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

As noted first by Sunyaev & Churazov (1984), the 3.071 mm hyperfine line from$^{57}Fe^{+23}$ might be observable in astrophysical plasmas. We assess theatomic processes which might contribute to the excitation of this line. Wedetermine the intensity of the hyperfine line from an isothermal, coronalplasma in collisional ionization equilibrium and for a coronal plasma coolingisobarically due to its own radiation. Comparisons of the hyperfine line toother lines emitted by the same ion, Fe$^{+23}$, are shown to be useful forderiving the isotopic fraction of $^{57}$Fe. We calculate the ratios of thehyperfine line to the 2s--2p EUV lines at 192 \AA and 255 \AA, and the 2s--3pX-ray doublet at 10.6 \AA.