Mid-Infrared Polycyclic Aromatic Hydrocarbon and H ₂ Emission as a Probe of Physical Conditions in Extreme Photodissociation Regions

Mid-infrared (mid-IR) observations of polycyclic aromatic hydrocarbons (PAHs) and molecular hydrogen emission are a potentially powerful tool to derive physical properties of dense environments irradiated by intense UV fields. We present new, spatially resolved, Spitzer mid-IR spectroscopy of the high UV field and dense photodissociation region (PDR) around Monoceros R2, the closest ultracompact H ii region, revealing the spatial structure of ionized gas, PAHs, and H 2 emissions. Using a PDR model and PAH emission feature fitting algorithm, we build a comprehensive picture of the physical conditions prevailing in the region. We show that the combination of the measurement of PAH ionization fraction and of the ratio between the H 2 0–0 S (3) and S (2) line intensities, respectively, at 9.7 and 12.3 μm, allows us to derive the fundamental parameters driving the PDR: temperature, density, and UV radiation field when they fall in the ranges T = 250–1500 K, n H = 10 4 –10 6 cm −3 , and G 0 = 10 3 –10 5 , respectively. These mid-IR spectral tracers thus provide a tool to probe the similar but unresolved UV-illuminated surface of protoplanetary disks or the nuclei of starburst galaxies.