The Color-Magnitude Diagram of NGC 2264

TL;DR

This study reexamines and homogenizes photometric data for NGC 2264, deriving its fundamental parameters and revealing new insights into its structure, age, and membership, including the discovery of its double cluster nature.

Contribution

It provides a revised, homogeneous photometric dataset for NGC 2264 and new estimates of its reddening, distance, age, and cluster structure, including the identification of its double cluster status.

Findings

Reddening E(B-V)=0.075±0.003

Distance modulus Vo-Mv=9.45±0.03 (d=777±12 pc)

Cluster age approximately 5.5 million years

Abstract

Existing photometry for NGC 2264 tied to the Johnson and Morgan (1953) UBV system is reexamined and, in the case of the original observations by Walker (1956), reanalyzed in order to generate a homogeneous data set for cluster stars. Color terms and a Balmer discontinuity effect in Walker's observations were detected and corrected, and the homogenized data were used in a new assessment of the cluster reddening, distance, and age. Average values of E(B-V)=0.075+-0.003 s.e. and Vo-Mv=9.45+-0.03 s.e. (d=777+-12 pc) are obtained, in conjunction with an inferred cluster age of ~5.5x10^6 yr from pre-main-sequence members and the location of the evolved, luminous, O7 V((f)) dwarf S Mon relative to the ZAMS. The cluster main sequence also contains gaps that may have a dynamical origin. The dust responsible for the initial reddening towards NGC 2264 is no more than 465 pc distant, and there are numerous, reddened and unreddened, late-type stars along the line of sight that are difficult to separate from cluster members by standard techniques, except for a small subset of stars on the far side of the cluster embedded in its gas and dust and background B-type ZAMS members of Mon OB2. A compilation of likely NGC 2264 members is presented. Only 3 of the 4 stars recently examined by asteroseismology appear to be likely cluster members. NGC 2264 is also noted to be a double cluster, which has not been mentioned previously in the literature.