Comment on Iglesias and Hansen (2017)

Noted below are factual errors in the Iglesias and Hansen paper "FeXVII opacity under solar interior conditions" (ApJ, 835, 2841, 2017; hereafter IH17) concerning the Nahar and Pradhan paper "Large Enhancement in High-Energy Photoionization of Fe XVII and Missing Continuum Plasma Opacity" (PRL 116, 235003, 2016; hereafter NP16a). A similar response to the Comment by Blancard et al. (PRL, 117, 249501, 2016, hereafter B16) has already been given by Nahar and Pradhan (PRL, 117, 249502, 2016; hereafter NP17b). A more detailed exposition will be given in publications submitted or in preparation.

1. As noted in the Reply NP16b to the Comment in B16, the results from existing opacity codes confirm, not contradict, the opacity enhancements demonstrated in NP16a relative to current Opacity Project Rosseland Mean Opacities ( Although there are discrepancies in monochromatic opacities at high energies, that is a minor contributor to overall FeXVII opacity.

2. IH17 claim that the discrepancy in high-energy monochromatic opacities is due to missing configurations in NP16a. From the rather simplistic description it is not clear where IH17 obtained information on NP16a calculations, since the details are not yet published, but it is incorrect and based on a misunderstanding of the coupled channel (CC) approximation in the R-matrix method. Specifically, the list of configurations in IH17 related to one- and two-electron excitations is wrong; most of the contributing transtions are indeed included by NP16a.

3. The confusion evident in IH17 is between target levels and target configurations in the R-matrix wavefunction expansion for the (e+ion) system. There two types of target ion configurations: (i) spectroscopic and (ii) correlation. Autoionizing resonances arise from coupling between open and closed channels belonging to target levels explicitly included in the spectroscopic set (i). However, correlation configurations in the much larger basis set (ii) are also coupled in the configuration-interaction expansion for ion wavefunctions. For example, 60CC BPRM refers to 60 fine structure levels upto and including n = 3 electronic configurations in the relativistic Breit-Pauli Approximation. Expansion over each fine structure level couples all terms of both sets of configurations (i) and (ii). In the 99CC calculation also reported in NP16a, 99 LS terms dominated by configurations upto and including n = 4 are included, explicitly those with one- and two-electron vacancies in the 2s sub-shell. In addition to (i) and (ii), there are bound-channel (e+ion) configurations including in the CC-RM calculations that also give rise to large number of excited levels. [IH17 refer to as yet unpublished data from NP16a; Ergo: their comments are not based on the original source]. The 60CC radiative data, including 2s-hole states have long been available at the NORAD database (, as described in Nahar et al. (Phys.Rev.A 83, 053417, 2011).

In summary, the NP16a calculations include most of the relevant transitions mentioned by IH17.

4. We attribute the discrepancies in the high-energy region not to missing configurations mentioned by IH17 but to two factors: (A) the OPAL equation-of-state has long been known to yield orders-of-magnitude larger occupation probabilies for excited levels than the Mihalas-Hummer-Dappen equation-of-state employed in the OP work and in NP16a (e.g. Hummer 1988, AIP Conf. Proc. 168, 1988; Badnell and Seaton, JPB, 36, 4367, 2003; Trampedach et al., ApJ, 646, 560, 2006), and (B) High-energy cutoffs and asymptotic forms of photoionization cross sections used in NP16a (under revision).

However, these would not significantly affect the results in NP16a for FeXVII RMOs.

5. The distorted-wave (DW) method employed in current opacities calculations does not yield accurate cross sections in energy regions dominated by autoionizing resonances. That is because channel coupling included in the CC-RM calculations is not considered in an ab initio manner in DW calculations; therefore a direct level-by-level comparison is not possible. However, the CC-RM resutls have been extensively benchmarked against synchrotron based light sources and storage ring experiments for photoionization and (e+ion) recombination. Historically, over half a century earlier, the CC-RM methodology was developed to redress the deficiencies in the DW calculations and neglect of channel coupling effects, primarily autoionizing resonance shapes and magnitudes.

Plasma perturbation effects broaden and dissolve resonances into the continuum. An approximate treatment of electron impact broadening employed in the new NP16a opacities calculations is described by Pradhan (2017, submitted). However, it remains to be seen if the elaborate CC-RM results verify the DW opacities in terms of detailed structures and magnitudes. Calculations are in progress for other iron ions of interest in the Sandia Z-pinch measurements (Bailey et al. 2015).

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