2012A&A...538A..41G

We investigate the structure of the Galactic HII region G353.2+0.9, by analyzing (sub-)mm molecular-line and -continuum observations. This region is excited by the massive open cluster Pismis-24. We study the detailed morphology, distribution, and physical parameters (column and volume densities, masses, temperatures and opacities) of the molecular gas and dust. We are also interested in the variation in these parameters across the photon-dominated region. We observed various molecules and transitions to derive the physical properties of the molecular gas through line ratios, and both LTE and non-LTE analyses. The physical properties of the gas were derived with a Bayesian approach for the non-LTE analysis. Based on the continuum data at 870µm, we derived the column density of molecular hydrogen from the surface brightness and thus molecular abundances from the molecular column densities. We determined the mass of the dust from the integrated flux. We also carried out the simplest possible analysis to identify the clump candidates for gravitational instability, determining their virial parameter α. The total mass of the gas in the region is 2000M_☉, while that of the dust is 21M_☉. The presence of a velocity gradient in the region, with clumps with redder V_LSR nearer Pis-24 suggests that the expansion of the ionized gas is pushing the molecular gas away from the observer. We unambiguously identify the ionization front in G353.2+0.9, at the location of which we detect an increase in gas density and temperature. Its location and position angle is consistent with Pis-24 being the main ionization source. Almost no molecular gas is found south of the ionization front, at the location of the intense, elongated continuum and atomic-line emission, strengthening the hypothesis that Pis-24 is associated with G353.2+0.9. We find at least 14 clumps at different positions and LSR velocities, and we determine their physical conditions. The typical excitation temperatures are in the range of about 10-25K, while H₂ column densities are in the range 10²⁰-10²³cm^–2. From the non-LTE analysis, we derive kinetic temperatures in the ranges 11-45K (CS) and 20-45K (CN). The H₂ number density is typically around 10⁵cm^–3 from CS and few x10⁵cm^–3 from CN, with maxima above 10⁶cm^–3. The abundances of the molecules observed are found to vary across the region, and appear to be higher in regions further away from the ionization front.