Hot stars and cool clouds: The photodissociation region M16

We present high-resolution spectroscopy and images of a photodissociation r egion (PDR) in M16 obtained during commissioning of the near-infrared spect rometer (NIRSPEC) on the Keck IT telescope. PDRs play a significant role in regulating star formation, and M16 offers the opportunity to examine the p hysical processes of a PDR in detail. We simultaneously observe both the mo lecular 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 vibrati onal levels of H-2. Variations in density-sensitive emission-line ratios de monstrate that the molecular cloud is clumpy, with an average density n = 3 x 10(5) cm(-3). We measure the kinetic temperature of the molecular region directly and find that T-H2 = 930 K. The observed density, temperature, an d UV flux imply a photoelectric heating efficiency of 4%. In the ionized re gion, n(i) = 5 x 10(3) cm(-3) and T-H II = 9500 K. In the brightest regions of the PDR, the recombination line widths include a nonthermal component, which we attribute to viewing geometry.