The abundance of HNCO and its use as a diagnostic of environment
  We aim to investigate the chemistry and gas phase abundance of HNCO and the
variation of the HNCO/CS abundance ratio as a diagnostic of the physics and
chemistry in regions of massive star formation. A numerical-chemical model has
been developed which self-consistently follows the chemical evolution of a hot
core. The model comprises of two distinct stages. An initial collapse phase is
immediately followed by an increase in temperature which represents the switch
on of a central massive star and the subsequent evolution of the chemistry in a
hot, dense gas cloud (the hot core). During the collapse phase, gas species are
allowed to accrete on to grain surfaces where they can participate in further
reactions. During the hot core phase surface species thermally desorb back in
to the ambient gas and further chemical evolution takes place. For comparison,
the chemical network was also used to model a simple dark cloud and
photodissociation regions. Our investigation reveals that HNCO is inefficiently
formed when only gas-phase formation pathways are considered in the chemical
network with reaction rates consistent with existing laboratory data. Using
currently measured gas phase reaction rates, obtaining the observed HNCO
abundances requires its formation on grain surfaces. However our model shows
that the gas phase HNCO in hot cores is not a simple direct product of the
evaporation of grain mantles. We also show that the HNCO/CS abundance ratio
varies as a function of time in hot cores and can match the range of values
observed. This ratio is not unambiguously related to the ambient UV field as
been suggested - our results are inconsistent with the hypothesis of Martin et
al (2008). In addition, our results show that this ratio is extremely sensitive
to the initial sulphur abundance.
