Detection of HF emission from the Orion Bar

TL;DR

This study detects HF emission in the Orion Bar, revealing that the emission originates from interclump gas with high electron density, challenging previous assumptions about the physical conditions of photon-dominated regions.

Contribution

It provides the first observational evidence that HF emission traces interclump gas with high electron density in the Orion Bar, using non-LTE radiative transfer modeling.

Findings

HF emission originates from interclump gas with high electron density

Collisional excitation by H2 underpredicts line intensity by factors of 3-5

Electron impact excitation explains observations at T=100 K and n(e)=10 cm^-3

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.