Physical conditions in star forming regions around S235
  Gas density and temperature in star forming regions around Sh2-235 are
derived from ammonia line observations. This information is used to evaluate
formation scenarios and to determine evolutionary stages of the young embedded
clusters S235 East1, S235 East2, and S235 Central. We also estimate the gas
mass in the embedded clusters and its ratio to the stellar mass. S235 East1
appears to be less evolved than S235 East2 and S235 Central. In S235 East1 the
molecular gas mass exceeds that in the other clusters. Also, this cluster is
more embedded in the parent gas cloud than the other two. Comparison with a
theoretical model shows that the formation of these three clusters could have
been stimulated by the expansion of the Sh2-235 HII region (hereafter S235) via
a collect-and-collapse process, provided the density in the surrounding gas
exceeds $3\cdot10^3$ cm$^{-3}$, or via collapse of pre-existing clumps. The
expansion of S235 cannot be responsible for star formation in the southern S235
A-B region. However, formation of the massive stars in this region might have
been triggered by a large-scale supernova shock. Thus, triggered star formation
in the studied region may come in three varieties, namely collect-and-collapse
and collapse of pre-existing clumps, both initiated by expansion of the local
HII regions, and triggering by an external large-scale shock. We argue that the
C235 A HII region expands into a highly non-uniform medium with increasing
density. It is too young to trigger star formation in its vicinity by a
collect-and-collapse process. There is an age spread inside the S235 A-B
region. Massive stars in the S235 A-B region are considerably younger than
lower mass stars in the same area. This follows from the estimates of their
ages and the ages of associated HII regions.
