NH$_3$ Observations of the S235 Star Forming Region: Dense Gas in
  Inter-core Bridges
  Star formation is thought to be driven by two groups of mechanisms;
spontaneous collapse and triggered collapse. Triggered star formation
mechanisms further diverge into cloud-cloud collision (CCC), "collect and
collapse" (C\&C) and shock induced collapse of pre-existing, gravitationally
stable cores, or 'radiation driven implosion' (RDI). To evaluate the
contributions of these mechanisms and establish whether these processes can
occur together within the same star forming region we performed mapping
observations of radio frequency ammonia, and water maser emission lines in the
S235 massive star forming region. Via spectral analyses of main, hyperfine and
multi-transitional ammonia lines we explored the distribution of temperature
and column density in the dense gas in the S235 and S235AB star forming region.
The most remarkable result of the mapping observations is the discovery of high
density gas in inter-core bridges which physically link dense molecular cores
that house young proto-stellar clusters. The presence of dense gas implies the
potential for future star formation within the system of cores and gas bridges.
Cluster formation implies collapse and the continuous physical links, also seen
in re-imaged archival CS and $^{13}$CO maps, suggests a common origin to the
molecular cores housing these clusters, i.e the structure condensed from a
single, larger parent cloud, brought about by the influence of a local
expanding H${\rm II}$ region. An ammonia absorption feature co-locating with
the center of the extended H${\rm II}$ region may be attributed to an older gas
component left over from the period prior to formation of the H${\rm II}$
region. Our observations also detail known and new sites of water maser
emission, highlighting regions of active ongoing star formation.
