Ultraviolet Survey of CO and H_2 in Diffuse Molecular Clouds: The Reflection of Two Photochemistry Regimes in Abundance Relationships

Abstract

(Abridged) We carried out a comprehensive far-ultraviolet (UV) survey of ^12CO and H_2 column densities along diffuse molecular Galactic sight lines in order to explore in detail the relationship between CO and H_2. We measured new CO abundances from HST spectra, new H_2 abundances from FUSE data, and new CH, CH^+, and CN abundances from the McDonald and European Southern Observatories. A plot of log N(CO) versus log N(H_2) shows that two power-law relationships are needed for a good fit of the entire sample, with a break located at log N(CO, cm^-2) = 14.1 and log N(H_2) = 20.4, corresponding to a change in production route for CO in higher-density gas. Similar logarithmic plots among all five diatomic molecules allow us to probe their relationships, revealing additional examples of dual slopes in the cases of CO versus CH (break at log N = 14.1, 13.0), CH^+ versus H_2 (13.1, 20.3), and CH^+ versus CO (13.2, 14.1). These breaks are all in excellent agreement with each other, confirming the break in the CO versus H_2 relationship, as well as the one-to-one correspondence between CH and H_2 abundances. Our new sight lines were selected according to detectable amounts of CO in their spectra and they provide information on both lower-density (< 100 cm^-3) and higher-density diffuse clouds. The CO versus H_2 correlation and its intrinsic width are shown to be empirically related to the changing total gas density among the sight lines of the sample. We employ both analytical and numerical chemical schemes in order to derive details of the molecular environments. In the low-density gas, where equilibrium-chemistry studies have failed to reproduce the abundance of CH^+, our numerical analysis shows that nonequilibrium chemistry must be employed for correctly predicting the abundances of both CH^+ and CO.

40 pages in emulateapj style, to appear in the Astrophysical Journal

References in corpus (50)

• The Milky Way in Molecular Clouds: A New Complete CO Survey
• Linear Regression for Astronomical Data with Measurement Errors and Intrinsic Scatter
• Far Infrared and Submillimeter Emission from Galactic and Extragalactic Photo-Dissociation Regions
• A model for atomic and molecular interstellar gas: The Meudon PDR code
• Large-Scale Structure of the Molecular Gas in Taurus Revealed by High Linear Dynamic Range Spectral Line Mapping
• The Definitive Abundance of Interstellar Oxygen
• A PDR-Code Comparison Study
• Enhanced cosmic-ray flux toward zeta Persei inferred from laboratory study of H3+ - e- recombination rate
• A FUSE Survey of Interstellar Molecular Hydrogen in Translucent Clouds
• The Millennium Arecibo 21-cm Absorption Line Survey. IV. Statistics of Magnetic Field, Column Density, and Turbulence
• Molecular hydrogen formation in the interstellar medium
• The distribution of cosmic-ray sources in the Galaxy, gamma-rays, and the gradient in the CO-to-H2 relation
• Ultraviolet Extinction Properties in the Milky Way
• Abundances and Behavior of 12CO, 13CO, and C2 in Translucent Sight Lines
• Direct Measurement of the Ratio of Carbon Monoxide to Molecular Hydrogen in the Diffuse Interstellar Medium
• HI Narrow Self-Absorption in Dark Clouds: Correlations with Molecular Gas and Implications for Cloud Evolution and Star Formation
• VLT/UVES Observations of Interstellar Molecules and Diffuse Bands in the Magellanic Clouds
• The Homogeneity of Interstellar Oxygen in the Galactic Disk
• The H II Region/PDR Connection: Self-Consistent Calculations of Physical Conditions in Star-Forming Regions
• Molecular Hydrogen in Star-forming regions: implementation of its micro-physics in Cloudy
• CN and HCN in Dense Interstellar Clouds
• Grain size distributions and photo-electric heating in ionized media
• Oxygen Gas Phase Abundance Revisited
• A Self-Absorption Census of Cold HI Clouds in the Canadian Galactic Plane Survey
• Formation, fractionation and excitation of carbon monoxide in diffuse clouds
• An Analysis of the Shapes of Ultraviolet Extinction Curves. IV. Extinction without Standards
• Cloud Structure and Physical Conditions in Star-Forming Regions from Optical Observations. II. Analysis
• Gas-phase recombination, grain neutralization and cosmic-ray ionization in diffuse gas
• Hubble Space Telescope Survey of Interstellar ^12CO/^13CO in the Solar Neighborhood
• Cloud Structure and Physical Conditions in Star-forming Regions from Optical Observations. I. Data and Component Structure
• Molecular gas in spiral galaxies: a new warm phase at large galactocentric distances?
