ABSORPTION-LINE CLOUD MODELS - DISTRIBUTION OF COLUMN DENSITIES AND VELOCITY PROFILES

A study is made of the properties of a randomly orientated distributio n of neutral hydrogen clouds, whose densities are constant on concentr ic similar spheroids. It is found that the distribution of column dens ities is relatively insensitive to the flattening of the clouds except at the highest column densities. If the clouds have an exponential su rface density profile, the theoretical column density distribution for a single type of cloud can agree with the observed distribution over many orders of magnitude in column density. For a spherical cloud, a s urface density profile can be found which gives complete agreement wit h observations. This implies that the column density distribution alon e provides no strong constraints on the shapes and sizes of clouds. Fo r rotating flattened clouds, the velocity width and velocity profile o f the absorbing atoms are calculated. If broadening mechanisms are ign ored, the velocity profiles can be highly asymmetric in a minority of cases. Although such asymmetric profiles could be a signature of rotat ion, the inclusion of Doppler broadening at a level observed in clouds removes the asymmetry, and it seems unlikely that rotation could be d educed from line profiles. However, the velocity at the line centre ma y differ from that of the disc centre by a significant fraction of the maximum rotation speed.