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 H _ { 2 } column densities range from 5 \times 10 ^ { 18 } -8 \times 10 ^ { 20 } cm ^ { -2 } and the CO from upper limits around 2 \times 10 ^ { 12 } cm ^ { -2 } to detections as high as 1.4 \times 10 ^ { 16 } cm ^ { -2 } .
CO increases with increasing H _ { 2 } , roughly following a power law of factor \sim 2 .
The CO/H _ { 2 } column density ratio is thus not constant , and ranges from 10 ^ { -7 } -10 ^ { -5 } , with a mean value of 3 \times 10 ^ { -6 } .
The sample segregates into ” diffuse ” and ” translucent ” regimes , the former having a molecular fraction less than \sim 0.25 and A _ { V } / d < 1 mag kpc ^ { -1 } .
The mean CO/H _ { 2 } for these two regimes are 3.6 \times 10 ^ { -7 } and 9.3 \times 10 ^ { -6 } , respectively .
These values are significantly lower than the canonical dark cloud value of 10 ^ { -4 } .
In six of the sightlines , the isotopic variant ^ { 13 } CO is observed , and the isotopic ratio we observe ( \sim 50 - 70 ) is consistent with , if perhaps a little below , the average ^ { 12 } C/ ^ { 13 } C for the interstellar medium at large .
The average H _ { 2 } rotational excitation temperature is 74 \pm 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 ^ { -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 .