UBVRI H-ALPHA PHOTOMETRY OF THE YOUNG OPEN CLUSTER NGC-2264

UBVRI H alpha CCD photometry has been obtained for the young open clus ter NGC 2264. Using (R-H alpha) color, a measure of H alpha emission, we selected member stars in the pre-main-sequence (PMS) stage. The rel iability of the (R-H alpha) color as a PMS star selection criterion wa s critically evaluated from their spatial distribution, near-IR photom etric data and spectroscopic characteristics. By constructing the HR d iagram, we determined the typical age and age spread of NGC 2264. Some discussions on the age discrepancy between turn-off age and turn-on a ge 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 N ebula Cluster. By assuming that the thickness (or amount of mass) of t he circumstellar shell is linearly proportional to the strength of the H alpha emission Delta(R-H alpha) (equivalent to(R-H alpha)()-(R-H a lpha)(MS)), it can be concluded that the amount of material in the cir cumstellar shell is not the main source of spread in magintude due to the lack of correlation between Delta(R-H alpha) and Delta M-V(equival ent to M-V()-M-V(T=2 Myr)). (C) 1997 American Astronomical Society.