Free Access

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 35 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).

Current usage metrics show cumulative count of Article Views (full-text article views including HTML views, PDF and ePub downloads, according to the available data) and Abstracts Views on Vision4Press platform.

Data correspond to usage on the plateform after 2015. The current usage metrics is available 48-96 hours after online publication and is updated daily on week days.

Initial download of the metrics may take a while.