Clustering of Emission-Line Stars in the W5E H II Region

We have made a new survey of emission-line stars in the W5E HII region to investigate the population of PMS stars near the OB stars by using the Wide Field Grism Spectrograph 2 (WFGS2). A total of 139 Hα emission stars were detected and their g' i'-photometry was performed. Their spatial distribution shows three aggregates, i.e., two aggregates near the bright-rimmed clouds at the edge of the W5E HII region (BRC 13 and BRC 14) and one near the exciting O7V star. The age and mass of each Hα star were estimated from an extinction-corrected color-magnitude diagram and theoretical evolutionary tracks. We found, for the first time in this region, that the young stars near the exciting star are systematically older (4Myr) than those near the edge of the HII region (1Myr). This result supports that the formation of stars proceeds sequentially from the center of HII region to the eastern bright rim. We further suggest a possibility that the birth of low-mass stars near the exciting star of the HII region precedes the production of massive OB stars in the pre-existing molecular cloud.