Through the analysis of
data provided by infrared sources of the DENIS (DEep Near-Infrared Survey)*,
an international team of astrophysicists** has recently
discovered a brown dwarf or a very low-mass star quite close to us, about
13 light-years away. The analysis of spectroscopic images provided by
the Keck I telescope located on the Mauna Kea in Hawaii shows that the
object is either a brown dwarf or a very low-mass star. An analysis of
older photographs (1975, 1986) where this star can be seen was performed
by the MAMA (Machine Automatique à Mesurer pour l’Astronomie
— Automatic Astronomic Measuring Machine)*** of
the Centre d’analyse des images (Center for Image Analysis).
This star is traveling very quickly, which is proof of its closeness to
us.

In the spring of 2000, two
astronomers from the Observatoire de Grenoble noticed a very red and unusually
shiny object on images provided by DENIS. This object could either be
a very low-mass star, located not too far away, or a very distant giant
star. Working in collaboration with a Spanish astrophysicist now at the
University of Hawaii, they obtained observation time on the Keck I telescope
on Mauna Kea, Hawai; they were thus able to perform a spectrum of the
star and determine its nature. The high-resolution spectrum, performed
in the night of May 30, 2000, clearly showed that the object is not a
giant red star but a very low-mass star or a brown dwarf located near
to us. In addition, the absence of lithium in the star’s surface or atmosphere
means that its mass is somewhere between 60 and 90 times that of Jupiter.
The lack of lithium shows the mass is more than 60 times that of Jupiter,
but this does not rule out the possibility that it is a brown dwarf (the
difference between brown dwarves and stars is 75 times Jupiter’s mass).

A star that is near moves much faster than distant stars. Thanks to MAMA,
astronomers from the Paris Observatory analyzed old snapshots of the sky
taken in 1975 and 1986. The star can be seen on the photographs and its
movement over the past two decades shows it is very large. Astrophysicists
have estimated the star’s distance from us at about 13 light-years. This
distance is not yet established for certain, since it is based on a comparison
between its luminosity with that of stars of the same spectral type. Measurements
show that it is probably situated between the 12th and 40th closest stars.

How is it that a star so close to us has remained unnoticed to date? Even
though the star is very near, its luminosity is nonetheless rather faint
(for instance, it is 10,000 time too faint to be seen by the naked eye).
The faint luminosity is due to the fact that the star is much colder and
smaller than the Sun. This is why these objects managed so far to escape
the attention of astronomers, especially in the southern hemisphere which
has been much less systematically explored than the northern hemisphere.
Thanks to the DENIS survey, however, they now can easily be identified
and it will very soon be possible to evaluate the number of very low-mass
stars and brown dwarves in the vicinity of the Sun.

Upon recommendation of the International Astronomical Union, this star
was named DENIS-P J104814.7-395606.1. It will now be easier for astronomers
to study a very low-mass object that is quite near to us.

Article soon to be published in Astronomy and Astrophysics Letters

*DENIS is a systematic mapping of the deep near infrared
southern sky (bands I, J, K). Three cameras were installed on a 1-meter
telescope of the European Southern Observatory (ESO) in La Silla, Chile.
The digital data are stored and made available to the community. It was
financed by the Institut National de Sciences de l’Univers of the
CNRS, the French Ministry of Research, the ESO, the Land of Baden-Württemberg
and the EEC. The main institutes participating in this sky mapping program
are: the Observatoire de Paris, the Leiden Observatory, the Institut d’Astrophysique
de Paris, the University of Innsbruck, and the ESO.

** This international team includes :
Xavier Delfosse (working since September 1, 2000 at the Observatoire de
Grenoble [joint CNRS research unit], who at the time of the observation
was working on a post-doc at the Astrophysics Institute of the Canary
Islands, Spain) ; Thierry Forveille (Observatoire de Grenoble [joint CNRS
research unit], seconded to the Canada-France-Hawaii telescope) ; Eduardo
Martin (University of Hawaii, USA) ; Jean Guibert (Département
d’astrophysique stellaire et galactique [joint CNRS research unit]
at the Observatoire de Paris, Centre d’analyse des Images – INSU)
; Jean Borsenberger (Institut d’Astrophysique de Paris – CNRS and
Centre d’analyse des Images – INSU), Françoise Crifo (Département
d’astrophysique stellaire et galactique [joint CNRS research unit]
of the Observatoire de Paris), Christophe Alard (Département de
recherche spatiale [joint CNRS research unit] of the Observatoire de Paris),
Nicolas Epchtein (director of the Denis project, Observatoire de la Côte
d’Azur [joint CNRS research unit]), Pascal Fouque (Département
de recherche spatiale [joint CNRS research unit] of the Observatoire de
Paris and ESO), Guy Simon (Département d’astrophysique stellaire
et galactique [joint CNRS research unit] of the Observatoire de Paris),
Françoise Tajahmady (Département d’astrophysique stellaire
et galactique [joint CNRS research unit] of the Observatoire de Paris,
Centre d’analyse des Images – INSU).

***The MAMA, of the Centre d’analyse des images,
is an instrument making it possible to digitalize astronomic photographs.
It is one of most rapid and precise microdensitometers in the world. The
digital images of the sky can thus be stored and then used by French and
foreign researchers. This instrument, financed and built by the Institut
National des Sciences de l’Univers of the CNRS, is located at the
Observatoire de Paris.

Researcher
Contact:
Xavier Delfosse
Observatoire de Grenoble
Tel. : +33 4 76 63 55 10
E-mail: delfosse@obs.ujf-grenoble.fr

Press contact :
Philippe Chauvin
Institut National des Sciences de l’Univers (INSU/CNRS)
Tel. : +33 4 44 96 43 36
E-mail : Philippe.Chauvin@obspm.fr