J/A+A/643/A91 ArH+ and p-H2O+ spectra towards 7 molecular clouds (Jacob+, 2020)
Extending the view of ArH+ chemistry in diffuse clouds.
Jacob A.M., Menten K.M., Wyrowski F., Winkel B., Neufeld D.A.
<Astron. Astrophys. 643, A91 (2020)>
=2020A&A...643A..91J 2020A&A...643A..91J (SIMBAD/NED BibCode)
ADC_Keywords: Molecular clouds ; Spectroscopy
Keywords: ISM: molecules - ISM: clouds - ISM: abundances - cosmic rays
Abstract:
One of the surprises of the Herschel mission was the detection of
ArH+, toward the Crab Nebula in emission and in absorption toward
strong Galactic background sources. Although, these detections were
limited to the first quadrant of the Galaxy, the existing data
suggests that ArH+ ubiquitously and exclusively probes the diffuse
atomic regions of the interstellar medium.
In this study we extend the coverage of ArH+ to other parts of the
Galaxy with new observations of its J=1-0 transition along seven
Galactic sight lines toward bright sub-millimetre continuum sources.
We aim to benchmark its efficiency as a tracer of purely atomic gas by
evaluating its correlation (or lack of correlation as suggested by
chemical models) with other well known atomic gas tracers like OH+ and
H2O+ and the molecular gas tracer CH.
The observations of the J=1-0 line of ArH+ near 617.5GHz were made
feasible with the new, sensitive SEPIA660 receiver on the APEX 12m
telescope. Further, the two sidebands of this receiver allowed us to
observe the NKaKc=11,0-10,1 transitions of para-H2O+ at
607.227GHz simultaneously with the ArH+ line.
We model the optically thin absorption spectra of the different
species and subsequently derived their column densities. By analysing
the steady state chemistry of OH+ and o-H2O+ we derive on average a
cosmic-ray ionisation rate of 2.3±0.3x10-16s-1, toward the
sightlines studied in this work. Using the derived values of the
cosmic-ray ionisation rates and the observed ArH+ abundances we
constrain the molecular fraction of the gas traced by ArH+ to lie
below 2x10-2 with a median value of 8.8x10-4. Combined, our
observations of ArH+, OH+, H2O+ and CH probe different regimes of the
interstellar medium, from diffuse atomic to diffuse and translucent
molecular clouds. Over Galactic scales, we see that the distribution
of N(ArH+) is associated with that of N(H), particularly in the inner
Galaxy (within 7 kpc of the Galactic center) with potentially even
contributions from the warm neutral medium phase of atomic gas at
larger galactocentric distances. We derive an average ortho-to-para
ratio for H2O+ of 2.1±1.0, which corresponds to a nuclear spin
temperature of 41K, consistent with the typical gas temperatures of
diffuse clouds.
Description:
The observations were performed in July-August 2019 using the SEPIA660
receiver on the APEX 12m telescope under the project id:
M-0103.F-9519C-2019. In addition to the results of the ArH+ and
p-H2O+ observations described above, we use complementary APEX data
of OH+, some of which has already been published by Wiesemeyer et al.
(2016A&A...585A..76W 2016A&A...585A..76W). The ancillary o-H2O+ data used here, was
procured as a part of the Water in star-forming regions with Herschel
(WISH) survey (van Dishoeck et al., 2011PASP..123..138V 2011PASP..123..138V) using the
HIFI instrument on the Herschel Space Observatory and retrieved using
the Herschel Science Archive. We also use archival data of HI
absorption and emission from the THOR survey for AGAL019.609-00.234,
data presented in Winkel et al. (2017A&A...600A...2W 2017A&A...600A...2W, Cat.
J/A+A/600/A2) for AGAL031.412+0.31 and data from the SGPS database
(McClure-Griffiths et al. 2005ApJS..158..178M 2005ApJS..158..178M) for the remaining
sources. Lastly, we also compare these line profiles with that of CH
presented in Jacob et al. (2019A&A...632A..60J 2019A&A...632A..60J, Cat. J/A+A/632/A60).
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
list.dat 154 7 List of fits files
fits/* . 7 Individual fits files
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See also:
J/A+A/600/A2 : Hydrogen in diffuse molecular clouds (Winkel+, 2017)
J/A+A/632/A60 : CH 2THz spectra towards 7 molecular clouds (Jacob+, 2019)
Byte-by-byte Description of file: list.dat
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Bytes Format Units Label Explanations
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1- 20 A20 --- Name Source name
23- 24 I2 h RAh Right Ascension (J2000)
26- 27 I2 min RAm Right Ascension (J2000)
29- 33 F5.2 s RAs Right Ascension (J2000)
35 A1 --- DE- Declination sign (J2000)
36- 37 I2 deg DEd Declination (J2000)
39- 40 I2 arcmin DEm Declination (J2000)
42- 46 F5.2 arcsec DEs Declination (J2000)
48- 54 F7.3 deg GLON Galactic longitude
56- 62 F7.3 deg GLAT Galactic latitude
64- 67 F4.1 kpc d Heliocentric distance to source
69- 73 F5.1 km/s vlsr Source LSR velocity
75- 78 F4.2 K Tc Main-beam brightness continuum temperature
at 617GHz
80- 82 F3.1 Kibyte size Size of FITS file
84-106 A23 --- FileName Name of the FITS file in subdirectory fits
108-154 A47 --- Title Title of the fits file
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Acknowledgements:
Arshia Maria Jaocb, ajacob(at)mpifr-bonn.mpg.de
(End) Patricia Vannier [CDS] 02-Oct-2020