UCLCHEM: A Gas-Grain Chemical Code
  We present a publicly available, open source version of the time dependent
gas-grain chemical code UCLCHEM. UCLCHEM propagates the abundances of chemical
species through a large network of chemical reactions in a variety of physical
conditions. The model is described in detail along with its applications. As an
example of possible uses, UCLCHEM is used to explore the effect of protostellar
collapse on commonly observed molecules and to study the behaviour of molecules
in C-type shocks. We find the collapse of a simple Bonnor-Ebert sphere
successfully reproduces most of the behaviour of CO,CS and NH$_3$ from cores
observed by \citet{Tafalla2004} but cannot predict the behaviour of N$_2$H$^+$.
In the C-shock application, we find that molecules can be categorized so that
they can be useful observational tracers of shocks and their physical
properties. Whilst many molecules are enhanced in shocked gas, we identify two
groups of molecules in particular. A small number of molecules are enhanced by
the sputtering of the ices as the shock propagates and then remain high in
abundance throughout the shock. A second, larger set are also enhanced by
sputtering but then are destroyed as the gas temperature rises. Through these
applications the general applicability of UCLCHEM is demonstrated.
