The young low-mass population of Orion's belt

I have conducted a BVRI survey covering ∼2.7 deg2 of the Orion OB1b sub-association, a ∼3 Myr old fossil star forming region located in the belt of Orion. I have developed a procedure to use single epoch photometry to estimate the number of PMS stars and field stars in the PMS locus as a function of color.; I detected ∼180 likely members of the σ Orionis cluster. The cluster has a radial profile which is consistent with a King model that has a King radius of r₀ ≤1 pc. The central density of the cluster is ∼30 stars pc−3 in the mass range 0.2 ≤ M ≤ 1.0 $M\odot$. The σ Orionis cluster is the richest region of Orion OB1b and the only region that where a clustered population has been detected.; The PMS locus for the σ Ori region is quite narrow for stars near 0.8 $M\odot$ but becomes wider at lower masses. At least part of this width must be due to a combination of the intrinsic variability of the PMS stars, binary stars, and observational errors. Part of this width may be caused by the stars in the cluster having formed over a period of several Myrs. It is impossible to interpret the width of the PMS locus in terms of an age-spread without knowing the expected width of a single age (isochronal) population. I constructed several simulated populations of PMS stars that were either coeval or had a range of ages. I compared my models with the observed width of the PMS locus and concluded that the age-spread among members of the σ Ori cluster is less than one million years.; I found a significant distributed population of low-mass PMS stars in the western belt with about 240 low-mass PMS stars per square degree. Roughly half the stars are members of the 11 Myr (D ∼ 330 pc) Orion OB1a sub-association which overlaps Orion OB1b. I found no evidence that these stars are arranged in clusters around δ Ori and &epsis; Ori.