Large‐Scale CO Observations of a Far‐Infrared Loop in Pegasus: Detection of a Large Number of Very Small Molecular Clouds Possibly Formed via Shocks

We have carried out large scale 12CO and 13CO observations with a mm/sub-mmtelescope NANTEN toward a loop-like structure in far infrared whose angularextent is about 20x20 degrees around (l, b) ~ (109, -45) in Pegasus. The 12COdistribution is found to consist of 78 small clumpy clouds whose masses rangefrom 0.04 Mo to 11 Mo. About 83% of the 12CO clouds have very small masses lessthan 1.0 Mo. 13CO emission shown in the 19 of the 78 12CO clouds was detectedin the region where the column density of H2 derived from 12CO is greater than5x10(20) cm(-2), corresponding to Av of ~ 1 mag, which takes into account thatof HI. We find no indication of star formation in these clouds in IRAS and2MASS Point Source Catalogs. The very low mass clouds, M < 1 Mo, identified areunusual in the sense that they have very weak 12CO peak temperature of 0.5 K to2.7 K and that they aggregate in a region of a few pc with no main massiveclouds of ~ 100 Mo. A comparison with a theoretical work on molecular cloudformation (Koyama & Inutsuka 2002) suggests that the very low-mass clouds mayhave been formed in the shocked layer through the thermal instability. The starHD886 (B2IV) may be the source of the mechanical luminosity via stellar windsto create shocks, forming the loop-like structure where the very low-massclouds are embedded.Comment: Accepted by ApJ, 35pages including 14 figure