Volume 537, January 2012
|Number of page(s)||4|
|Published online||20 January 2012|
Detection of HF emission from the Orion Bar⋆
1 SRON Netherlands Institute for Space Research, Landleven 12, 9747 AD Groningen, The Netherlands
2 Kapteyn Institute, University of Groningen, The Netherlands
3 KOSMA, I. Physik. Inst., Universität zu Köln, Germany
4 UJF-Grenoble 1/CNRS, Institut de Planétologie et d’Astrophysique de Grenoble, France
5 Dept. of Astronomy, University of Michigan, Ann Arbor, USA
Received: 21 October 2011
Accepted: 15 December 2011
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 ne = 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.
Key words: astrochemistry / ISM: molecules
© ESO, 2012
Current usage metrics show cumulative count of Article Views (full-text article views including HTML views, PDF and ePub downloads, according to the available data) and Abstracts Views on Vision4Press platform.
Data correspond to usage on the plateform after 2015. The current usage metrics is available 48-96 hours after online publication and is updated daily on week days.
Initial download of the metrics may take a while.