The Young Cluster and Star Forming Region NGC 2264

Abstract

NGC 2264 is a young Galactic cluster and the dominant component of the Mon OB1 association lying approximately 760 pc distant within the local spiral arm. The cluster is hierarchically structured, with subclusters of suspected members spread across several parsecs. Associated with the cluster is an extensive molecular cloud complex spanning more than two degrees on the sky. The combined mass of the remaining molecular cloud cores upon which the cluster is superposed is estimated to be at least ∼3.7×10 4 M ⊙ . Star formation is ongoing within the region as evidenced by the presence of numerous embedded clusters of protostars, molecular outflows, and Herbig-Haro objects. The stellar population of NGC 2264 is dominated by the O7 V multiple star, S Mon, and several dozen B-type zero-age main sequence stars. X-ray imaging surveys, Hα emission surveys, and photometric variability studies have identified more than 600 intermediate and low-mass members distributed throughout the molecular cloud complex, but concentrated within two densely populated areas between S Mon and the Cone Nebula. Estimates for the total stellar population of the cluster range as high as 1000 members and limited deep photometric surveys have identified ∼230 substellar mass candidates. The median age of NGC 2264 is estimated to be ∼3 Myr by fitting various pre-main sequence isochrones to the low-mass stellar population, but an apparent age dispersion of at least ∼5 Myr can be inferred from the broadened sequence of suspected members. Infrared and millimeter observations of the cluster have identified two prominent sites of star formation activity centered near NGC 2264 IRS1, a deeply embedded early-type (B2-B5) star, and IRS2, a star forming core and associated protostellar cluster. NGC 2264 and its associated molecular clouds have been extensively examined at all wavelengths, from the centimeter regime to X-rays. Given its relative proximity, well-defined stellar population, and low foreground extinction, the cluster will remain a prime candidate for star formation studies throughout the foreseeable future.