• Further Evidence for Chemical Fractionation from Ultraviolet Observations of Carbon Monoxide
• FUSE Measurements of Interstellar Fluorine
• New Insights on Interstellar Gas-Phase Iron
• Non-Thermal Chemistry in Diffuse Clouds with Low Molecular Abundances
• "Missing Link" Clouds in the Southern Galactic Plane Survey
• Oscillator Strengths and Predissociation Rates for Rydberg Transitions in 12C16O, 13C16O, and 13C18O Involving the E 1Pi, B 1Sigma+, and W 1Pi States
• Oscillator Strengths for B-X, C-X, and E-X Transitions in Carbon Monoxide
• Effects of turbulent diffusion on the chemistry of diffuse clouds
• FUSE Measurements of Far Ultraviolet Extinction. I. Galactic Sight Lines
• Abundances and Physical Conditions in the Interstellar Gas toward HD 192 639
• Density Variations over Subparsec Scales in Diffuse Molecular Gas
• Physical conditions in the ISM towards HD185418
• The Nature of Interstellar Gas toward the Pleiades Revealed in Absorption Lines
• Physical Conditions in the Foreground Gas of Reflection Nebulae: NGC 2023, vdB 102, and NGC 7023
• Reanalysis of Copernicus Measurements on Interstellar Carbon Monoxide
• FUSE Measurements of Rydberg Bands of Interstellar CO between 925 and 1150 A
• Oscillator strengths for transitions to Rydberg levels in ${}^{12}{C}^{16}O$, ${}^{13}{C}^{16}O$ and ${}^{13}{C}^{18}O$ between 967 and 972 A
• CO in HI Self-Absorbed Clouds in Perseus
• CH 3 GHz Observations of Molecular Clouds Along the Galactic Plane

Cited by in corpus (182)

• The CO-to-H2 Conversion Factor
• A Unified Representation of Gas-Phase Element Depletions in the Interstellar Medium
• The Dark Molecular Gas
• The photodissociation and chemistry of CO isotopologues: applications to interstellar clouds and circumstellar disks
• The Cosmic Origins Spectrograph
• Investigating the Cosmic-Ray Ionization Rate in the Galactic Diffuse Interstellar Medium through Observations of H3+
• The relation between gas and dust in the Taurus Molecular Cloud
• CO-dark gas and molecular filaments in Milky Way type galaxies
• Modeling CO Emission: I. CO as a Column Density Tracer and the X-Factor in Molecular Clouds
• The Plateau de Bure + 30m Arcsecond Whirlpool Survey reveals a thick disk of diffuse molecular gas in the M51 galaxy
• Photodissociation and X-Ray Dominated Regions
• Water in star-forming regions (WISH): Physics and chemistry from clouds to disks as probed by Herschel spectroscopy
• A Herschel [C II] Galactic plane survey II: CO-dark H2 in clouds
• The First Positive Detection of Molecular Gas in a GRB Host Galaxy
• Models of turbulent dissipation regions in the diffuse interstellar medium
• Surface chemistry in the interstellar medium - I - H2 formation by Langmuir-Hinshelwood and Eley-Rideal mechanisms
• Interstellar Hydrides
• The Rest-Frame Submillimeter Spectrum of High-Redshift, Dusty, Star-Forming Galaxies
• Comparative study of CH+ and SH+ absorption lines observed towards distant star-forming regions
• A translucent interstellar cloud at z=2.69: CO, H2 and HD in the line-of-sight to SDSS J123714.60+064759.5
• The CO luminosity and CO-H2 conversion factor of diffuse ISM: does CO emission trace dense molecular gas?
