Star formation in the remnant clouds associated with OB Associations

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The main body of the molecular cloud has been studied in detail, however, there has been only a little information about the surrounding area including On OB1a, b associations. We have detected more than forty X-ray sources in the On OB1b region, where the activity of star formation is low, by the ASCA satellite. The X-ray luminosities of each source are 1030-1031 erg/s and the average temperature is 4 keV.Eight sources are identified as known low-mass pre-main sequence stars (CTTS), and most of them lie in the field where the Horsehead nebula is located. For other X-ray sources, we have made infrared photometry and showed that their position on the near-infrared two color diagram is consistent that most of them are WTTS.We also found X-ray emission from possible extragalactic sources in this region.We also made statistical analysis of the surface distribution of emission line stars and clustering properties. As the site of star formation outside of the main giant molecular clouds, we … More have studied "remnant clouds" in the Orion region as well. CO observations of the "remnant clouds" including reflection clouds, cometary globules, and bright-rimmed clouds revealed molecular line emission from 60% of the 24 target clouds. Many of them appear associated with the candidates of pre-main sequence stars. These results suggest that the star forming activity is rather low but still cotinueing.Furthermore, we have carried out X-ray observations of another GB association, Mon OB1. Mon OB1 is more distant than On OB1, but one field of ASCA covered the main part of this association. The detailed data analysis is still going on, we have detected twelve X-ray sources. Particularly two X-ray sources coincide well with the cluster of near-infrared sources and dense molecular cloud cores. The peaks of bard X-ray emission of these two sources were identified as NGC 2264 IRS1 and 2, class I sources with intermediate bolometric luminosities. Our X-ray data may provide the important information on the formation of stars with intermediate mass. Less