Molecular Gas and Dark Neutral Medium in the Outskirts of Chamaeleon

Abstract

%context More gas is inferred to be present in molecular cloud complexes than can be accounted for by HI and CO emission, a phenomenon known as dark neutral medium (DNM) or CO-dark gas for the molecules. %aims To see if molecular gas can be detected in Chamaeleon when gas column densities in the DNM were inferred and CO emission was not detected. % methods We took 3mm absorption profiles of HCO+ and other molecules toward quasars across Chamaeleon, 1 of which had detectable CO emission. We derived N(H2) assuming N(HC+)/N(H2) = 3x10^{-9}. %results With the possible exception of 1 weak continuum target HCO+ absorption was detected in all directions, \cch\ in 8 and HCN in 4 directions. The sightlines divide in 2 groups according to their DNM content with 1 group of 8 directions having N(DNM) \ga 2x10^{20} \pcc and another group of 5 directions having N(DNM) < .5x10^{20}\pcc. The groups have comparable <N(HI)> in Chamaeleon 6-7 x 10^{20}\pcc and <N(H)/E(B-V) ~ 6-7x10^{21}\pcc/mag. They differ in having quite different <E(B-V)> 0.33 vs 0.18 mag, <N(DNM)> 3.3 vs .14 x 10^{20}\pcc and <2N(H2)> = 5.6 vs 0.8 x 10^{20} \pcc. Gas at more positive velocities is enriched in molecules and DNM. %conclusion Overall the quantity of H2 inferred from HCO+ fully accounts for the previously-inferred DNM along the studied sightlines. H2 is concentrated in the high-DNM group, where the molecular fraction is 46% vs. 13% otherwise and 38% overall. Thus, neutral gas in the outskirts of the complex is mostly atomic but the DNM is mostly molecular. Saturation of the HI emission may occur along 3 of the 4 sightlines having the largest DNM column densities but there is no substantial reservoir of 'dark' atomic or molecular gas that remains undetected as part of the inventory of dark neutral medium.

Accepted for A&A

References in corpus (16)

• Ultraviolet Survey of CO and H_2 in Diffuse Molecular Clouds: The Reflection of Two Photochemistry Regimes in Abundance Relationships
• Abundances and Behavior of 12CO, 13CO, and C2 in Translucent Sight Lines
• GASS: The Parkes Galactic All-Sky Survey. Update: improved correction for instrumental effects and new data release
• Direct Measurement of the Ratio of Carbon Monoxide to Molecular Hydrogen in the Diffuse Interstellar Medium
• Optically thick HI dominant in the local interstellar medium; an alternative interpretation to "dark gas"
• HI, CO, and Dust in the Perseus Cloud
• On the distance to the North Polar Spur and the local CO-H2 factor
• Cosmic-rays, gas, and dust in nearby anticentre clouds : II -- Interstellar phase transitions and the Dark Neutral Medium
• The detectability of mm-wave molecular rotational transitions
• Formation and Fractionation of CO (carbon monoxide) in diffuse clouds observed at optical and radio wavelengths
• The diffuse molecular component in the nuclear bulge of the Milky Way
• Molecular gas in absorption and emission along the line of sight to W31C G10.62-0.38
• New Detections of Galactic Molecular Absorption Systems toward ALMA Calibrator Sources
• ALMA observations of molecular absorption in four directions toward the Galactic bulge
• ALMA hints at the existence of an unseen reservoir of diffuse molecular gas in the Galactic bulge
• Sensitive CO(1-0) Survey in Pegasus-Pisces Reduces CO-Dark Gas Inventory by Factor of Two

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• Molecular hydrogen and its proxies HCO${}^{+}$ and CO in the diffuse interstellar medium
• Gas and Dust Properties in the Chamaeleon Molecular Cloud Complex based on the Optically Thick HI
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• The Dark Neutral Medium is (Mostly) Molecular Hydrogen
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