Diagnostics of the molecular component of photon-dominated regions with mechanical heating

Abstract

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.