Astronomers are honing the skills they need to take a
census of the most elusive stars in the Galaxy. A team
from Keele University and Exeter University applied a
relatively simple test to pick out objects they thought
might be faint red dwarfs and their even dimmer cousins,
brown dwarfs, belonging to a young star cluster in
Orion. Then, after much more detailed scrutiny, they
found they had achieved a 90% hit rate. Their bounty
consisted of 67 very low mass objects in the cluster,
about half of which are brown dwarfs. Studies like
this are very important for refining theories on how
stars form and estimating how much of the material
in a galaxy is in stars that are impossible to see
individually very far beyond our own stellar
neighbourhood. Mike Kenyon, a doctoral student from
Keele, will present the team’s results to date at the
UK/Ireland National Astronomy Meeting in Dublin on 9
April.

Stars are born in clusters and the majority of stars
are less massive than the Sun: that much is already
understood. But when a cluster forms, what is the
breakdown in numbers among stars of different masses?
The answer to that critical question will help
astronomers understand the stellar population of the
Milky Way and other galaxies. However, taking the
census of the least significant members of a stellar
family is still a difficult task to complete. Low
mass stars and brown dwarfs shine so weakly,
detecting them at all is a real challenge.

With the aim of tackling this problem, Mike Kenyon,
Rob Jeffries, and Joana Oliveira of Keele University
and Tim Naylor of the University of Exeter observed
a large sample of objects suspected of being
low-mass members of an association of young stars
surrounding the massive hot star, Sigma Orionis.
From these, they chose a selection for further
examination on the basis that their luminosities
and colours hinted that they were likely cluster
members. Using the William Herschel Telescope on
the island of La Palma, they took spectra of 75 of
these candidates.

From the spectra, the researchers could tell whether
each object was indeed a cluster member, or an
unrelated object that happens to lie in the direction
of Sigma Orionis. The outcome was confirmation that
at least 90% are cluster members. "We have discovered
67 low-mass members of Sigma Orionis, approximately
half of which are brown dwarfs," said Mike Kenyon.
"This validates our belief that surveys of colour
and luminosity do a reasonable job of detecting
cluster members. It gives us a way of counting
low-mass stars without having to take detailed
spectra, which is difficult and sometimes impossible
for the faintest stars."

However, spectra are valuable because they can reveal
intimate details of individual stars. "Three of the
low-mass objects we have studied so far show very
strong evidence that they are accreting matter from
a circumstellar disk," says Mike Kenyon, and several
may be binary systems. We have more work to do on
this but one brown dwarf is a strong contender for
being a binary system."

NOTE

The determination of cluster membership was performed
using several techniques. Firstly, radial velocities
were measured. The expectation is that all members
will exhibit similar velocities, so it is relatively
simple to discriminate the cluster population from
polluting objects. Secondly, the spectra were analysed
for the presence of lithium. This element acts as an
indicator of stellar age; low-mass stars will preserve
their initial abundance of lithium far longer than
higher mass objects. The predominant contaminating
objects in a survey of Sigma Orionis are believed
to be foreground dwarf type stars. However, such
interlopers would have expended their reserves of
lithium via nuclear reactions, and can be rejected
by the "lithium test". As final confirmation of an
object’s membership status, one further spectral
feature was measured; the yellow sodium doublet ("D
lines"). Indicative of the gravitational strength
possessed by a star, this line strengthens for stars
with higher gravity. This is useful in the detection
of low-mass stars and brown dwarfs, which have
particularly strong gravitational fields.