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

¹ Sterrewacht Leiden, PO Box 9513, 2300 RA Leiden, The Netherlands
e-mail: mher@strw.leidenuniv.nl
² Kapteyn Astronomical Institute, PO Box 800, 9700 AV Groningen, The Netherlands

Received: 14 December 2011
Accepted: 20 March 2012

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 modeling PDRs. To this end, the effect of dissipated turbulence on the thermal and chemical properties of PDRs is explored.

Methods. Clouds are modeled as 1D semi-infinite slabs whose thermal and chemical equilibrium is solved for using the Leiden PDR-XDR code, where mechanical heating is added as a constant term throughout the cloud. An extensive parameter space in hydrogen gas density, FUV radiation field and mechanical heating rate is considered, covering almost all possible cases for the ISM relevant to the conditions that are encountered in galaxies. Effects of mechanical heating on the temperature profiles, column densities of CO and H₂O and column density ratios of HNC, HCN and HCO⁺ are discussed.

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 H₂O 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 ~ 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 (SFR/n^3/2) is close to, or exceeds, 7 × 10^-6 M_⊙ yr^-1 cm^4.5. If mechanical heating is not included, predicted column densities (such as those of CO) are underestimated, sometimes (as in the case of HCN and HCO⁺) 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.