UBVRI H(alpha) Photometry of the Young Open Cluster NGC 2264

UBVRI Hα CCD photometry has been obtained for the young open cluster NGC 2264. Using (R -Hα) color, a measure of Hα emission, we selected member stars in the pre-main-sequence (PMS) stage. The reliability of the (R -Hα) color as a PMS star selection criterion was critically evaluated from their spatial distribution, near-IR photometric data and spectroscopic characteristics. By constructing the HR diagram, we determined the typical age and age spread of NGC 2264. Some discussions on the age discrepancy between turn-off age and turn-on age in a cluster were made. The shape of the initial mass function (IMF) of NGC 2264 is very similar to that of the Orion Nebula Cluster, but the surface density is far lower than that of NGC 6231 or the Orion Nebula Cluster. By assuming that the thickness (or amount of mass) of the circumstellar shell is linearly proportional to the strength of the Hα emission Δ (R - Hα) (≡ (R -Hα)<SUB>*</SUB> - ( R - Hα)<SUB>MS</SUB>), it can be concluded that the amount of material in the circumstellar shell is not the main source of spread in magnitude due to the lack of correlation between Δ(R - Hα) and ΔM<SUB>v</SUB> (≡M<SUB>v</SUB><SUP>*</SUP>-M<SUP>t</SUP><SUB>V</SUB>=Myr<SUP>2</SUP>).

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