J/A+A/632/A8 Orion Bar photon-dominated region water lines (Putaud+, 2019)
The water line emission and ortho-to-para ratio in the Orion Bar
photon-dominated region.
Putaud T., Michaut X., Le Petit F., Roueff E., Lis D.C.
<Astron. Astrophys. 632, A8 (2019)>
=2019A&A...632A...8P 2019A&A...632A...8P (SIMBAD/NED BibCode)
ADC_Keywords: Molecular clouds ; Interstellar medium ; Spectroscopy
Keywords: ISM: molecules - ISM: individual objects: Orion Bar -
photon-dominated region - ISM: lines and bands - submillimeter: ISM
Abstract:
The ortho-to-para ratio (OPR) of water in the interstellar medium
(ISM) is often assumed to be related to the formation temperature of
water molecules, making it a potentially interesting tracer of the
thermal history of interstellar gas.
A very low OPR of 0.1-0.5 was previously reported in the Orion Bar
photon-dominated region (PDR), based on observations of two optically
thin H218O lines which were analyzed by using a single-slab large
velocity gradient (LVG) model. The corresponding spin temperature does
not coincide with the kinetic temperature of the molecular gas in this
UV-illuminated region. This was interpreted as an indication of water
molecules being formed on cold icy grains which were subsequently
released by UV photodesorption.
A more complete set of water observations in the Orion Bar, including
seven H216O lines and one H218O line, carried out using
Herschel/HIFI instrument, was reanalyzed using the Meudon PDR code to
derive gas-phase water abundance and the OPR. The model takes into
account the steep density and temperature gradients present in the
region. The model line intensities are in good agreement with the
observations assuming that water molecules formed with an OPR
corresponding to thermal equilibrium conditions at the local kinetic
temperature of the gas and when solely considering gas-phase chemistry
and water gas-grain exchanges through adsorption and desorption.
Gas-phase water is predicted to arise from a region deep into the
cloud, corresponding to a visual extinction of AV∼9, with a
H216O fractional abundance of ∼2x10-7 and column density of
(1.4±0.8)x1015cm-2 for a total cloud depth of AV=15. A
line-of-sight average OPR of 2.8±0.2 is derived.
The observational data are consistent with a nuclear spin isomer
repartition corresponding to the thermal equilibrium at a temperature
of 36±2K, much higher than the spin temperature previously reported
for this region and close to the gas kinetic temperature in the
water-emitting gas.
Description:
The observations of water lines in the Orion Bar were made with HIFI
onboard Herschel from February 2010 to September 2012.
The observations id (obsid) are: 1342215923, 1342218527, 1342215922,
1342217720, 1342215970, 1342229840, 1342218426, 1342251054,
1342218628, 1342218217, 1342216378, 1342190848, 1342190849,
1342203221, 1342203222 and 1342216355.
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 Ori 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 34 List of fits files
fits/* . 34 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)
J/A+A/631/A117 : Perpendicular HF map to the Orion Bar (Kavak, 2019)
Byte-by-byte Description of file:list.dat
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Bytes Format Units Label Explanations
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1- 8 F8.5 deg RAdeg Right Ascension of center (J2000)
10- 17 F8.5 deg DEdeg Declination of center (J2000)
19- 44 A26 "datime" Obs.dat Observation date
46- 49 I4 Kibyte size Size of FITS file
51- 75 A25 --- FileName Name of FITS file, in subdirectory fits
77-119 A43 --- Title Title of the FITS file
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Acknowledgements:
Thomas Putaud, thomas.putaud(at)obspm.fr
(End) Patricia Vannier [CDS] 13-Nov-2019