The ratio of carbon monoxide to molecular hydrogen in interstellar dark clouds

Abstract

Carbon monoxide and molecular hydrogen column densities are compared at various locations within 38 interstellar dark clouds. CO column densities were obtained from radio observations of the / = 1-> 0 transitions of the 12 C 16 0 and 13 C 16 0 isotopic species of the molecule. Corresponding H 2 column densities were inferred by means of visual extinctions derived from star counts, since it is argued that the standard gas-to-extinction ratio can be expected to remain valid in the clouds studied. For locations in the sources possessing line-of-sight visual extinctions in the approximate range 1.5 < < 10 mag, we find, on the average, the relationship JVh 2 (cm" 2) = (5.0 ± 2.5) x lO 6 #^ between molecular hydrogen and 13 CO LTE column densities. The carbon monoxide molecule can therefore be used as a quantitative "tracer" for the (directly unobservable) H 2 content of dark clouds. The above relationship implies that at least ~ 127 0 of the gas-phase carbon in the clouds studied is in the form of CO, provided that the clouds are assumed to be chemically homogeneous. Danger's ion-molecule chemistry for dark clouds appears to agree well with the present work if the fractionation channel of Watson, Anicich, and Huntress is included.