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

Abstract

We have performed large-scale CO observations with the millimeter/submillimeter telescope NANTEN toward a FIR looplike structure whose angular extent is ~20° × 20° around (l,b) ~ (109°, - 45°) in Pegasus. Its diameter corresponds to ~25 pc at a distance of 100 pc, adopted from that of the star HD 886 (B2 IV) near the center of the loop. We covered the looplike structure in the 12CO (J = 1-0) emission at 4'-8' grid spacing and in the 13CO (J = 1-0) emission at 2' grid spacing for the 12CO-emitting regions. The 12CO distribution is found to consist of 78 small clumpy clouds of mass 0.04-11 M☉, of which ~83% have very small masses <1.0 M☉. 13CO observations revealed significant 13CO emission in 18 of the 78 12CO clouds. 13CO emission was detected in the region where the column density of H2 derived from 12CO is >5 × 1020 cm-2, corresponding to AV ~ 1 mag, which takes into account that of H I. We find no indication of star formation in these clouds in the IRAS and 2MASS point source catalogs. The very low mass clouds, M ≤ 1 M☉, identified are unusual in that they have a very weak 12CO peak temperature of 0.5-2.7 K and that they aggregate in a region of a few parsecs with no main massive clouds; in contrast, similar low-mass clouds <1 M☉ in other regions previously observed including those at high Galactic latitude are all associated with more massive main clouds of ~100 M☉. Comparison with a theoretical work on molecular cloud formation suggests that the very low mass clouds may have been formed in the shocked layer through thermal instability. HD 886 may be the source of the mechanical luminosity via stellar winds to create shocks, forming the looplike structure where the very low mass clouds are embedded.