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

Abstract

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 carbon monoxide and molecular hydrogen, respectively, along 23 sight lines to Galactic O and B stars. The reddening values range from E(B À V ) ¼ 0:07 to 0.62, sampling the diffuse to translucent interstellar medium (ISM ). We find that the H 2 column densities range from 5 ; 10 18 to 8 ; 10 20 cm À2 and the CO from upper limits around 2 ; 10 12 cm À2 to detections as high as 1:4 ; 10 16 cm À2 . CO increases with increasing H 2 , roughly following a power law of factor $2. The CO/ H 2 column density ratio is thus not constant, ranging from 10 À 7 to 10 À5 , with a mean value of 3 ; 10 À6 . The sample segregates into ''diffuse'' and ''translucent'' regimes, the former with molecular fraction P0.25 and A V /d < 1 mag kpc À1 . The mean CO/ H 2 for these two regimes are 3:6 ; 10 À7 and 9:3 ; 10 À6 , respectively, significantly lower than the canonical dark cloud value of 10 À4 . Six sight lines show the isotopic variant 13 CO, and the isotopic ratio we observe ($50Y70) is consistent with, if perhaps a little below, the average 12 C/ 13 C for the ISM at large. The average H 2 rotational excitation temperature is 74 AE 24 K, agreeing well with previous studies, and the average CO temperature is 4.1 K, with some sight lines 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 20Y100 cm À3 range. We discuss the implications for the structure of the diffuse/translucent regimes of the ISM and the estimation of molecular mass in galaxies.

The evolution of the cosmic microwave background temperature

“…The excitation temperatures for the five CO-bearing systems are given in Table 1. At high redshift, the excitation of CO is dominated by radiative excitation as predicted in diffuse interstellar clouds (Warin et al 1996;Burgh et al 2007). Indeed, the excitation temperatures we measured are well above the mean temperature measured in the Galaxy for similar CO column densities ( T ex = 3.6 K, see Burgh et al 2007).…”

“…At high redshift, the excitation of CO is dominated by radiative excitation as predicted in diffuse interstellar clouds (Warin et al 1996;Burgh et al 2007). Indeed, the excitation temperatures we measured are well above the mean temperature measured in the Galaxy for similar CO column densities ( T ex = 3.6 K, see Burgh et al 2007). This is further supported by the low volume density of the gas derived from the analysis of H 2 and C 0 lines in two of these high-z absorption systems (Srianand et al 2008;Noterdaeme et al 2010).…”

“…Instead, we used a technique similar to that presented in Burgh et al (2007) and Prochaska et al (2009): a single excitation temperature was used as an external parameter to model the CO profile directly, again, using a Boltzmann distribution of the rotational level populations. The best fit-model was chosen from the minimum χ 2 obtained for a grid of total column densities and excitation temperatures (see right-hand side panels of Figs.…”

How does C+recombine in diffuse molecular gas?

“…Burgh et al (2010) include a summary of measurements of N(CO), seven of which are new in their work. For the remainder of the sightlines considered here, CO column densities were taken from the recent work of Sonnentrucker et al (2007), Burgh et al (2007) and Sheffer et al (2008), which had previously been summarized in Liszt (2007). We also include a small sample of sightlines observed in mm-wave CO emission and absorption, placing them in N(H 2 ) using observations of N(CO) and N(HCO + ) (Liszt & Lucas 1998) with X(HCO + ) = N(HCO + )/N(H 2 ) = 3 × 10 −9 (Liszt et al 2010) as in the models for CO formation discussed in Sect.…”

The Far-Ultraviolet “Continuum” in Protoplanetary Disk Systems. Ii. Carbon Monoxide Fourth Positive Emission and Absorption*

“…We provide this very brief review of A-X observations in other astrophysical environments because the literature in these fields is considerably more developed than what exists for protoplanetary disks. The absorption bands of CO have been long studied in the interstellar medium (ISM) (Federman et al 1980;Morton & Noreau 1994;Burgh et al 2007;Sheffer et al 2008). Absorption lines from cold CO have been seen in older debris disks (e.g., β Pic, age ∼ 8-20 Myr; Vidal-Madjar et al 1994;Roberge et al 2000) and in the disk of the Herbig Ae star AB Aur (age ≈ 2 Myr; Roberge et al 2001).…”