[SWW2004] J053931.594-024908.10 , the SIMBAD biblio

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 ∼5x10⁵ 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.