On the Hydrodynamic Interaction of Shock Waves with Interstellar Clouds. II. The Effect of Smooth Cloud Boundaries on Cloud Destruction and Cloud Turbulence

摘要

The effect of smooth cloud boundaries on the interaction of steady planarshock waves with interstellar clouds is studied using a high-resolution localAMR technique with a second-order accurate axisymmetric Godunov hydrodynamicscheme. A 3D calculation is also done to confirm the results of the 2D ones. Weconsider an initially spherical cloud whose density distribution is flat nearthe cloud center and has a power-law profile in the cloud envelope. When anincident shock is transmitted into a smooth cloud, velocity gradients in thecloud envelope steepen the smooth density profile at the upstream side,resulting in a sharp density jump having an arc-like shape. Such a ``slipsurface'' forms immediately when a shock strikes a cloud with a sharp boundary.For smoother boundaries, the formation of slip surface and therefore the onsetof hydrodynamic instabilities are delayed. Since the slip surface is subject tothe Kelvin-Helmholtz and Rayleigh-Taylor instabilities, the shocked cloud iseventually destroyed in $\sim 3-10$ cloud crushing times. After complete clouddestruction, small blobs formed by fragmentation due to hydrodynamicinstabilities have significant velocity dispersions of the order of 0.1 $v_b$,where $v_b$ is the shock velocity in the ambient medium. This suggests thatturbulent motions generated by shock-cloud interaction are directly associatedwith cloud destruction. The interaction of a shock with a cold HI cloud shouldlead to the production of a spray of small HI shreds, which could be related tothe small cold clouds recently observed by Stanimirovic & Heiles (2005). Thelinewidth-size relation obtained from our 3D simulation is found to betime-dependent. A possibility for gravitational instability triggered by shockcompression is also discussed.