• Sulphur-bearing molecules in diffuse molecular clouds: new results from SOFIA/GREAT and the IRAM 30 m telescope
• A Simple and Accurate Network for Hydrogen and Carbon Chemistry in the ISM
• Detection of hydrogen fluoride absorption in diffuse molecular clouds with Herschel/HIFI: a ubiquitous tracer of molecular gas
• [CII] absorption and emission in the diffuse interstellar medium across the Galactic Plane
• Diffuse interstellar bands in Upper Scorpius: Probing variations in the DIB spectrum due to changing environmental conditions
• Photodissociation of interstellar N2
• Chemical probes of turbulence in the diffuse medium: the TDR model
• GASKAP -- The Galactic ASKAP Survey
• Discovery of interstellar mercapto radicals (SH) with the GREAT instrument on SOFIA
• Non-equilibrium chemistry and cooling in the diffuse interstellar medium - II. Shielded gas
• Diffuse molecular gas at high redshift: Detection of CO molecules and the 2175 Å dust feature at z=1.64
• Investigating the Cosmic-Ray Ionization Rate Near the Supernova Remnant IC 443 Through H3+ Observations
• Nitrogen hydrides in interstellar gas II. Analysis of Herschel/HIFI observations towards W49N and G10.6-0.4 (W31C)
• Full-disc ${}^{13}$CO(1-0) mapping across nearby galaxies of the EMPIRE survey and the CO-to-H${}_2$ conversion factor
• Carbon Fractionation in PDRs
• SILCC-Zoom: H${}_2$ and CO-dark gas in molecular clouds -- The impact of feedback and magnetic fields
• The Temperature of Interstellar Clouds from Turbulent Heating
• Cosmic rays, gas and dust in nearby anticentre clouds : I -- CO-to-H2 conversion factors and dust opacities
• The X_CO conversion factor from galactic multiphase ISM simulations
• CO-dark molecular gas at high redshift: very large H${}_2$ content and high pressure in a low metallicity damped Lyman-alpha system
• H2 distribution during formation of multiphase molecular clouds
• Metallicity dependence of the H/H${}_2$ and C${}^{+}$/C/CO distributions in a resolved self-regulating interstellar medium
• Interstellar CN and CH+ in Diffuse Molecular Clouds: 12C/13C Ratios and CN Excitation
• An ALMA Early Science survey of molecular absorption lines toward PKS1830-211 -- Analysis of the absorption profiles
• Imaging diffuse clouds: Bright and dark gas mapped in CO
• The abundance of C3H2 and other small hydrocarbons in the diffuse interstellar medium
• CO/H2 Abundance Ratio ~ 10^{-4} in a Protoplanetary Disk
• UV-driven chemistry in simulations of the interstellar medium. I. Post-processed chemistry with the Meudon PDR code
• Atomic and Molecular Carbon as a Tracer of Translucent Clouds
• The Far-Ultraviolet "Continuum" in Protoplanetary Disk Systems II: CO Fourth Positive Emission and Absorption
• The MPIfR-MeerKAT Galactic Plane survey I -- System setup and early results
• Anomalous Diffuse Interstellar Bands in the Spectrum of Herschel 36. I. Observations of Rotationally Excited CH and CH+ Absorption and Strong, Extended Redward Wings on Several DIBs
• The Physical Drivers and Observational Tracers of CO-to-H2 Conversion Factor Variations in Nearby Barred Galaxy Centers
• Thermal Pressure in the Cold Neutral Medium of Nearby Galaxies
• Diffuse Interstellar Bands vs. Known Atomic and Molecular Species in the Interstellar Medium of M82 toward SN 2014J
• A Far Ultraviolet Spectroscopic Explorer Survey of Interstellar Molecular Hydrogen in the Galactic Disk
• Diffuse Molecular Cloud Densities from UV Measurements of CO Absorption
• CO excitation, molecular gas density and interstellar radiation field in local and high-redshift galaxies
• The Galactic Dust-Up: Modeling Dust Evolution in FIRE
• Gamma-ray observations of the Orion Molecular Clouds with the Fermi Large Area Telescope
• Chemical Analysis of a Diffuse Cloud along a Line of Sight Toward W51: Molecular Fraction and Cosmic-Ray Ionization Rate
• Spatial distribution of small hydrocarbons in the neighborhood of the Ultra Compact HII region Monoceros R2
