K-shell dielectronic resonances in photoabsorption: Differential oscillator strengths for Li-like C IV, O VI, and Fe XXIV

Recently x-ray photoabsorption in $\mathrm{KLL}$ resonances of O VI was predicted [A. K. Pradhan, Astrophys. J. Lett. 545, L165 (2000)], and detected by the Chandra X-Xay Observatory [J. C. Lee et al., Astrophys. J. Lett. (to be published)]. The required resonance oscillator strengths $f_r$ are evaluated in terms of the differential oscillator strength $df/d\epsilon$ that relates bound and continuum absorption. We present the $f_r$ values from radiatively damped and undamped photoionization cross sections for Li-like C, O, and Fe calculated using relativistic the close-coupling Breit-Pauli R-matrix method. The $\mathrm{KLL}$ resonances of interest here are ${1s2p(}^3{P}^o\left)2s{[}^4{P}_{1/2,3/2}^o{,}^2{P}_{1/2,3/2}^o\right]$ and ${1s2p(}^1{P}^o\left)2s{[}^2{P}_{1/2,3/2}^o\right].$ The $\mathrm{KLL}$ photoabsorption resonances in Fe XXIV are fully resolved up to natural autoionization profiles. It is demonstrated that the undamped $f_r$ independently yield the resonance radiative decay rates, and thereby provide a precise check on the resolution of photoionization calculations in general. The predicted photoabsorption features should be detectable by the x-ray space observatories and enable column densities in highly ionized astrophysical plasmas to be determined from the calculated $f_r.$ The dielectronic satellites may appear as redward broadening of resonance lines in emission and absorption.