Emission-Line Stars Associated with the Nebulous Cluster NGC 2264.

Eighty-four faint stars having the Ha line in emission have been found in and around the nebulous cluster NGC 2264. The photographic magnitudes of these stars range from about 14.0 to 19.5, corresponding to Mpg = +4.5 to + 10; many, if not all, are variable. With the exception of one Ae-type star, those objects for which slit spectrograms have been obtained are late-type stars of the T Tauri group. Over the entire obscured area surveyed, the mean surface density of emission-Ha stars is 0.38 star/psc2 down to Mpg = +8.5, but within the most populous region the surface density rises to 1.6 stars/psc2. Volume densities may be obtained if a model of the dark cloud is assumed: the highly idealized model adopted yields emission-Ha star densities of 0.25 and 1.1 stars/psc3 for the entire obscured area and for the central region, respectively, down to Mpg = +8.5. These correspond to about 20 and 85 times, respectively, the star density in the solar neighborhood for the same interval of absolute magnitude. The survey is increasingly incomplete for fainter magnitudes, so that these densities are all minimum values. It was found that 75 per cent of the known variable stars in the region exhibited Ha emission during the rather short period of observation.