The interstellar medium in our Galaxy is highly: inhomogeneous
and is kept in a highly disturbed state by the large input
of1 energy by supernova explosions. Under such circumstances,
supernova remnants and interstellar clouds co-exist in a
symbiotic relationship the parameters of each being determined by those of the other through the agency of the various
interactions between them.
I here develop a simple model of such a supernova dominated
cloudy interstellar medium (SDC-ISM). Although based on
the work of McKee and Ostriker (l977) it has been extended
in several important respects. This model is subjected
to an exhaustive parameter search in order to locate the
boundaries of its domain of validity and to establish the
sensitivity of the model to its various input parameters.
This representation of the SDC-ISM is then combined with a
simple model of the structure of the galaxy constructed
within the framework of the spiral density wave theory.
It is shown that the high temperature component of the ISM
is ubiquitous both within the spiral arms and in the interarm
region. It then follows that the strong spiral shock
assumed can only be supported if quite restrictive conditions are imposed on the galactic supernova rate and
the interstellar gas density.
During this investigation the need for a more detailed
description of the dynamical interaction between an SNR
shock and an interstellar cloud was identified. To meet
this need I have constructed a highly approximate but simpl
model of this process which is then used to explore the
behaviour of a cloud exposed to conditions prevalent in
the SDC-ISM. I find that having been shocked a cloud is
rarely allowed sufficient time to return to pressure equili
brium with its surroundings before encountering a second
shock. The disruption of a cloud by its passage through
a blast wave is found to be quite effective and the half
life of clouds cannot greatly exceed the mean interval
between shocks striking a given cloud.