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.”

IMAGE
An image of Sigma Orionis cluster, showing it’s low-mass/brown dwarf members
is available at:

http://www.astro.keele.ac.uk/~mk/NAM

CONTACTS
Mr. Mike Kenyon, Keele University)
Phone: (+44) (0)1782 583530
E-mail: mk@astro.keele.ac.uk

Dr. Robin Jeffries, Keele University
(+44) (0)1782 583892
rdj@astro.keele.ac.uk

Both will be attending the NAM.

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.