Table 1
Spectroscopic parameters of the lines studied in this work (from Endres et al. 2016, and references therein), critical densities for collisions with p-H2 and electrons at 20 K (if LTE prevails, 99.82% of H2 is in para form), and critical fractional abundance of electrons (see text).
| Species | Transition | Frequency (GHz) | Eu/kB (K) | Aul (s−1) |  |
 |
 |
Ref. collisional rates para-H2, e− |
|---|---|---|---|---|---|---|---|---|
| HCN | J = 1−0 | 88.63185 | 4.25 | 2.41 × 10−5 | 2.7 × 105 | 3.2 | 1.2 × 10−5 | a,b |
| HCN | J = 2−1 | 177.26122 | 12.76 | 2.31 × 10−4 | 2.3 × 106 | 38 | 1.7 × 10−5 | |
| HCN | J = 3−2 | 265.88650 | 25.52 | 8.36 × 10−4 | 5.8 × 106 | 168 | 2.9 × 10−5 | |
| HCN | J = 4−3 | 354.50548 | 42.53 | 2.05 × 10−3 | 1.6 × 107 | 485 | 3.0 × 10−5 | |
| HNC | J = 1−0 | 90.66357 | 4.35 | 2.69 × 10−5 | 7.0 × 104 | 3.6 | 5.2 × 10−5 | a,b |
| HNC | J = 3−2 | 271.98114 | 26.11 | 9.34 × 10−4 | 2.2 × 106 | 191 | 8.6 × 10−5 | |
| HCO+ | J = 1−0 | 89.18852 | 4.28 | 4.19 × 10−5 | 4.5 × 104 | 2.0 | 4.4 × 10−5 | c,d |
| HCO+ | J = 2−1 | 178.37506 | 12.84 | 4.02 × 10−4 | 4.0 × 105 | 21 | 5.3 × 10−5 | |
| HCO+ | J = 3−2 | 267.55763 | 25.68 | 1.45 × 10−3 | 1.5 × 106 | 84 | 5.8 × 10−5 | |
| HCO+ | J = 4−3 | 356.73422 | 42.80 | 3.57 × 10−3 | 3.1 × 106 | 223 | 7.1 × 10−5 | |
| CO | J = 1−0 | 115.27120 | 5.53 | 7.20 × 10−8 | 5.7 × 102 | … | … | e |
| C0 | 3P1−3P0 | 492.16065 | 23.62 | 7.88 × 10−8 | 5.5 × 102 | 410 | 7.0 × 10−1 | f,g |
Notes. We define the critical density as the H2 (or e) density at which Au1 equals the sum of all upward and downward collisional rates from the upper level. That is, ncr = Au1/ Σi+u yui. For collisions with electrons, we consider only dipole-allowed transitions. We define the critical fractional abundance of electrons as 
.
References. (a) Hernandez Vera et al. (2017); (b) Faure et al. (2007b); (c) Denis-Alpizar et al. (2020); (d) Faure et al. (2007a, 2009); (e) Yang et al. (2010); (f) Schroder et al. (1991); (g) Johnson et al. (1987).
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