range than in the infrared, meaning it is
a very cool low-mass sub-stellar object. Comparing its characteristics with
theoretical models, the astronomers infer that the object has a mass between 20
and 60 Jupiter masses, and a temperature between 500 and 600 degrees Celsius.
It is thus ten times colder and 300 000 less luminous than the Sun. These
properties place it in the category of cool T-type brown dwarfs.
"Due to
their faintness even in the infrared, these cool T dwarfs are very difficult to
find", said Mugrauer. "Only two other brown dwarfs with similar brightness are
presently known. Their study will provide important insights into the
atmospheric properties of cool sub-stellar objects."
The separations (top) and position angles (bottom) of HD 3651B relative to HD 3651 over a three year period. The solid, red line indicates the expected variation of separation and position angle in case HD 3651B is a non-moving background star. (source: ESO)
More than 170 stars are currently known to
host exoplanets. In some cases, these stars were also found to have one or
several stellar companions, showing that planet formation can also take place
in a dynamically more complex environment than our own Solar System where
planet formation occurred around an isolated single star.
In 2001,
Mugrauer and his colleagues started an observational programme to find out
whether exoplanet host stars are single or married. In this programme, known
exoplanet host stars are systematically imaged at two different epochs, at
least several months apart. True companions can be distinguished from
coincidental background objects as only they move together with the stars over
time. With this effective search strategy several new companions of exoplanet
host stars have been detected. Most of the detected companions are low-mass
stars in the same evolutionary state as the Sun. In two cases, however, the
astronomers found the companions to be white dwarfs, that is, stars at the end
of their life. These intriguing systems bear evidence that planets can even
survive the troubled last moments in the life of a nearby star.
The
planet host star HD 3651 is thus surrounded by two sub-stellar objects. The
planet, HD 3651b, is very close, while the newly found brown dwarf companion
revolves around the star 1500 times farther away than the planet. This system
is the first imaged example that planets and brown dwarfs can form around the
same star.
These results were first presented in August at the IAU
General Assembly in Prague and are in press in the Monthly Notices of the Royal
Astronomical Society (Mugrauer et al., astro-ph/0608484). The discovery was
later confirmed by another team, using the Spitzer space telescope (Luhman et
al., astro-ph/0609464). The spectral classification was confirmed by additional
follow-up spectroscopy of the companion (Burgasser,
astro-ph/0609556).
The team comprises Markus Mugrauer and Ralph
Neuhauser (Astrophysical Institute and University observatory of the
Friedrich-Schiller University of Jena, Germany), Andreas Seifahrt (ESO), and
Tsevi Mazeh (Tel Aviv University, Israel).
(source: European Southern
Observatory)