The impressive rho Ophiuchi cloud is one of the heavenly meeting points
for astronomers in search of young stars. Located 540 light-years away
in the constellation of Ophiucus, in the celestial equator, this dusty
region is the nest of more than one hundred newborn stars. But ESA’s
Infrared Space Observatory, ISO, has also found a surprise hidden in the
dust: 30 brown dwarfs, elusive and ambiguous objects considered to be
‘failed stars’ because they have too little mass to shine as stars.
Relatively few of these brown dwarfs have been identified so far, so
finding one is like winning a trophy. With this discovery ISO has turned
the rho Ophiuchi region into a favourite game reserve for brown-dwarf

“ISO gives us a new, really rich sample of young brown dwarfs in the rho
Ophiuchi region. We will clearly have to go back and search for more of
these sub-stellar objects with current and future infrared telescopes,
both in space and from the ground with the 10-metre class telescopes,”
says Sylvain Bontemps (Observatoire de Bordeaux, Floirac, France), a
member of an international team led by Lennart Nordh (Stockholm
Observatory, Sweden) that observed the rho Ophiuchi cloud with ISO.

Brown dwarfs are elusive because they are very faint, and ambiguous
because their true nature is still unclear. Some astronomers say that at
least some of them, the less massive ones, could be better described as
giant planets, like Jupiter, instead of as failed stars. The minimum mass
for a star to shine as such is 8 per cent of the mass of the Sun, or 80
times the mass of Jupiter — below that limit, the ‘nuclear oven’ that
provides the star’s energy cannot be ignited at the star’s core.

In the case of the brown dwarfs found in the rho Ophiuchi region, “the
less massive are about 5 per cent of the mass of the Sun, or 50 Jupiter
masses. But certainly there could still be less massive objects hidden
in the dust,” Bontemps says.

This brown dwarf population has the added value of its youth. They are
typically a million years old, and as a consequence they are still
relatively bright. This makes them easier to study than other older
brown dwarfs, whose light is weakened due to their very cold atmosphere.

ISO performed similar surveys in other nearby regions of star formation,
such as Chamaeleon I and Serpens, which have also revealed the presence
of young brown dwarf populations. All these results contribute to
solving the question of what the true nature of brown dwarfs is.

This note is based on the results published in the scientific paper
ISOCAM observations of the rho Ophiuchi cloud: Luminosity and mass
functions of the pre-main sequence embedded cluster by S. Bontemps et
al., published in Astronomy & Astrophysics 372, 173, 2001.

About this image

Three of the most massive young stars in this stellar nursery are easy
to find in this image: one in the centre of the right-hand-side border;
a second one in the middle of a comet-shaped nebula in the lower-right
of the image; and finally, one in the middle of the small nebula close
to the centre-right. Other point-like sources are also young stars and
‘protostars’ — stars that are still ‘growing’ by ‘sucking in’ gas from
the cloud.

In the dust surrounding the newborn stars there are plenty of small
carbonaceous grains. The exact nature of these grains is still a matter
of debate. The young stars heat these grains and make them radiate
infrared light (seen in the image as extended halos).

This image was taken by Alain Abergel (Institut d’Astrophysique Spatiale,
Paris) with the infrared camera, ISOCAM, on board ISO.

The colour image was constructed from a 7.7 micron infrared exposure
(shown as blue), and a 14.5 micron infrared exposure (shown as red). The
green colour is a combination of the blue and red exposures.

Credit: ESA ISO/ ISOCAM/ Alain Abergel (Abergel et al. (1996) A&A 315,

About ISO

The European Space Agency’s infrared space telescope, ISO, operated from
November 1995 till May 1998. As an unprecedented observatory for infrared
astronomy ISO made nearly 30,000 scientific observations.


* More about ISO

* The complete text and images for this photo release

* Plain text version of this photo release

* The scientific paper on the rho Ophiuchi observations (Bontemps et al.)
[pdf file]

A game reserve for brown-dwarf hunters — ISO finds 30 ‘failed stars’ in
nearby stellar nursery.


Sylvain Bontemps

Observatoire de Bordeaux (Floirac, France)

Tel: +33 557 776164

Leo Metcalfe ISO project scientist

European Space Agency, Vilspa, Spain

Tel: +34 91 8131372

ESA Science Communication Service

Tel: +31 71 5653223