Mid-infrared PAH and H2 emission as a probe of physical conditions in extreme PDRs

TL;DR

This study demonstrates how mid-infrared observations of PAHs and H2 emissions can effectively determine physical conditions like temperature, density, and UV radiation in dense, UV-irradiated regions such as PDRs, starburst nuclei, and protoplanetary disks.

Contribution

The paper introduces a method combining PAH ionization and H2 line ratios to accurately derive physical parameters in extreme PDRs using mid-IR spectroscopy.

Findings

PAH ionization fraction and H2 line ratios reveal physical conditions.

The method applies to dense PDRs with specific temperature, density, and UV field ranges.

Spatially resolved Spitzer spectra provide detailed PDR structure insights.

Abstract

Mid-infrared (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, \emph{Spitzer} mid-IR spectroscopy of the high UV-field and dense photodissocation region (PDR) around Monoceros R2, the closest ultracompact \hII 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 $\mu$m, allows 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$^{-3}$, $G_0=10^3-10^5$ 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.