J/A+A/631/A117 Perpendicular HF map to the Orion Bar (Kavak, 2019)
Origin of hydrogen fluoride emission in the Orion Bar.
An excellent tracer for CO-dark H2 gas clouds.
Kavak U., van der Tak F.F.S., Tielens A.G.G.M., Shipman R.F.
<Astron. Astrophys. 631, A117 (2019)>
=2019A&A...631A.117K 2019A&A...631A.117K (SIMBAD/NED BibCode)
ADC_Keywords: Molecular clouds ; Interstellar medium ; Radio lines
Keywords: astrochemistry - photon-dominated region (PDR) - ISM: molecules -
ISM: individual objects: Orion Bar
Abstract:
The hydrogen fluoride (HF) molecule is seen in absorption in the
interstellar medium (ISM) along many lines of sight. Surprisingly, it
is observed in emission toward the Orion Bar, which is an interface
between the ionized region around the Orion Trapezium stars and the
Orion molecular cloud. We aim to understand the origin of HF emission
in the Orion Bar by comparing its spatial distribution with other
tracers. We examine three mechanisms to explain the HF emission:
thermal excitation, radiative dust pumping, and chemical pumping. We
used a Herschel/HIFI strip map of the HF J=1-0 line, covering 0.5'
by 1.5' that is oriented perpendicular to the Orion Bar. We used the
RADEX non-local thermodynamic equilibrium (non-LTE) code to construct
the HF column density map. We use the Meudon PDR code to explain the
morphology of HF. The bulk of the HF emission at 10km/s emerges from
the CO-dark molecular gas that separates the ionization front from the
molecular gas that is deeper in the Orion Bar. The excitation of HF is
caused mainly by collisions with H2 at a density of 105cm-3
together with a small contribution of electrons in the interclump gas
of the Orion Bar. Infrared pumping and chemical pumping are not
important. We conclude that the HF J=1-0 line traces CO-dark
molecular gas. Similarly, bright photodissociation regions associated
with massive star formation may be responsible for the HF emission
observed toward active galactic nuclei.
Description:
The observations were made with HIFI onboard Herschel on 2012 August
28 with observation id (obsid) 1342250409.
The HF map of the Orion Bar was centered on the CO+ peak, that is,
RA=05:35:20.8, DE=-05:25:17.10 (J2000). Reference spectra have been
taken ∼5.5' away at RA=05:35:45.0, DE=-05:26:16.9 (J2000). The total
integration time (OTF + Reference observation) is 105min. The
dobule-sideband system temperature (Tsys) is 920K. The full width at
half maximum (FWHM) beam size at 1232.476GHz is 18.1" which
corresponds to 7500AU or 0.036pc at the distance of the Orion Bar.
Objects:
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RA (2000) DE Designation(s)
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05 35 22.30 -05 24 33.0 Orion Bar = NAME Orion Bar
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File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
list.dat 119 1 Information on fits files
fits/* . 1 Individual fits files
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See also:
J/ApJ/838/152 : Deep NIR spectrum of the Orion Bar PDR (Kaplan+, 2017)
J/A+A/617/A77 : Orion Bar Herschel/PACS CO maps (Parikka+, 2018)
Byte-by-byte Description of file: list.dat
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Bytes Format Units Label Explanations
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1- 9 F9.5 deg RAdeg Right Ascension of center (J2000)
10- 18 F9.5 deg DEdeg Declination of center (J2000)
20- 21 I2 --- Nx Number of pixels along X-axis
23- 24 I2 --- Ny Number of pixels along Y-axis
26 I1 Kibyte size Size of FITS file
28- 47 A20 --- FileName Name of FITS file, in subdirectory fits
49-119 A71 --- Title Title of the FITS file
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Acknowledgements:
Umit Kavak, kavak(at)astro.rug.nl
(End) Patricia Vannier [CDS] 30-Sep-2019