CO emission and CO hotspots in diffuse molecular gas

Abstract

We observed $\lambda$3mm \cotw, \coth, \coei, \hcop, HCN and CS emission from diffuse molecular gas along sightlines with \EBV\ $\approx$ 0.1 - 1 mag. Directions were mostly chosen for their proximity to sightlines toward background mm-wave continuum sources studied in \hcop\ absorption, at positions where maps of \cotw\ at 1\arcmin\ resolution showed surprisingly bright integrated CO J=1-0 emission \WCO\ = 5-12 K-\kms\ but we also observed in L121 near \zoph. Coherence emerges when data are considered over a broad range of \cotw\ and \coth\ brightness. \WCO/\Wth\ and N(\cotw)/N(\coth) are 20-40 for \WCO\ $\la 5$ K-\kms\ and N(CO) $\la 5\times 10^{15}\pcc$, increasing with much scatter for larger \WCO\ or N(CO). N(\coth)/N(\coei) $>20-40$ ($3\sigma$) vs. an intrinsic ratio ${}^{13}$C/${}^{18}$O = 8.4, from a combination of selective photodissociation and enhancement of \coth. The observations are understandable if \cotw\ forms from the thermal recombination of \hcop\ with electrons, after which the observed \coth\ forms via endothermic carbon isotope exchange with ${}^{13}$C\p. \WCS/\WCO\ increases abruptly for \WCO\ $\ga 10$ K-\kms\ and \WCS/\Whcop\ is bimodal, showing two branches having N(CS)/N(\hcop) $\approx 5$ and 1.25. Because CO formation and \hcop\ excitation both involve collisions between \hcop\ and ambient electrons, comparison of the CO and \hcop\ emission shows that the CO hotspots are small regions of enhanced N(CO) occupying only a small fraction of the column density of the medium in which they are embedded. \hcop/CO and HCN/CO brightness ratios are 1-2\% with obvious implications for determinations of the true dense gas fraction.

Accepted to ApJ

References in corpus (15)

• The Critical Density and the Effective Excitation Density of Commonly Observed Molecular Dense Gas Tracers
• A Large Catalog of Accurate Distances to Local Molecular Clouds: The Gaia DR2 Edition
• Isotopic fractionation of carbon, deuterium and nitrogen : a full chemical study
• Chemical probes of turbulence in the diffuse medium: the TDR model
• Galactic interstellar 18O/17O ratios - a radial gradient?
• Formation, fractionation and excitation of carbon monoxide in diffuse clouds
• Modeling the Anomalous Microwave Emission with Spinning Nanoparticles: No PAHs Required
• Waves on the surface of the Orion molecular cloud
• ${}^{13}$CO/C${}^{18}$O Gradients Across the Disks of Nearby Spiral Galaxies
• Striations in the Taurus molecular cloud: Kelvin-Helmholtz instability or MHD waves?
• Chemical Abundances in a Turbulent Medium -- H${}_2$, OH${}^{+}$, H${}_2$O${}^{+}$, ArH${}^{+}$
• HD/H2 as a probe of the roles of gas, dust, light, metallicity and cosmic rays in promoting the growth of molecular hydrogen in the diffuse interstellar medium
• Standing in the shadow of dark gas: ALMA observations of absorption from dark CO in the molecular DNM of Chamaeleon
• Formation and Fractionation of CO (carbon monoxide) in diffuse clouds observed at optical and radio wavelengths
• A CO J=1-0 Survey of common optical/uv absorption sightlines

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