Ionized Gas Atomic Spectral Line Intensity Diagnostics for H II Regions

Observations of fine-structure line ratios arising in ionized gas can be used to estimate the electron density, \(n_e\), and gas temperature \(T_e\), in an H II region. In general, lines that arise from different energies above ground give estimates of the gas temperature, while lines that arise from similar levels above ground but with different collision strengths give estimates of the gas density. We focus on lines that are expected to be bright in JWST observations, namely those arising from \({\rm Fe^+}, {\rm Ar^{+2}}\), and \({\rm Ar^{+4}}\). In particular, \({\rm Fe^+}\) has great potential for producing diagnostic line ratios due to the large number of levels excited in an H II region, but with strong caveats as noted here. Low level \({\rm Fe^+}\) line emission is also produced in the neutral gas within the PDR and thus the same species could trace physical conditions continuously from ionized to neutral gas.

We assume that the line emission is in the optically thin limit so that the ratio of intensities is given by the ratio of volume emissivity. For \({\rm Ar^{+2}}\) , and \({\rm Ar^{+4}}\) , we use CHIANTI ( Dere et al (1997); DelZanna et al. (2022) ) using the default values for the A values and collision strengths. For \({\rm Fe^+}\) we substituted the default values in CHIANTI with Einstein A values from Deb & Hibbert (2011) and collision strengths from Smyth et al. (2019) . The emissivity ratios are found in the temperature range from \(T_e=10^3\) K to \(10^4\) K, and the density range from \(n_e = 10^2~{\rm cm^{-3}}\) to \(10^6~{\rm cm^{-3}}\).

We created FITS files of emissivity and emissivity ratio in the \( (n_e,T_e) \) plane from which the phase space plots below were made. We note, however, that for [Fe II] fine-structure lines the published A values and collision strengths vary between different authors and in some cases do not agree with the observations (e.g., Koo et al. (2016) ) so the [Fe II] plots must be considered tentative until the atomic data can be further verified by observations, laboratory work, or quantum calculations.

Here is a Python script to make these figures.