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 HST/STIS and FUSE to determine the column densities and rotational excitation temperatures for CO and H2, 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, sampling the diffuse to translucent interstellar medium. We find that the H2 column densities range from 5x10^18-8x10^20 cm^-2 and the CO from upper limits around 2x10^12 cm^-2 to detections as high as 1.4x10^16 cm^-2. 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^-7 - 10^-5, with a mean value of 3x10^-6. The sample segregates into "diffuse" and "translucent" regimes, the former having a molecular fraction less than ~0.25 and A_V/d<1 mag kpc^-1. The mean CO/H2 for these two regimes are 3.6x10^-7 and 9.3x10^-6, respectively, significantly lower than the canonical dark cloud value of 10^-4. In six of the sightlines, 13CO is observed, and the isotopic ratio we observe (~50-70) is consistent with, if perhaps a little below, the average 12C/13C for the ISM 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 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^-3 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. 
Eric B. Burgh (UW-Madison), Kevin France (JHU/CITA), Stephan R.   McCandliss (JHU)