Abstract
The giant molecular cloud extending parallel to the galactic plane southwest of M17 to 85 pc is investigated in the H I 21-cm absorption line by use of the Maryland-Green Bank Survey. The H2/H I abundance ratio is estimated to be 830 and 450 for two of the CO fragments. An anomalous region, characterized by a large line width of about 10 km/s and two-dip profiles, is found near the southwestern edge of the cloud. These findings are examined in the light of current theories on chemical evolution of dark clouds and on cloud formation. It is suggested that in the preshock region, cloud fragments appear to line up in order of age as in the postshock region represented by OB subgroups. Cloud-cloud collisions induced by a galactic density wave may play an important role in the formation of the giant cloud.
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