Ultraviolet Survey of CO and H2 in Diffuse Molecular Clouds: The Reflection of Two Photochemistry Regimes in Abundance Relationships

By Y. Sheffer, M. Rogers, S. Federman, N. Abel, R. Gredel, D. Lambert, G. Shaw. 2008

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We carried out a comprehensive far-UV survey of 12CO and H2 column densities along diffuse molecular Galactic sight lines. This sample includes new measurements of CO from HST spectra along 62 sight lines and new measurements of H2 from FUSE data along 58 sight lines. In addition, high-resolution optical data were obtained at the McDonald and European Southern Observatories, yielding new abundances for CH, CH+, and CN along 42 sight lines to aid in interpreting the CO results. These new sight lines were selected according to detectable amounts of CO in their spectra and provide information on both lower density (≤100 cm−3) and higher density diffuse clouds. A plot of log N(CO) versus log N(H2) 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(H2) = 20.4, corresponding to a change in production route for CO in higher density gas. Similar logarithmic plots among all five diatomic molecules reveal additional examples of dual slopes in the cases of CO versus CH (break at log N = 14.1, 13.0), CH+ versus H2 (13.1, 20.3), and CH+ versus CO (13.2, 14.1). We employ both analytical and numerical chemical schemes in order to derive details of the molecular environments. In the denser gas, where C2 and CN molecules also reside, reactions involving C+ and OH are the dominant factor leading to CO formation via equilibrium chemistry. 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.

Subfields: Astronomy and Astrophysics, Atmospheric Science, Spectroscopy

View Paper: https://www.semanticscholar.org/paper/1fafeb429a0c68f76c384c232d8c633a2558a98b

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Citations in the corpus

CO-dark gas and molecular filaments in Milky Way-type galaxies (2014, methodology): “…involved in reproducing the observed NCO–NH2 relationship at low NH2 with standard PDR models (see e.g. the detailed discussion in Sonnentrucker et al. 2007), a problem which may point to the importance of non-thermal chemistry in these regions (Zsargó & Federman 2003; Sheffer et al. 2008).”
Molecular Clouds in the Milky Way (2015, background): “The primary chemical precursors are CO+ in the diffuse gas regime and OH in regions of higher gas density (Sheffer et al. 2008, Visser et al. 2009).”
THE FIRST POSITIVE DETECTION OF MOLECULAR GAS IN A GRB HOST GALAXY (2009, methodology): “These values are consistent with detailed models of CO photodissociation in translucent environments (e.g., model T3 in van Dishoeck & Black 1988), and with the transition region between diffuse and dark clouds in the correlation plot of CO versus H2 (Sheffer et al. 2008).”
The Nine Lives of Cosmic Rays in Galaxies (2015, background): “Large XCO fluctuations may result from local imbalance between CO formation and dissociation in the optically thin envelopes (Glover & Clark 2012, Lee et al. 2014, Sheffer et al. 2008), but they may average out along sightlines (Liszt, Pety & Lucas 2010).”
The photodissociation and chemistry of CO isotopologues: applications to interstellar clouds and circumstellar disks (2009, methodology): “7 shows the 100-point means of the full set, revealing the same two distinct regimes as found by Sheffer et al. (2008).”
Modelling CO emission – I. CO as a column density tracer and the X factor in molecular clouds (2010, background): “In particular, non-thermal chemistry in C-shocks or turbulent vortices may play an important role in the chemistry of the diffuse gas (see e.g. Federman et al. 1996; Sheffer et al. 2008; Godard et al. 2009).”
THE DARK MOLECULAR GAS (2010, background): “A mixture of H2 and C + is also inferred to exist in diffuse clouds where the H2 and CO columns are measured by UV absorption spectroscopy and the CO accounts for only a trace amount of C (Sonnentrucker et al. 2007; Burgh et al. 2007; Sheffer et al. 2008).”
THE RELATION BETWEEN GAS AND DUST IN THE TAURUS MOLECULAR CLOUD (2010, background): “The excitation temperature of the gas observed by Sheffer et al. (2008) does not show a large variation from Tex=5K while we observe values between 4 and 15K.”
A UNIFIED REPRESENTATION OF GAS-PHASE ELEMENT DEPLETIONS IN THE INTERSTELLAR MEDIUM (2009, unknown): “5 Vn GHS77 0.40 2.70 0.01 21.30 21.36 21.42 C++08 20.55 20.63 20.70 C++08 116658 α Vir 316.11 50.84 1.04 B1 IV B++08 0.10 0.08 0.06 18.90j 19.00j 19.10j YR76 12.65 12.95 13.25 Y76 116781 V967 Cen 307.05 −0.07 7.45 B0 IIIne B++08 0.31 2.2 0.00 21.08 21.18 21.28 JS07a 20.03 20.08 20.13 S++08a 116852 H”
INVESTIGATING THE COSMIC-RAY IONIZATION RATE IN THE GALACTIC DIFFUSE INTERSTELLAR MEDIUM THROUGH OBSERVATIONS OF H+3 (2011, methodology): “The preferred method uses column densities of CH determined from observations in combination with the relation N(CH)/N(H2) = 3.5 +2.1 −1.4 × 10−8 from Sheffer et al. (2008).”
Physical Processes in the Interstellar Medium (2014, background): “Indeed, this presents something of a problem, as the resulting CH+ formation rate is too slow to explain the observed CH+ abundance in the diffuse atomic ISM, possibly indicating that some form of non-thermal chemistry is active there (see e.g. Sheffer et al., 2008; Godard et al., 2009).”
The CO-to-H2 Conversion Factor (2013, methodology): “Indeed, Sheffer et al. (2008) analyze ZCO in Milky Way lines-of-sight, finding a steep ZCO ≈ 4.7× 10−6(N(H2)/1021 cm−2)2.07 for N(H2) > 2.5 × 1020 cm−2, with an order of magnitude scatter (see also Sonnentrucker et al. 2007).”