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

Recently x-ray photoabsorption in 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/dɛ 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 KLL resonances of interest here are 1s2p(³P^o)2s [⁴P^o_1/2,3/2,²P^o_1/2,3/2] and 1s2p(¹P^o)2s [²P^o_1/2,3/2]. The 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.