CO emission and CO hotspots in diffuse molecular gas
Abstract
We observed λ3mm , , , , HCN and CS emission from diffuse molecular gas along sightlines with \ ≈ 0.1 - 1 mag. Directions were mostly chosen for their proximity to sightlines toward background mm-wave continuum sources studied in \ absorption, at positions where maps of \ at 1\ resolution showed surprisingly bright integrated CO J=1-0 emission \ = 5-12 K-\ but we also observed in L121 near . Coherence emerges when data are considered over a broad range of \ and \ brightness. /\ and N()/N() are 20-40 for \ 5 K-\ and N(CO) 5× 1015, increasing with much scatter for larger \ or N(CO). N()/N() > 20-40 (3σ) vs. an intrinsic ratio 13C/18O = 8.4, from a combination of selective photodissociation and enhancement of . The observations are understandable if \ forms from the thermal recombination of \ with electrons, after which the observed \ forms via endothermic carbon isotope exchange with 13C. /\ increases abruptly for \ 10 K-\ and /\ is bimodal, showing two branches having N(CS)/N() ≈ 5 and 1.25. Because CO formation and \ excitation both involve collisions between \ and ambient electrons, comparison of the CO and \ 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. /CO and HCN/CO brightness ratios are 1-2\% with obvious implications for determinations of the true dense gas fraction.
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