Detection of HF emission from the Orion Bar

Abstract : Context. 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.
Aims: The aim of this paper is to constrain the physical and chemical conditions in the Orion Bar, a prototypical nearby photon-dominated region.
Methods: 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.
Results: 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 ~10⁹ 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.
Conclusions: 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.

Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.