Extending the view of ArH+ chemistry in diffuse clouds

Abstract

One of the surprises of the Herschel mission was the detection of ArH+ towards the Crab Nebula in emission and in absorption towards strong Galactic background sources. Although these detections were limited to the first quadrant of the Galaxy, the existing data suggest that ArH+ ubiquitously and exclusively probes the diffuse atomic regions of the ISM. 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 towards bright sub-mm continuum sources. We aim to benchmark its efficiency as a tracer of purely atomic gas by evaluating its correlation (or lack there of) with other well-known atomic and molecular gas tracers. The observations of ArH+ near 617.5 GHz were made feasible with the new, sensitive SEPIA660 receiver on the APEX 12 m telescope. The two sidebands of this receiver allowed us to observe p-H2O+ transitions of at 607.227 GHz simultaneously with the ArH+ line. By analysing the steady state chemistry of OH+ and o-H2O+, we derive on average a cosmic-ray ionisation rate (CRIR), of 2.3e-16 s^-1 towards the sight lines studied in this work. Using the derived values of the CRIR and the observed ArH+ abundances we constrain the molecular fraction of the gas traced by ArH+ to lie below 2e-2 with a median value of 8.8e-4. Combined, our observations of ArH+, OH+, H2O+, and CH probe different regimes of the ISM, 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 with potentially even contributions from the warm neutral medium phase of atomic gas at larger galactocentric distances. We derive an average o/p-ratio for H2O+ of 2.1, which corresponds to a nuclear spin temperature of 41 K, consistent with the typical gas temperatures of diffuse clouds.

arXiv commentAccepted for publication in Astronomy & Astrophysics (24 pages, 30 figures)

References in corpus (52)

• The NumPy array: a structure for efficient numerical computation
• The Herschel-SPIRE instrument and its in-flight performance
• The Neutral Atomic Phases of the ISM in the Galaxy
• Trigonometric Parallaxes Of High-Mass Star Forming Regions: Our View Of The Milky Way
• ATLASGAL - The APEX Telescope Large Area Survey of the Galaxy at 870 microns
• A KInetic Database for Astrochemistry (KIDA)
• The Atacama Large Millimeter/submillimeter Array
• Complex organic molecules in the interstellar medium: IRAM 30 m line survey of Sagittarius B2(N) and (M)
• Investigating the Cosmic-Ray Ionization Rate in the Galactic Diffuse Interstellar Medium through Observations of H3+
• The Southern Galactic Plane Survey: HI Observations and Analysis
• ATLASGAL --- properties of a complete sample of Galactic clumps
• A Herschel [CII] Galactic plane survey I: the global distribution of ISM gas components
• Ultraviolet Survey of CO and H_2 in Diffuse Molecular Clouds: The Reflection of Two Photochemistry Regimes in Abundance Relationships
• Herschel Survey of Galactic OH+, H2O+, and H3O+: Probing the Molecular Hydrogen Fraction and Cosmic-Ray Ionization Rate
• Water in Star-Forming Regions with the Herschel Space Observatory (WISH): Overview of key program and first results
• Interstellar Carbon in Translucent Sightlines
• In-flight calibration of the Herschel-SPIRE instrument
• The ATLASGAL survey: a catalog of dust condensations in the Galactic plane
• First astrophysical detection of the helium hydride ion (HeH${}^{+}$)
• Detection of a Noble Gas Molecular Ion, {36}ArH^+, in the Crab Nebula
• The Chemistry of Interstellar OH+, H2O+, and H3O+: Inferring the Cosmic Ray Ionization Rates from Observations of Molecular Ions
• Interstellar OH+, H2O+ and H3O+ along the sight-line to G10.6-0.4
• The importance of the ortho:para H2 ratio for the deuteration of molecules during pre-protostellar collapse
• The RMS Survey: Galactic distribution of massive star formation
• Herschel/HIFI observations of interstellar OH+ and H2O+ towards W49N: a probe of diffuse clouds with a small molecular fraction
• The cosmic ray ionization rate in the Galactic disk, as determined from observations of molecular ions
• The Dawn of Chemistry
• Ubiquitous argonium (ArH${}^{+}$) in the diffuse interstellar medium -- a molecular tracer of almost purely atomic gas
• Herschel observations of EXtra-Ordinary Sources: the present and future of spectral surveys with Herschel/HIFI
• ATLASGAL - Kinematic distances and the dense gas mass distribution of the inner Galaxy
• Trigonometric parallaxes to star-forming regions within 4 kpc of the galactic center
• Physical conditions in the central molecular zone inferred by H3+
• SEPIA - a new single pixel receiver at the APEX Telescope
• Spectrophotometric Distances to Galactic H\,{\sc{ii}} Regions
• Detection of extragalactic argonium, ArH${}^{+}$, toward PKS~1830$-$211
• The Solar Argon Abundance
• The HI/OH/Recombination line survey of the inner Milky Way (THOR)
• The chemistry of interstellar argonium and other probes of the molecular fraction in diffuse clouds
• Ortho-to-para ratio of interstellar heavy water
• Chemical Analysis of a Diffuse Cloud along a Line of Sight Toward W51: Molecular Fraction and Cosmic-Ray Ionization Rate
• Photodissociation of interstellar ArH+
• Herschel observations of ortho- and para-oxidaniumyl (H2O+) in spiral arm clouds toward Sgr B2(M)
• Far-infrared study of tracers of oxygen chemistry in diffuse clouds
• OH+ and H2O+ absorption toward PKS1830-211
• Precipitable Water Vapor, Temperature, and Wind Statistics At Sites Suitable for mm and Submm Wavelength Astronomy in Northern Chile
• ArH2+ and NeH2+ as Global Minima in the Ar+/Ne+ + H2 Reactions: Energetic, Spectroscopic, and Structural Data
• Chemical Abundances in a Turbulent Medium -- H${}_2$, OH${}^{+}$, H${}_2$O${}^{+}$, ArH${}^{+}$
• Modelling the ArH${}^{+}$ emission from the Crab Nebula
• Unveiling the chemistry of interstellar CH: Spectroscopy of the 2 THz $N=2\leftarrow 1$ ground state line
• Infall, outflow, and rotation in the G19.61-0.23 hot molecular core
• Recognizing the fingerprints of the Galactic bar: a quantitative approach to comparing model (l,v) distributions to observation
• Fingerprinting the effects of hyperfine structure on CH and OH far infrared spectra using Wiener filter deconvolution

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• HyGAL: Characterizing the Galactic ISM with observations of hydrides and other small molecules -- I. Survey description and a first look toward W3(OH), W3 IRS5 and NGC 7538 IRS1
• The Role of Terahertz and Far-IR Spectroscopy in Understanding the Formation and Evolution of Interstellar Prebiotic Molecules
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• HyGAL: Characterizing the Galactic ISM with observations of hydrides and other small molecules II. The absorption line survey with the IRAM 30 m telescope
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