Interstellar Clouds

Four topics relating to interstellar clouds are studied in this paper: (1) We have obtained an analytical solution to the mass and velocity distribution of interstellar clouds based on the Oort (1954) model in which clouds increase in mass because of inelastic collisions. We confirm the massdistribution obtained by Field & Saslaw (1965) in which $$N(M)\,\propto\,{M}^{-3/2}$$ and we further show that a Maxwellian velocity distribution is set up. (2) We have simulated the Oort model by a Monte Carlo method on a computer, obtaining a favourable comparison with the above and predicting the existence of ‘ rogue ’ clouds of high mass and velocity. (3) We have analysed Adams' (1949) data on interstellar line velocities and intensities to compare the predictions of (1) and (2) with observations. This shows good agreement when corrections are made for observational selection. (4) We have also examined these observations to obtain the motion of the clouds relative to the Sun. From this we find the effect of non-gravitational forces on the motion of the interstellar medium is to cause the gas to lag behind the stars by 3 km s ^–1 in the direction of galactic rotation. We compare this lag with that which might be expected from magnetic fields, cosmic ray pressure and turbulence.