2000ApJ...533L..53L

We present high-resolution spectroscopy and images of a photodissociation region (PDR) in M16 obtained during commissioning of the near-infrared spectrometer (NIRSPEC) on the Keck II telescope. PDRs play a significant role in regulating star formation, and M16 offers the opportunity to examine the physical processes of a PDR in detail. We simultaneously observe both the molecular and ionized phases of the PDR and resolve the spatial and kinematic differences between them. The most prominent regions of the PDR are viewed edge-on. Fluorescent emission from nearby stars is the primary excitation source, although collisions also preferentially populate the lowest vibrational levels of H₂. Variations in density-sensitive emission-line ratios demonstrate that the molecular cloud is clumpy, with an average density n=3x10⁵ cm^–3. We measure the kinetic temperature of the molecular region directly and find that T_H2=930 K. The observed density, temperature, and UV flux imply a photoelectric heating efficiency of 4%. In the ionized region, n_i=5x10³ cm^–3 and T_HII_=9500 K. In the brightest regions of the PDR, the recombination line widths include a nonthermal component, which we attribute to viewing geometry.