The X‐Ray Spectrum of Fe¹⁶⁺Revisited with a Multi‐Ion Model

Abstract

The theoretical intensities of the soft X-ray Fe XVII lines arising from\n2l-3l' transitions are reexamined using a three-ion collisional-radiative model\nthat includes the contributions to line formation of radiative recombination\n(RR), dielectronic recombination (DR), resonant excitation (RE), and\ninner-shell collisional ionization (CI), in addition to the usual contribution\nof collisional excitation (CE). These additional processes enhance mostly the\n2p-3s lines and not the 2p-3d lines. Under coronal equilibrium conditions, in\nthe electron temperature range of 400 to 600 eV where the Fe XVII line\nemissivities peak, the combined effect of the additional processes is to\nenhance the 2p-3s lines at 16.78, 17.05, and 17.10 A, by ~ 25%, 30%, and 55%,\nrespectively, compared with their traditional, single-ion CE values. The weak\n2p-3d line at 15.45 A is also enhanced by up to 20%, while the other 2p-3d\nlines are almost unaffected. The effects of DR and RE are found to be dominant\nin this temperature range (400 - 600 eV), while that of CI is 3% at the most,\nand the contribution of RR is less than 1%. At lower temperatures, where the Fe\nXVII / Fe XVIII abundance ratio is high, the RE effect dominates. However, as\nthe temperature rises and the Fe XVIII abundance increases, the DR effect takes\nover.\n The newly calculated line powers can reproduce most of the often observed\nhigh values of the (I17.05 + I17.10) / I15.01 intensity ratio. The importance\nof ionization and recombination processes to the line strengths also helps to\nexplain why laboratory measurements in which CE is essentially the sole\nmechanism agree well with single-ion calculations, but do not reproduce the\nastrophysically observed ratios.\n