ON THE DISTANCE DETERMINATION AND IONIZATION OF THE HIGH-VELOCITY CLOUDS

We present a study of the ionization and thermal structure of neutral hydrogen clouds located in the Galactic halo, immersed in the extragal actic background radiation field, and supposed to be in pressure equil ibrium with the surrounding (presumably) hot medium. The problem is so lved numerically, but an useful analytical approximation has been deri ved as well. We discuss the two main parameters of the problem, i.e., the radiation field and the halo pressure, in the framework of the cur rent models for the halo/disk interaction. We find a well defined rela tion between a critical column density at which the cloud starts to de velop a cold central core with the cloud linear size. Making use of th is relation, we suggest a straightforward method to derive the distanc e to the cloud. We discuss the possible sources of error in this deter mination and it is found that the method is particularly suitable for those clouds which are subsonic with respect to the surrounding medium , while for the supersonic ones, the method can only give a lower limi t to the distance. The Halpha emission from the partially ionized edge of the cloud is calculated and compared with the available observatio ns; this measure is a powerful indicator of the possible presence of a shock in the cloud.