• 3D chemical structure of diffuse turbulent ISM. I. Statistics of the HI-to-H${}_2$ transition
• Optically thick HI does not dominate dark gas in the local ISM
• Hydrogen Fluoride in High-Mass Star-forming Regions
• Characterizing the transition from diffuse atomic to dense molecular clouds in the Magellanic clouds with [CII], [CI], and CO
• Far-infrared study of tracers of oxygen chemistry in diffuse clouds
• Molecular gas and star formation in an absorption-selected galaxy: Hitting the bull's eye at z = 2.46
• Diffuse Atomic and Molecular Gas in the Interstellar Medium of M82 toward SN 2014J
• Molecular ion abundances in the diffuse ISM : CF+, HCO+, HOC+, and C3H+
• OH+ and H2O+ absorption toward PKS1830-211
• Planck intermediate results. XXVIII. Interstellar gas and dust in the Chamaeleon clouds as seen by Fermi LAT and Planck
• Herschel observations of extra-ordinary sources: Detecting spiral arm clouds by CH absorption lines
• First detection of 13CH in the interstellar medium
• Chemical complexity in local diffuse and translucent clouds: ubiquitous l-C3H and CH3CN, a detection of HC3N and an upper limit on the abundance of CH2CN
• Hydrogen chloride in diffuse interstellar clouds along the line of sight to W31C (G10.6-0.4)
• Synthetic CO, H2 and HI surveys of the Galactic 2nd Quadrant, and the properties of molecular gas
• The EDIBLES survey IV. Cosmic ray ionization rates in diffuse clouds from near-ultraviolet observations of interstellar OH${}^{+}$
• The Behavior of Selected Diffuse Interstellar Bands with Molecular Fraction in Diffuse Atomic and Molecular Clouds
• Direct Determination of the HF/H2 Abundance Ratio in Interstellar Gas
• Constraining the Environment of CH+ Formation with CH3+ Observations
• DIGIT: Herschel and Spitzer spectro-imaging of SMM3 and SMM4 in Serpens
• OH+ in Diffuse Molecular Clouds
• Unveiling the chemistry of interstellar CH: Spectroscopy of the 2 THz $N=2\leftarrow 1$ ground state line
• The C${}^{14}$N/C${}^{15}$N Ratio in Diffuse Molecular Clouds
• Herschel/HIFI Spectral Mapping of C${}^{+}$, CH${}^{+}$, and CH in Orion BN/KL: The Prevailing Role of Ultraviolet Irradiation in CH${}^{+}$ Formation
• PDFchem: A new fast method to determine ISM properties and infer environmental parameters using probability distributions
• Small-scale structure in the interstellar medium: time-varying interstellar absorption towards κ Velorum
• CH in absorption in IRAS16293-2422
• A large amount of diffuse molecular gases in the bar of the strongly barred galaxy NGC1300: Cause of the low star formation efficiency
• Simulating the atomic and molecular content of molecular clouds using probability distributions of physical parameters
• Thermal Pressures in the Interstellar Medium of the Magellanic Clouds
• Cosmic-rays, gas, and dust in nearby anticentre clouds : II -- Interstellar phase transitions and the Dark Neutral Medium
• CH as a Molecular Gas Tracer and C-Shock Tracer Across a Molecular Cloud Boundary in Taurus
• Sensitivity of rotational transitions in CH and CD to a possible variation of fundamental constants
• Spatially extended and high-velocity dispersion molecular component in spiral galaxies: single-dish vs. interferometric observations
• A Herschel/HIFI Legacy Survey of HF and H2O in the Galaxy: Probing Diffuse Molecular Cloud Chemistry
• The CH+ Abundance in Turbulent, Diffuse Molecular Clouds
• An Outer Arm in the Second Galactic Quadrant: Structure
• Observational Properties of Rotationally Excited Molecular Hydrogen In Translucent Lines of Sight
• Ultraviolet Measurements of Interstellar C2
• A Unique View of AGN-Driven Molecular Outflows: The Discovery of a Massive Galaxy Counterpart to a $z=2.4$ High-Metallicity Damped Lyman-$\alpha$ Absorber
• Stacking Analysis of 12CO and 13CO Spectra of NGC3627: Existence of non-optically thick 12CO emission?
