We observed \lambda 3mm \mathrm { { } ^ { 12 } CO } , \mathrm { { } ^ { 13 } CO } , \mathrm { C ^ { 18 } O } , \mathrm { HCO ^ { + } } , HCN and CS emission from diffuse molecular gas along sightlines with E _ { B - V } \approx 0.1 - 1 mag .
Directions were mostly chosen for their proximity to sightlines toward background mm-wave continuum sources studied in \mathrm { HCO ^ { + } } absorption , at positions where maps of \mathrm { { } ^ { 12 } CO } at 1 ^ { \prime } 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 \zeta Oph .
Coherence emerges when data are considered over a broad range of \mathrm { { } ^ { 12 } CO } and \mathrm { { } ^ { 13 } CO } brightness . { W } _ { CO } / { W } _ { 13 } and N ( \mathrm { { } ^ { 12 } CO } ) /N ( \mathrm { { } ^ { 13 } CO } ) are 20-40 for { W } _ { CO } \la 5 K-km s ^ { -1 } and N ( CO ) \la 5 \times 10 ^ { 15 } ~ { } { cm } ^ { -2 } , increasing with much scatter for larger { W } _ { CO } or N ( CO ) .
N ( \mathrm { { } ^ { 13 } CO } ) /N ( \mathrm { C ^ { 18 } O } ) > 20 - 40 ( 3 \sigma ) vs. an intrinsic ratio ^ { 13 } C/ ^ { 18 } O = 8.4 , from a combination of selective photodissociation and enhancement of \mathrm { { } ^ { 13 } CO } .
The observations are understandable if \mathrm { { } ^ { 12 } CO } forms from the thermal recombination of \mathrm { HCO ^ { + } } with electrons , after which the observed \mathrm { { } ^ { 13 } CO } forms via endothermic carbon isotope exchange with ^ { 13 } C ^ { + } . { W } _ { CS } / { W } _ { CO } increases abruptly for { W } _ { CO } \ga 10 K-km s ^ { -1 } and { W } _ { CS } /W _ { \mathrm { HCO ^ { + } } } is bimodal , showing two branches having N ( CS ) /N ( \mathrm { HCO ^ { + } } ) \approx 5 and 1.25 .
Because CO formation and \mathrm { HCO ^ { + } } excitation both involve collisions between \mathrm { HCO ^ { + } } and ambient electrons , comparison of the CO and \mathrm { HCO ^ { + } } 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 . \mathrm { HCO ^ { + } } /CO and HCN/CO brightness ratios are 1-2 % with obvious implications for determinations of the true dense gas fraction .