Hα emission-line stars in molecular clouds.: I.: The NGC 2264 region

We present a deep survey of Halpha emission-line stars in the NGC 2264 region using wide-field objective prism films. We find 357 Halpha emission-line stars, of which 113 were previously detected, within an area of 3degrees x 3degrees centered on the Cone Nebula, with a majority of stars being concentrated in a dense cluster at the center of the region. We present a large-scale CO map of NGC 2264 and find a strong correlation between the cluster of Halpha emission-line stars and the most massive core in the cloud complex. A more extended halo of stars poorly correlated with the gas may represent stars that have drifted away from their birth place, suggesting that star formation has taken place in NGC 2264 for several million years.