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.