Excitation of the 3.071 mm Hyperfine Line in Li‐like57Fe in Astrophysical Plasmas

Abstract

As noted first by Sunyaev & Churazov, the 3.071 mm hyperfine line from 57Fe+23 might be observable in astrophysical plasmas. We assess the atomic processes that might contribute to the excitation of this line. The distorted wave approximation was used to compute the direct electron collision strength between the two hyperfine sublevels of the ground configuration; it was found to be small. Proton collisional excitation was calculated and found to be negligible. We determine the rate of line excitation by electron collisional excitation of more highly excited levels, followed by radiative cascades. The branching ratios for hyperfine sublevels for allowed radiative decays and electron collisional excitation or deexcitation are derived. We show that the dominant line excitation process is electron collisional excitation of the 2p levels followed by radiative decay, as first suggested by Sunyaev & Churazov. We calculate an effective collision strength for excitation of the hyperfine line, including all of these effects and correcting for resonances. Because the hyperfine line is near the peak in the cosmic microwave background radiation spectrum, induced radiative processes are also very important. The effect of background radiation on the level populations and line excitation is determined. We determine the intensity of the hyperfine line from an isothermal, coronal plasma in collisional ionization equilibrium. Because of the variation in the ionization fraction of Fe+23, the emissivity peaks at a temperature of about 1.8 × 107 K. We have also derived the hyperfine line luminosity emitted by a coronal plasma cooling isobarically by its own radiation. Comparisons of the hyperfine line to other lines emitted by the same ion, Fe+23, are shown to be useful for deriving the isotopic fraction of 57Fe. We calculate the ratios of the hyperfine line to the 2s-2p EUV lines at 192 A and 255 A and the 2s-3p X-ray doublet at 10.6 A.