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W. H. Sherry (SUNY Stony Brook)
I have completed a BVRI survey of 5 deg2 of the belt of Orion. I found that the pre-main sequence (PMS) population dominates a distinct locus of the V versus V-I color-magnitude diagram (CMD). I have statistically identified the low-mass PMS population by fitting the distribution of field and PMS stars on the V versus V-I CMD. This allowed me to count the number of PMS stars as a function of V magnitude and V-I color. While photometry alone can not select individual PMS stars with certainty, most of the stars which I identified as likely PMS stars have an 80% or greater likelihood of being PMS stars.
The PMS stars of the belt consist of 2 populations. The low-mass PMS stars around \epsilon and \sigma Orionis are consistent with membership in the Orion OB1b sub-association. The PMS population is best fit by a 3 Myr isochrone. The width of the PMS locus is inconsistent with an age spread greater than 2 Myrs. The low-mass stars near \delta Orionis are primarily from the 10 Myr old Orion OB1a sub-association.
The highest density of PMS stars is found in the \sigma Orionis cluster. In the region around \sigma Ori I identified ~350 PMS stars in the mass range 1.4>M>0.2M\odot. The spatial distribution of low-mass PMS stars around \sigma Orionis is consistent with a King model. The radius of the cluster is about 30' or ~4pc (d=450pc). I have used the initial mass function of Kroupa (2002) to estimate the total mass, number of stars, and the number of brown dwarfs in the cluster. The total mass is ~320M\odot. There are about 750 stars and 400 brown dwarfs (0.075>M>0.01M\odot) in the cluster. Given the total mass of the cluster, it is probably unbound.
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Bulletin of the American Astronomical Society, 34, #4
© 2002. The American Astronomical Soceity.