2000/02/26 by N. A. Levenson, James R. Graham, Ian S. McLean +8 · 1 citation
Chemistry · Earth and Planetary Sciences · Physics and Astronomy · #Astronomy #Astrophysics #Astrophysics and Star Formation Studies #Atmospheric Ozone and Climate #Chemistry #Emission spectrum #Flux (metallurgy) #Ion #Ionization #Line (geometry) #Materials science #Mean kinetic temperature #Molecular cloud #Photochemistry #Photodissociation #Physics #Spectral line #Spectroscopy #Spectroscopy and Laser Applications #Stars #astro-ph
paper · pdf · doi:10.1086/312601
published as ApJ Letters, 533, L53 · 5 pages including 2 Postscript figures. To appear in ApJ Letters, April 2000
arxiv created 2000/02/26 · openalex publication_date 2000/04/10 · arxiv updated 2009/12/01 · openalex created_date 2025/10/10 · openalex updated_date 2026/08/05
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 TH2=930 K. The observed density, temperature, and UV flux imply a photoelectric heating efficiency of 4%. In the ionized region, ni=5x103 cm-3 and THii=9500 K. In the brightest regions of the PDR, the recombination line widths include a nonthermal component, which we attribute to viewing geometry.