Abstract
The clumpy density structure of photon-dominated regions is well established, but the physical properties of the clumps and of the surrounding interclump medium are only approximately known. The aim of this paper is to constrain the physical and chemical conditions in the Orion Bar, a prototypical nearby photon-dominated region. We present observations of the HF J=1-0 line, which appears in emission toward the Orion Bar, and compare the brightness of the line to non-LTE radiative transfer calculations. The large width of the HF line suggests an origin of the emission in the interclump gas, but collisional excitation by H2 in the interclump gas underpredicts the observed line intensity by factors of 3-5. In contrast, an origin of the line in the dense clumps requires a density of ~10^9 cm^-3, 10-100 times higher than previous estimates, which is unlikely. However, electron impact excitation reproduces our observations for T = 100 K and n(e) = 10 cm^-3, as expected for the interclump gas. We conclude that HF emission is a signpost of molecular gas with a high electron density. Similar conditions may apply to active galactic nuclei where HF also appears in emission.
Highlights
Photon-dominated regions (PDRs) are the surface regions of molecular clouds, where ultraviolet radiation with photon energies between a few and 13.6 eV drives the thermal and chemical balance of the gas (Hollenbach & Tielens 1999)
The width of the HF line is close to the values measured for C+ and CH+ (Nagy et al, in prep.), suggesting an origin of the observed HF emission in the low-density interclump gas of the Orion Bar, which is a surprise given the large critical density (∼5 × 109 cm−3) of the line
We have presented observations of the HF J = 1–0 line toward the Orion Bar, which is the first time that this line is seen in emission from the Galactic interstellar medium
Summary
Photon-dominated regions (PDRs) are the surface regions of molecular clouds, where ultraviolet radiation with photon energies between a few and 13.6 eV drives the thermal and chemical balance of the gas (Hollenbach & Tielens 1999). This situation occurs in regions of high-mass star formation, and in protoplanetary disks and in the nuclei of active galaxies. Absorption of the impinging ultraviolet radiation by dust and gas in the PDR creates a layered structure, where chemical transitions such as H+ → H → H2 and C+ → C → CO occur. The mean temperature of the molecular gas in the Bar is 85 K, while the temperature rises to several 100 K toward the ionization front, where
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