| | |  | 30 'failed stars' discovered in the rho Ophiuchi cloud | |
A game reserve for brown-dwarf hunters - ISO finds 30 'failed stars' in nearby stellar nursery
25 October 2001
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 hunters.
"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.
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.
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