• A proto-pseudobulge in ESO 320-G030 fed by a massive molecular inflow driven by a nuclear bar
• Small-scale physical and chemical structure of diffuse and translucent molecular clouds along the line of sight to Sgr B2
• The Molecular Chemistry of Diffuse and Translucent Clouds in the Line-of-Sight to Sgr B2
• Standing in the shadow of dark gas: ALMA observations of absorption from dark CO in the molecular DNM of Chamaeleon
• Molecular Gas and Dark Neutral Medium in the Outskirts of Chamaeleon
• HD 62542: Probing the Bare, Dense Core of a Translucent Interstellar Cloud
• The Parsec-Scale Relationship Between I_CO and A_V in Local Molecular Clouds
• Reproducing the CO-to-H${}_2$ conversion factor in cosmological simulations of Milky Way-mass galaxies
• Widespread Galactic CF+ absorption: detection toward W49 with the Plateau de Bure Interferometer
• 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
• Abundances and Depletions of Neutron-Capture Elements in the Interstellar Medium
• Dense molecular clouds in the SN2008fp host galaxy
• A High-Resolution Study of the CO-H2 Conversion Factor in the Diffuse Cloud MBM 40
• First Extragalactic Detection of Far-Infrared CH Rotational Lines from the Herschel Space Observatory
• Herschel CHESS discovery of the fossil cloud that gave birth to the Trapezium and Orion KL
• Carbon isotope fractionation and depletion in TMC1
• The Distribution of Metallicities in the Local Galactic Interstellar Medium
• Fingerprinting the effects of hyperfine structure on CH and OH far infrared spectra using Wiener filter deconvolution
• Formation and Fractionation of CO (carbon monoxide) in diffuse clouds observed at optical and radio wavelengths
• Molecular hydrogen and its proxies HCO${}^{+}$ and CO in the diffuse interstellar medium
• Dependence of Chemical Abundance on the Cosmic Ray Ionization Rate in IC 348
• CH radio emission from heiles cloud 2 as a tracer of molecular cloud evolution
• The Abundance of Boron in Diffuse Interstellar Clouds
• New constraints on the physical conditions in H${}_2$-bearing GRB-host damped Lyman-$\alpha$ absorbers
• SUNRISE: The rich molecular inventory of high-redshift dusty galaxies revealed by broadband spectral line surveys
• Herschel/HIFI observations of spectrally resolved methylidyne signatures toward the high-mass star-forming core NGC6334I
• Abundance ratios of OH/CO and HCO+/CO as probes of the cosmic ray ionization rate in diffuse clouds
• The CH radical at radio wavelengths: Revisiting emission in the 3.3GHz ground state lines
• Interstellar Extinction and Elemental Abundances: Individual Sight Lines
• Extending the view of ArH+ chemistry in diffuse clouds
• Molecular gas in absorption and emission along the line of sight to W31C G10.62-0.38
• Fourier-transform spectroscopy of ${}^{13}$C${}^{17}$O and deperturbation analysis of the A${}^1\Pi$ ($\nu$ = 0 - 3) levels
• Where is the warm H2 ? A search for H2 emission from disks around Herbig Ae/Be stars
• Interstellar H2 toward HD 147888
• Cosmo-tomography toward PKS1830-211: Variability of the quasar and of its foreground molecular absorption monitored with ALMA
• CO+ as a probe of the origin of CO in diffuse interstellar clouds
• Hunting for the elusive methylene radical
• The Transition from Diffuse Molecular Gas to Molecular Cloud Material in Taurus
• A Reexamination of Phosphorus and Chlorine Depletions in the Diffuse Interstellar Medium
• First Detection of HCO${}^{+}$ Absorption in the Magellanic System
• Three-dimensional distribution of hydrogen fluoride gas toward NGC6334 I and I(N)
• The 7Li/6Li Isotope Ratio Near the Supernova Remnant IC 443
• Revisiting the OH-CH correlation in diffuse clouds
• Diffuse Interstellar Bands: How Are They Related to Known Gas-Phase Constituents of the ISM?
