2020ApJ...897..104L

We observed λ3 mm ¹²CO, ¹³CO, C¹⁸O, HCO⁺, HCN, and CS emission from diffuse molecular gas along sight lines with E_B–V ≃ 0.1-1 mag. Directions were mostly chosen for their proximity to sight lines toward background millimeter-wave continuum sources studied in HCO⁺ absorption at positions where maps of ¹²CO at 1' resolution showed surprisingly bright integrated CO J = 1-0 emission, W_CO = 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 ¹²CO and ¹³CO brightness. In this work W_CO/W₁₃ and N(¹²CO)/N(¹³CO) are 20-40 for W_CO <= 5 K km s^–1 and N(CO) ≲ 5×10¹⁵cm^–2, increasing with much scatter for larger W_CO or N(CO). Here N(¹³CO)/N(C¹⁸O) > 20-40 (3σ) versus an intrinsic ratio ¹³C/¹⁸O = 8.4 from a combination of selective photodissociation and enhancement of ¹³CO. The observations are understandable if ¹²CO forms from the thermal recombination of HCO⁺ with electrons, after which the observed ¹³CO forms via endothermic carbon isotope exchange with ¹³C⁺. W_CS/W_CO increases abruptly for W_CO >= 10 K km s^–1, and W_CS/W_HCO⁺ is bimodal, showing two branches having N(CS)/N(HCO⁺) ≃5 and 1.25. Because CO formation and HCO⁺ excitation both involve collisions between HCO⁺ and ambient electrons, comparison of the CO and HCO⁺ 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 HCO⁺/CO and HCN/CO brightness ratios are 1%-2%, with obvious implications for determinations of the true dense gas fraction.