Molecular Cloud Formation in Shock‐compressed Layers

We investigate the propagation of a shock wave into a warm neutral medium andcold neutral medium by one-dimensional hydrodynamic calculations with detailedtreatment of thermal and chemical processes. Our main result shows that thermalinstability inside the shock-compressed layer produces a geometrically thin,dense layer in which a large amount of hydrogen molecules is formed. Linearstability analysis suggests that this thermally-collapsed layer will fragmentinto small molecular cloudlets. We expect that frequent compression due tosupernova explosions, stellar winds, spiral density waves, etc., in the galaxymake the interstellar medium occupied by these small molecular cloudlets.Comment: 21 pages, 11 figures. Accepted by Ap