CO Emission and CO Hot Spots in Diffuse Molecular Gas

Abstract

We observed λ 3 mm , , , , HCN, and CS emission from diffuse molecular gas along sight lines with  ≈ 0.1–1 mag. Directions were mostly chosen for their proximity to sight lines toward background millimeter-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 km s −1 , but we also observed in L121 near ζ Oph. Coherence emerges when the data are considered over a broad range of and brightness. In this work / and N ( )/ N ( ) are 20–40 for  ≲ 5 K km s −1 and N (CO) , increasing with much scatter for larger or N (CO). Here N ( )/ N ( ) > 20–40 (3 σ ) versus an intrinsic ratio 13 C/ 18 O = 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 13 C + . / increases abruptly for  ≳ 10 K km s −1 , 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 hot spots are small regions of enhanced N (CO) occupying only a small fraction of the column density of the medium in which they are embedded. The /CO and HCN/CO brightness ratios are 1%–2%, with obvious implications for determinations of the true dense gas fraction.