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Table 1

Basic information on the CO rotational lines and [C I] 3P fine structure lines used in this paper.

Molecule Transition νrest Eup/k AUL ncrit
JU → JL (GHz)  (K) (s-1) (cm-3)
CO 1 → 0 115.271 5.5 7.20 × 10-8 2.4 × 102
2 → 1 230.538 16.6 6.91 × 10-7 2.1 × 103
3 → 2 345.796 33.2 2.50 × 10-6 7.6 × 103
4 → 3 461.041 55.3 6.12 × 10-6 1.8 × 104
5 → 4 576.268 83.0 1.22 × 10-5 3.6 × 104
6 → 5 691.473 116.2 2.14 × 10-5 6.3 × 104
7 → 6 806.652 154.9 3.42 × 10-5 1.0 × 105
8 → 7 921.800 199.1 5.13 × 10-5 1.5 × 105
9 → 8 1036.912 248.9 7.33 × 10-5 2.1 × 105
10 → 9 1151.985 304.2 1.00 × 10-4 2.9 × 105
11 → 10 1267.014 365.0 1.34 × 10-4 3.9 × 105
[C I] 3P1 → 3P0 492.161 23.6 7.88 × 10-8 4.9 × 102
3P2 → 3P1 809.342 62.4 2.65 × 10-7 9.3 × 102

Notes. Critical density ncrit,UL ≡ AUL/ Σi ≠ UγUi (e.g. Tielens 2005). AUL is the Einstein coefficient for spontaneous emission from level U to L, and γUi is the collision rate coefficient. The critical densities (ncrit) are calculated by assuming a gas temperature Tk = 100 K, and an ortho-H2 to para-H2 ratio of 3 and an optically thin regime. The rest-frame frequencies (νrest), upper-level energies (Eup/k) and Einstein A coefficients are taken from the LAMDA database (Schöier et al. 2005). The collision rate coefficients are from Yang et al. (2010). Throughout this paper, we refer to [C I](3P1 → 3P0) as [C I](1–0) and [C I](3P2 → 3P1) as [C I](2–1).

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