Astronomy and Astrophysics, volume 305, 558-571 (1996/1-2)
Millimeter observations of 12CO and ^ 13^CO in the diffuse region around ζ Ophiuchi.
KOPP M., GERIN M., ROUEFF E. and LE BOURLOT J.
Abstract (from CDS):
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.
radio lines: ISM - ISM: abundances - ISM: molecules - ISM: clouds - ISM: individual objects: ζ Oph