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R. I. Klein (UC Livermore, LLNL and UC Berkeley, Dept. of Astronomy), T. Woods (U. California, Space Science Laboratory), C. F. McKee (UC Berkeley Dept. of Physics)
An important example of interstellar shock-cloud interactions occurs when shocks are generated by the collision of two clouds. We study the problem of interstellar cloud-cloud collisions with extreme high resolution local Adaptive Mesh Refinement three dimensional Hydrodynamics. The problem is of great significance because of the possibility that such an interaction may trigger star formation in clouds, as well as provide a mechanism for developing complex structure in the interstellar medium. Previous work on this problem has been lacking in two fundamental ways; (1) inadequacy of the resolution necessary to capture the key hydrodynamic instabilities whose presence may destroy the clouds and (2) absence of any clear delineation of those conditions under which a gravitational collapse leading to star formation will occur during cloud-cloud collisions of initially stable clouds. Our calculations have established two critical criteria that must be satisfied: (1) the Jeans criterion that demands that the Jean's length be well-resolved and (2) a minimal resolution of the cloud interaction region which addresses whether competition between gravitational collapse and pressure re-expansion of the compressed cloud will determine whether or not a collapse will occur. We present results for collisions for several different masses and a range of velocities to identify the region of parameter space for which a collision can lead to a triggered star formation. We determine this phase space for both head-on and off-center collisions. Key results are that for masses below 0.3 Jeans mass there is a lower velocity limit to the collapse region; there is a minimum mass for a collapse to occur regardless of how fast or how slow clouds collide and there is a high velocity limit to the collapse region. A crucial finding of our models is that the parameter space for induced star formation by cloud collisions is surprisingly small.