Photometric Identification of the Low-Mass Population of Orion OB1b. I. The σ Orionis Cluster

We report an optical photometric survey of 0.89 deg2 of the Orion OB1b association centered on σ Ori. This region includes most of the σ Ori cluster, the highest-density region within Orion OB1b. We have developed a statistical procedure to identify the young, low-mass, pre–main-sequence population of the association. We estimate that the cluster has ~160 members in the mass range (0.2 ≤ M ≤ 1.0 M⊙). The cluster has a radius of ~3–5 pc and an estimated age of 2.5 ± 0.3 Myr. We estimate that the total mass of the cluster is 225 ± 30 M⊙. This mass is similar to the estimated mass of the ~5 × 105 yr old cluster NGC 2024. NGC 2024 and σ Ori appear to be a well-matched pair of clusters, except for the ~2 Myr difference in their ages.