HIGH-VELOCITY RAIN - THE TERMINAL VELOCITY MODEL OF GALACTIC INFALL

A model is proposed for determining the distances to falling interstel lar clouds in the galactic halo by measuring the cloud velocity and co lumn density and assuming a model for the vertical density distributio n of the Galactic interstellar medium. It is shown that falling clouds with N(H I) less than or similar to 10(19) cm(-2) may be decelerated to a terminal velocity which increases with increasing height above th e Galactic plane, This terminal velocity model correctly predicts the distance to high-velocity cloud Complex M and several other interstell ar structures of previously determined distance. It is demonstrated ho w interstellar absorption spectra alone may be used to predict the dis tances of the clouds producing the absorption. If the distance, veloci ties, and column densities of enough interstellar clouds are known ind ependently, the procedure can be reversed, and the terminal velocity m odel can be used to estimate the vertical density structure (both the mean density and the porosity) of the interstellar medium. Using the d ata of Danly and assuming a drag coefficient of C-D congruent to 1, th e derived density distribution is consistent with the expected density distribution of the warm ionized medium, characterized by Reynolds. T here is also evidence that for z greater than or similar to 0.4 kpc on e or more of the following occurs: (1) the neutral fraction of the clo ud decreases to similar to 31 +/- 14%, (2) the density drops off faste r than characterized by Reynolds, or (3) there is a systematic decreas e in C-D with increasing z. Current data do not place strong constrain ts on the porosity of the interstellar medium.