Diagnostics of the Molecular Component of PDRs with Mechanical Heating
  Context. Multitransition CO observations of galaxy centers have revealed that
significant fractions of the dense circumnuclear gas have high kinetic
temperatures, which are hard to explain by pure photon excitation, but may be
caused by dissipation of turbulent energy.
  Aims. We aim to determine to what extent mechanical heating should be taken
into account while modelling PDRs. To this end, the effect of dissipated
turbulence on the thermal and chemical properties of PDRs is explored. Methods.
Clouds are modelled as 1D semi-infinite slabs whose thermal and chemical
equilibrium is solved for using the Leiden PDR-XDR code.
  Results. In a steady-state treatment, mechanical heating seems to play an
important role in determining the kinetic temperature of the gas in molecular
clouds. Particularly in high-energy environments such as starburst galaxies and
galaxy centers, model gas temperatures are underestimated by at least a factor
of two if mechanical heating is ignored. The models also show that CO, HCN and
H2 O column densities increase as a function of mechanical heating. The HNC/HCN
integrated column density ratio shows a decrease by a factor of at least two in
high density regions with n \sim 105 cm-3, whereas that of HCN/HCO+ shows a
strong dependence on mechanical heating for this same density range, with
boosts of up to three orders of magnitude.
  Conclusions. The effects of mechanical heating cannot be ignored in studies
of the molecular gas excitation whenever the ratio of the star formation rate
to the gas density is close to, or exceeds, 7 \times 10-6 M yr-1 cm4.5 . If
mechanical heating is not included, predicted column densities are
underestimated, sometimes even by a few orders of magnitude. As a lower bound
to its importance, we determined that it has non-negligible effects already
when mechanical heating is as little as 1% of the UV heating in a PDR.
