All Figures

Figure 1: Top: variation of CO and H2 column densities observed in uv (rectangles; Sonnentrucker et al. 2007; and Burgh et al. 2007) and mm-wave (triangles; Liszt & Lucas 1998) CO absorption. Lines of sight represented in Fig. 5 are outlined and shown in green. For the mm-wave data N(H2) = N(HCO+)/2  10-9. The curves represent models of CO formation via recombination of HCO+ and are labelled by their density n(H)  n(H I) + 2n(H2) (see Sect. 4). The lowest curve shows the result of halving the HCO+ abundance at n(H) = . Bottom: variation of CH and H2 column densities from the summary tables of Sonnentrucker et al. (2007) and a few high-N(CH) datapoints from Gredel et al. (1993) and Welty et al. (2003).

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In the text

	Figure 2: Variation of X(12CO) = N(12CO)/N(H2) with N(H2). The regression line shown has slope 1.014  0.13 and passes through X(12CO) = 4.05  10-7 for N(H2) = . Lines of sight having uv absorption measurements of both 12CO and 13CO are noted (red in appropriate media); they have relatively high X(12CO) at a given N(H2).
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In the text

Figure 3: Left: variation of N(12CO)/N(13CO) with N(H2) ( left) and N(12CO). Where possible points are labeled with estimates of K from H2 ( at left) and C2 ( right) as tabulated by Burgh et al. (2007) and Sheffer et al. (2007) for H2 and Sonnentrucker et al. (2007) for H2 and C2. Radiofrequency data are those of Liszt & Lucas (1998). There is no tendency for the column density ratio to vary monotonically with the H2 temperature and only a very loose trend with that of C2.

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In the text

Figure 4: Variation of N(12CO)/N(13CO) with X(CO) = N(12CO)/N(H2) compared with the model chemistry (curves) described in Sect. 4.5 of the text. Models are for I=3, K = 45 K, n(H2) = 50  ; I=1, K = 45 K, n(H2) = 125  ; I=1/3, K = 22.5 K, n(H2) = 125  (upper of two curves) and 250  . As in Fig. 3 optical data are labelled with the kinetic temperature derived from C2 as tabulated by Sonnentrucker et al. (2007).

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In the text

Figure 5: Variation of 12CO J=1-0 rotational excitation temperature plotted against N(H2) ( left) and N(12CO). Radio data are shown as lower limits because they involve the assumption of a beam efficiency. Data from the work of Burgh et al. (2007) and Sonnentrucker et al. (2007) are shown separately; sightlines with measurements in both references are shown chained, note that the former typically derives slightly stronger excitation in these cases. Data labelled "radio'' are those of Liszt & Lucas (1998). Superposed on the data are calculated results for uniform-density spherical models having total densites n(H) = 64 and 128  like those used to determine N(CO) in Fig. 1, where only a handful of the sightlines represented here require n(H) < . The lower curves at each density employ an older but likely more nearly correct set of rotational excitation rates for H-atom + CO collisions (see Sect. 6 of the text). A minimum excitation temperature of 4.0 K is required to produce a  mm emission line having a brightness temperature B = 1 K.

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In the text

Figure 6: Integrated 12CO J=1-0  mm brightness temperatures predicted from uv absorption and/or observed at  mm, plotted against N(H2) ( left) and N(12CO) ( right). The shaded line at left represents a CO-H2 conversion factor N(H2) = 2  /K-km s-1. Direct radio observations toward Oph are also shown chained to the equivalent optical datapoint. Data from the work of Burgh et al. (2007) and Sonnentrucker et al. (2007) are shown separately and the radio observations are those of (Liszt & Lucas 1998) at right; see Sect. 6.1 for an explanation of the radio data plotted at left.

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In the text