Table 1
Excitation conditions of the fine-structure lines [Cii], [Oi]63, and [Oiii]88, with Col. 2 providing the ionization potential (IP) to create the species.
| Line | IPa | Eu/k | ncrit,H | ncrit,e | Ref | Origin? |
| [eV] | [K] | [cm-3] | [cm-3] | |||
|
|
||||||
| [CII] | 11.3 | 91 | 1.6 × 103 | 44 | 1 | PDRs, diffuse Hi clouds, diffuse ionized gas, Hii regions |
| [OI] 63 | – | 228 | 5 × 105 | – | 2 | warm and/or dense PDRs |
| [OIII] 88 | 35.1 | 163 | – | 510 | 3 | low excitation, highly ionized gas |
Notes. Columns 3−5 give the energy of the upper state and critical density for line excitation through collisions with hydrogen atoms and electrons, calculated for a kinetic temperature Tkin = 8000 K. Column 6 provides the reference to the literature work reporting the excitation conditions of each fine-structure line. Column 7 specifies the ISM phase(s) from which the lines might originate.
The ionization potential refers to the ionization energy of an atom to create the species in Col. 1. For example, the ionization potential of 35.1 eV for [Oiii]88 indicates the energy required to remove another electron from O+ and, thus, ionize O+ in order to create O++.
Reference. (1) Goldsmith et al. (2012); (2) Tielens & Hollenbach (1985a); (3) Aggarwal & Keenan (1999).
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