• Lifetimes and Oscillator Strengths for Ultraviolet Transitions in Singly-Ionized Lead
• HYACINTH: HYdrogen And Carbon chemistry in the INTerstellar medium in Hydro simulations
• Absorption Line Observations of H${}_3^{+}$ and CO in Sight Lines Toward the Vela and W28 Supernova Remnants
• Diffuse Atomic and Molecular Gas near IC443
• The Interstellar Line of Sight to the Interacting Galaxy NGC 5195
• CO enhancement by magnetohydrodynamic waves; Striations in the Polaris Flare
• Using deuterated H3+ and other molecular species to understand the formation of stars and planets
• Thermal Pressure in Diffuse H2 Gas Measured by Herschel [C II] Emission and FUSE UV H2 Absorption
• High-velocity interstellar absorption associated with the supernova remnant W28
• Mapping the Cosmic-Ray Ionization Rate in the Local Galaxy with H${}_3^{+}$
• Vibrationally Excited Molecular Hydrogen Near Herschel 36
• High-Resolution M-band Spectroscopy of CO towards the Massive Young Stellar Binary W3 IRS5
• A new measurement of the Galactic ${}^{12}$C/${}^{13}$C gradient from sensitive HCO${}^{+}$ absorption observations
• Precise Measurements of CH Maser Emission and Its Abundance in Translucent Clouds
• The evolution of CH in Planck Galactic Cold Clumps
• The Connection Between Different Tracers Of The Diffuse Interstellar Medium: Kinematics
• A CO-to-H${}_2$ ratio of $\approx 10^{-5}$ towards the Herbig Ae star HK Ori
• Revisiting rotationally excited CH at radio wavelengths: A case study towards W51
• Shocks in the warm neutral medium II -- Origin of neutral carbon at high pressure
• ALMACAL X: Constraints on molecular gas in the low-redshift circumgalactic medium
• Investigating the OH-H2 relation in diffuse Galactic clouds
• Revisiting the Temperature of the Diffuse ISM with CHESS Sounding Rocket Observations
• Boron Abundances in Diffuse Interstellar Clouds
• A sub-ppm upper limit on the cosmological variations of the fine structure constant alpha
• First detection of deuterated methylidyne (CD) in the interstellar medium
• Spectral survey of the diffuse gas toward BL Lac in the Q band
• Physical Properties and HI-to-H2 Transition Across Taurus Linear Edge
• What microwave astronomical spectroscopy can tell you about the carriers of the DIBS
• First detections of CO absorption in the Magellanic Clouds and direct measurement of the CO-to-H${}_2$ ratio
• First Search for Low-Frequency CH with a Square Kilometre Array Precursor Telescope
• Heteronuclear diatomics in diffuse and translucent clouds
• First Search for Pulsed CH Maser Emission Stimulated by a Pulsar
• Interstellar Hydrocarbons: Degradation Chemistry in Diffuse Clouds
• A Definitive Determination of the Interstellar Carbon Abundance toward rho Ophiuchi A and B