1996A&A...305..558K

Using the 15-m SEST submillimeter telescope, we have observed ¹²CO and ¹³CO in the diffuse molecular cloud lying close to ζ Oph, and searched for C¹⁸O, CS, CN and HCO⁺ at 2 particular positions. The ¹²CO(1-0) measurements show lines of the order of a few Kelvins in regions where the ¹³CO data seem to indicate diffuse material. We measure ¹²CO(1-0)/¹³CO(1-0) ratios of the order of 30 toward the positions of peak CO emission and lower limits of this ratio up to 65 in more diffuse regions. We compare these data with the predictions of a chemical model which solves consistently the thermal and chemical equilibrium of an interstellar cloud and computes the submillimeter and millimeter emergent line intensities of various atomic and molecular transitions. For the positions of peak CO emission, the model and observations agree satisfactorily for an isobaric cloud of pressure p=7.5x10⁴Kcm^–3 or an isochoric cloud of density n(H₂)=2500cm^–3 permeated by an UV radiation field enhanced by a factor of 5 compared to the standard interstellar radiation field (ISRF) of Mathis et al.. The excitation properties of CO are consistent with a visual magnitude of the order of 1.2. The upper limit found for CS is also reproduced if the gas phase sulfur abundance is lower than the recent HST determinations.