Astrochemical modelling of infrared dark clouds
  Infrared dark clouds (IRDCs) are cold, dense regions of the interstellar
medium (ISM) that are likely to represent the initial conditions for massive
star formation. It is thus important to study the physical and chemical
conditions of IRDCs to provide constraints and inputs for theoretical models of
these processes. We aim to determine the astrochemical conditions, especially
cosmic ray ionisation rate (CRIR) and chemical age, in different regions of the
massive IRDC G28.37+00.07 by comparing observed abundances of multiple
molecules and molecular ions with the predictions of astrochemical models. We
have computed a series of single-zone astrochemical models with a gas-grain
network that systematically explores the parameter space of density,
temperature, CRIR, and visual extinction. We have also investigated the effects
of choices of CO ice binding energy and temperatures achieved in transient
heating of grains when struck by cosmic rays. We selected 10 positions across
the IRDC that are known to have a variety of star formation activity. We
utilised mid-infrared (MIR) extinction maps and sub-mm emission maps to measure
the mass surface densities of these regions, needed for abundance and volume
density estimates. The sub-mm emission maps were also used to measure
temperatures. We then used IRAM-30m observations of various tracers to estimate
column densities and thus abundances. Using estimates of the abundances of CO,
HCO$^+$ and N$_2$H$^+$ we find consistency with astrochemical models that have
relatively low CRIRs of $\zeta \sim10^{-18}$ to $\sim10^{-17}\:{\rm s}^{-1}$,
with no evidence for systematic variation with the level of star formation
activity. Astrochemical ages are found to be < 1 Myr. We discuss potential
sources of systematic uncertainties in these results and the overall
implications for IRDC evolutionary history and astrochemical models.(abridged
for arXiv)
