Comment on “Large Enhancement in High-Energy Photoionization of Fe XVII and Missing Continuum Plasma Opacity”

Comment on "Large enhancement in high-energy photoionization of Fe XVII and missing continuum plasma opacity" Recent R-matrix calculations claim to provide a significant enhancement in the opacity of Fe XVII due to atomic core excitations [1] and assert that this enhancement is consistent with recent measurements of higher-than-predicted iron opacities [2].This comment shows that the standard opacity models [3-7] which have already been directly compared with experimental data [2,7] produce photon absorption cross-sections for Fe XVII that are effectively equivalent to the R-matrix opacities reported in [1].Thus, the new R-matrix results cannot be expected to significantly impact the existing discrepancies between theory and experiment because they produce neither a "large enhancement" nor account for "missing continuum plasma opacity" relative to standard models.All models that satisfy the f-sum rule [7] and include the same initial and final electronic configurations can be expected to produce similar opacities (e.g.[8]).This is demonstrated in Fig. 1, which compares photo-absorption cross sections for Fe XVII from five standard models to the R-matrix and OP results from [1].The standard models have been restricted to the Fe XVII ion and normalized to a 0.195 abundance but are otherwise unchanged from previously published results [2,7].Both R-matrix and standard models include spectral features associated with autoionizing states that are evident in measured data but neglected in OP [1,9].Thus the opacity enhancement of R-matrix over OP reported in [1] illustrates the deficiencies of OP rather than the merits of R-matrix.Table 1 gives relative opacities for Fe XVII to help quantify the similarities between Rmatrix and standard models and their mutual differences with measurements.Both R-matrix and standard models yield larger total Rosseland mean opacities than OP, confirming the importance of transitions missing in OP.However, the Rosseland weighting function peaks near 17 Å while the most profound discrepancies between theory and experiment are in the 7 -9 Å monochromatic continuum.Here, the average opacities from all models (as well as cold reference opacities [10]) are significantly smaller than the experimental data.In this critical range, R-matrix is smaller even than OP.Thus the results reported in [1] thus appear unlikely to resolve the profound discrepancy between theory and experiments.