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M. Franklin, D. Dinge, T. Jones (U. of Minnesota), B. Benjamin (U. of Wisconsin)
Clouds of neutral hydrogen falling toward the Galactic plane with a speed of about 100 km/s or more are among those considered to be "high velocity clouds" (HVCs). As HVCs are often observed approaching the midplane, the collision of such clouds with the gaseous disk of the Galaxy has been proposed as a precursor event to the phenomena known as "supershells" and as a catalyst to star formation. While many previous analytic calculations have assumed that ram pressure of the resisting medium was negligible, and a ballistic approximation was valid, observations showing a correlation between speed and increased height above the plane, the opposite of what is expected for free fall, suggest otherwise. Benjamin & Danly suggested in 1997 that clouds falling at terminal velocity provide a simple explanation for the observed velocity distribution. In this work, numerical models are used to test the above hypotheses with clouds falling through a more modern model of the interstellar medium than that used in the seminal work by Tenorio-Tagle et al. (TT) in 1987. With the addition of more dense material to the model background, clouds were still able to form supershell-like remnants, though star formation does not appear to be triggered. Further, though agreement was not perfect, the terminal velocity model was found to be a better approximation for these clouds' fall than the ballistic case. Cooling was a physical process included in TT's work which was not included here, but was found to be non-negligible. Simulations which include a cooling algorithm must be done to confirm these results. This work was supported in part by NSF grant AST96-19438.
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