Journal
NEW ASTRONOMY
Volume 8, Issue 4, Pages 295-311Publisher
ELSEVIER SCIENCE BV
DOI: 10.1016/S1384-1076(02)00222-1
Keywords
hydrodynamics; instabilities; ISM : structure
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Observations of molecular clouds point to the existence of supersonic, turbulent flows. Therefore, any theory which attempts to describe molecular cloud evolution and star formation must include a consideration of the dynamics of colliding flows. Previous studies have considered the collision of supersonic streams or clouds. The resultant instabilities provide a mechanism which may give rise to observable cloud morphologies and enhance the star formation rate. One such instability is the nonlinear thin shell instability (NTSI) of a shock-bounded slab. This process is driven by ram pressure and efficient cooling. In this study, I use numerical simulations to examine the head-on collision of supersonic gas streams in a cold, molecular gas. A dense slab forms in the collision midplane and is prone to a number of instabilities, including the NTSI. The thermodynamic processes involved are found to have a controlling influence upon the instability and fragmentation of the slab. Although some minimal amount of cooling is needed to drive the instability, too rapid a cooling rate gives rise to smaller wavelength instabilities which wipe out the NTSI. The growth rate of the NTSI in a gas undergoing molecular cooling corresponds to a timescale of order 10(12) s, in general agreement with the theoretical value for an isothermal gas. The NTSI may provide a viable mechanism for the instigation of rapid star formation. (C) 2002 Elsevier Science B.V. All rights reserved.
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