Hydrogen in diffuse molecular clouds in the Milky Way: Atomic column densities and molecular fraction along prominent lines of sight

Abstract

Recent submillimeter and far-infrared wavelength observations of absorption in the rotational ground-state lines of various simple molecules against distant Galactic continuum sources have opened the possibility of studying the chemistry of diffuse molecular clouds throughout the Milky Way. In order to calculate abundances, the column densities of molecular and atomic hydrogen, HI, must be known. We aim at determining the atomic hydrogen column densities for diffuse clouds located on the sight lines toward a sample of prominent high-mass star-forming regions that were intensely studied with the HIFI instrument onboard Herschel. Based on Jansky Very Large Array data, we employ the 21 cm HI absorption-line technique to construct profiles of the HI opacity versus radial velocity toward our target sources. These profiles are combined with lower resolution archival data of extended HI emission to calculate the HI column densities of the individual clouds along the sight lines. We employ Bayesian inference to estimate the uncertainties of the derived quantities. Our study delivers reliable estimates of the atomic hydrogen column density for a large number of diffuse molecular clouds at various Galactocentric distances. Together with column densities of molecular hydrogen derived from its surrogates observed with HIFI, the measurements can be used to characterize the clouds and investigate the dependence of their chemistry on the molecular fraction, for example.

arXiv comment36 pages, 26 figures; A&A in press

References in corpus (72)

• Astropy: A Community Python Package for Astronomy
• The NumPy array: a structure for efficient numerical computation
• HI4PI: A full-sky HI survey based on EBHIS and GASS
• Trigonometric Parallaxes of High Mass Star Forming Regions: the Structure and Kinematics of the Milky Way
• The Millennium Arecibo 21-CM Absorption Line Survey. II. Properties of the Warm and Cold Neutral Media
• Complex organic molecules in the interstellar medium: IRAM 30 m line survey of Sagittarius B2(N) and (M)
• The VLA Galactic Plane Survey
• The Southern Galactic Plane Survey: HI Observations and Analysis
• Interstellar water chemistry: from laboratory to observations
• Parallaxes and proper motions of interstellar masers toward the Cygnus X star-forming complex. I. Membership of the Cygnus X region
• A FUSE Survey of Interstellar Molecular Hydrogen in Translucent Clouds
• Herschel Survey of Galactic OH+, H2O+, and H3O+: Probing the Molecular Hydrogen Fraction and Cosmic-Ray Ionization Rate
• GREAT: the SOFIA high-frequency heterodyne instrument
• Molecular Hydrogen in the FUSE Translucent Lines of Sight: The Full Sample
• The Spiral Structure of the Outer Milky Way in Hydrogen
• Interstellar OH+, H2O+ and H3O+ along the sight-line to G10.6-0.4
• The Millennium Arecibo 21-cm Absorption Line Survey. I. Techniques and Gaussian Fits
• Trigonometric Parallax of W51 Main/South
• Discovery of Interstellar Hydrogen Fluoride
• Parallaxes for W49N and G048.60+0.02: Distant Star Forming Regions in the Perseus Spiral Arm
• A Wide-field High Resolution HI Mosaic of Messier 31: I. Opaque Atomic Gas and Star Formation Rate Density
• The chemistry of fluorine-bearing molecules in diffuse and dense interstellar gas clouds
• Interstellar Hydrides
• 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
• Comparative study of CH+ and SH+ absorption lines observed towards distant star-forming regions
• The Parallax of W43: a Massive Star Forming Complex near the Galactic Bar
• Sulphur-bearing molecules in diffuse molecular clouds: new results from SOFIA/GREAT and the IRAM 30 m telescope
• Submillimeter absorption from SH+, a new widespread interstellar radical, 13CH+ and HCl
• Detection of hydrogen fluoride absorption in diffuse molecular clouds with Herschel/HIFI: a ubiquitous tracer of molecular gas
• Magnetically Dominated Strands of Cold Hydrogen in the Riegel-Crutcher Cloud
• [CII] absorption and emission in the diffuse interstellar medium across the Galactic Plane
• Trigonometric parallaxes to star-forming regions within 4 kpc of the galactic center
• First interstellar detection of OH+
• Fitting Together the HI Absorption and Emission in the SGPS
• A Self-Absorption Census of Cold HI Clouds in the Canadian Galactic Plane Survey
• The Outer Disk of the Milky Way Seen in 21-cm Absorption
• GHIGLS: HI mapping at intermediate Galactic latitude using the Green Bank Telescope
• Detection of interstellar oxidaniumyl: abundant H2O+ towards the star-forming regions DR21, Sgr B2, and NGC6334
• Nitrogen hydrides in interstellar gas: Herschel/HIFI observations towards G10.6-0.4 (W31C)
• Nitrogen hydrides in interstellar gas II. Analysis of Herschel/HIFI observations towards W49N and G10.6-0.4 (W31C)
• Cold and Warm Atomic Gas around the Perseus Molecular Cloud. II. The Impact of High Optical Depth on the HI Column Density Distribution and Its Implication for the HI-to-H2 Transition
• Gas dynamics in the Milky Way: a low pattern speed model
• Water absorption in Galactic translucent clouds: conditions and history of the gas derived from Herschel/HIFI PRISMAS observations
• Trigonometric Parallaxes of Massive Star Forming Regions: G012.88+0.48 and W33
• Annual Parallax Measurements of an Infrared Dark Cloud MSXDC G034.43+00.24 with VERA
• H2 distribution during formation of multiphase molecular clouds
• Cold and warm atomic gas around the Perseus molecular cloud I: Basic Properties
• THOR - The HI, OH, Recombination Line Survey of the Milky Way - The pilot study: HI observations of the giant molecular cloud W43
• Terahertz ammonia absorption as a probe of infall in high-mass star forming clumps
• Giant Molecular clouds: what are they made from, and how do they get there?
• Herschel/HIFI measurements of the ortho/para ratio in water towards Sagittarius B2(M) and W31C
• VLA HI Zeeman Observations Toward the W49 Complex
• The ATCA HI Galactic Center Survey
• The chemistry of interstellar argonium and other probes of the molecular fraction in diffuse clouds
• Evolution of Molecular and Atomic Gas Phases in the Milky Way
• A High-Resolution Survey of HI Absorption toward the Central 200 pc of the Galactic Center
• Herschel/HIFI Observations of Hydrogen Fluoride Toward Sagittarius B2(M)
• Spatial and Temporal Variations in Small-Scale Galactic HI Structure Toward 3C~138
• The chemistry of C3 & Carbon Chain Molecules in DR21(OH)
• Far-infrared study of tracers of oxygen chemistry in diffuse clouds
• 1420 MHz Continuum Absorption Towards Extragalactic Sources in the Galactic Plane
• The Myth of the Molecular Ring
• Herschel observations of extra-ordinary sources: Detecting spiral arm clouds by CH absorption lines
• Accurate measurement of the HI column density from HI 21cm absorption-emission spectroscopy
• Direct Determination of the HF/H2 Abundance Ratio in Interstellar Gas
• An Automated Method for the Detection and Extraction of HI Self-Absorption in High-Resolution 21cm Line Surveys
• A Herschel/HIFI Legacy Survey of HF and H2O in the Galaxy: Probing Diffuse Molecular Cloud Chemistry
• Cygrid: A fast Cython-powered convolution-based gridding module for Python
• High-resolution absorption spectroscopy of the OH 2Pi 3/2 ground state line
• Spatial Variations in Galactic H I Structure on AU-Scales Toward 3C 147 Observed with the Very Long Baseline Array
• Molecular gas in absorption and emission along the line of sight to W31C G10.62-0.38