Distribution of Young Stellar Populations in the Orion Region

Based on the survey of emission-line stars in a wide field of the Orion star forming region, surface distributions of young stellar populations and gaseous clouds are compared as a whole in an attempt to make a speculation on the star formation process in this region. Existence of a primeval molecular cloud is suggested as the birth site of older members of emission-line stars and probably of X-ray detected pre-main sequence stars. This revised version was published online in July 2006 with corrections to the Cover Date.