Shock-driven implosion of interstellar gas clouds and star formation

Two-dimensional hydrodynamical calculations are presented to demonstrate a mechanism of star formation which, within the context of the spiral density wave theory, can explain the narrowness of the spiral arms of galaxies delineated by the classic spiral tracers-the bright, young stars and their associated H ii regions. The implosion of a standard interstellar cloud has been followed numerically after it encounters a shock in the intercloud medium. Parameters have been chosen to represent a cloud flowing into a spiral arm which is delineated by a shock in the interstellar gas. Although this work is motivated by spiral wave theory, the results should also be indicative of the evolution of a cloud struck, for example, by a supernova shock. The asymmetrical surface pressure distribution tends to flatten the cloud into a general pancake shape, so that gravitational collapse of the cloud as a whole cannot occur. The low sound speed and rapid cooling within the cloud cause the implosion to be completed before the cloud is accelerated appreciably in the direction of relative flow of the shocked intercloud gas. The compressed cloud gas is therefore subjected to a very large effective surface pressure (ram pressure), which makes gravitational collapse of subregions of the cloud possible. Gravitationally bound subregions are formed from the action of the Rayleigh-Taylor instability of the front cloud surface. Where the Kelvin-Helmholtz instability also has a chance to act, dense tongues of compressed material are formed which project outward from the front of the cloud. Densities sufficient to produce observable CO emission are attained in the thin shell at the front of the cloud. If observed CO sources are such thin shells rather than spheres, their mass estimates then fall in a range which is theoretically tractable. Ordered gas motions in such shells give rise to supersonic broadening of CO line profiles. In contrast to spherical collapse models for CO sources, this model implies that only about 5 percent of the visible material ends up in stellar form. A time for shock-initiated star formation of order l0 years, as previously estimated observationally, is obtained from the calculation. Subject headings: hydrodynamics - nebulae: general - shock waves - stars: formation