Direct Measurement of the Ratio of Carbon Monoxide to Molecular Hydrogen in the Diffuse Interstellar Medium

We have used archival far-ultraviolet spectra from observations made by the Space Telescope Imaging Spectrograph (STIS) of the Hubble Space Telescope and the Far Ultraviolet Spectroscopic Explorer (FUSE ) to determine the column densities and rotational excitation temperatures for carbon monoxide and molecular hydrogen, respectively, along the lines of sight to 23 Galactic O and B stars. The sightlines have reddening values in the range E(B − V ) = 0.07–0.62, thus sampling the diffuse to translucent interstellar medium. We find that the H2 column densities range from 5× 10 18 − 8× 10 cm and the CO from upper limits around 2×10 cm to detections as high as 1.4×10 cm. CO increases with increasing H2, roughly following a power law of factor ∼ 2. The CO/H2 column density ratio is thus not constant, and ranges from 10 − 10, with a mean value of 3× 10. The sample segregates into ”diffuse” and ”translucent” regimes, the former having a molecular fraction less than ∼ 0.25 andAV /d < 1 mag kpc . The mean CO/H2 for these two regimes are 3.6×10 −7 and 9.3×10, respectively. These values are significantly lower than the canonical dark cloud value of 10. In six of the sightlines, the isotopic variant CO is observed, and the isotopic ratio we observe (∼ 50−70) is consistent with, if perhaps a little below, the average C/C for the interstellar medium at large. The average H2 rotational excitation temperature is 74± 24 K, in good agreement with previous studies, and the average CO temperature is 4.1 K, with some sightlines showing temperatures as high as 6.4 K. The higher excitation CO is observed with higher column densities, consistent with the effects of photon trapping in clouds with densities in the 20-100 cm range. We discuss the implications for the structure of the diffuse/translucent regimes of the interstellar medium and the estimation of molecular mass in galaxies. Subject headings: ISM: abundances, ISM: clouds, ISM: lines and bands, ISM: molecules, ISM: structure