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The chemical element nickel is of particular interest in stellar physics. In the layers in which the Fe-peak elements dominate the mean opacity (the so-called Z-bump), Ni is the second contributor to the Rosseland opacity after iron, according to the Opacity Project data. Reliable nickel cross sections are therefore mandatory for building realistic stellar models, especially for main-sequence pulsators such as $\beta$ Cep and slowly pulsating B stars, whose oscillations are triggered by the $\kappa$-mechanism of the Fe-peak elements. Unfortunately, the Opacity Project data for Ni were extrapolated from those of Fe, and previous studies have shown that they were underestimated in comparison to detailed calculations. We investigate the impact of newly computed monochromatic cross sections on the Rosseland mean opacity of Ni and on the structure of main-sequence massive pulsators. We compare our results with the widely used Opacity Project and OPAL data. Monochromatic cross sections for Ni were obtained with the SCO-RCG code. The Toulouse-Geneva evolution code was used to build the stellar models. With the new data, the Rosseland opacities of Ni are roughly the same as those of the Opacity Project or OPAL at high temperatures ($\log\ T>6$). At lower temperatures, significant departures are observed; the ratios are up to six times higher with SCO-RCG. These discrepancies span a wider temperature range in the comparison with OPAL than in comparison with the Opacity Project. For massive star models, the results of the comparison with a structure computed with Opacity Project data show that the Rosseland mean of the global stellar mixture is only marginally altered in the Z-bump. The maximum opacity is shifted towards slightly more superficial layers. A new maximum appears in the temperature derivative of the mean opacity, and the driving of the pulsations should be affected., Comment: 7 pages, 10 figures, accepted on November 18th 2021

Recent R-matrix calculations claim to produce a significant enhancement in the opacity of Fe XVII due to atomic core excitations [S. N. Nahar & A.K. Pradhan, Phys. Rev. Letters 116, 235003 (2016), arXiv:1606.02731] and assert that this enhancement is consistent with recent measurements of higher-than-predicted iron opacities [J. E. Bailey et al., Nature 517, 56 (2015)]. This comment shows that the standard opacity models which have already been directly compared with experimental data produce photon absorption cross-sections for Fe XVII that are effectively equivalent to (and in fact larger than) the new R-matrix opacities. 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., Comment: three pages, one table, one figure

Opacity is an important ingredient of the evolution of stars. The calculation of opacity coefficients is complicated by the fact that the plasma contains partially ionized heavy ions that contribute to opacity dominated by H and He. Up to now, the astrophysical community has greatly benefited from the work of the contributions of Los Alamos [1], Livermore [2] and the Opacity Project (OP) [3]. However unexplained differences of up to 50% in the radiative forces and Rosseland mean values for Fe have been noticed for conditions corresponding to stellar envelopes. Such uncertainty has a real impact on the understanding of pulsating stellar envelopes, on the excitation of modes, and on the identification of the mode frequencies. Temperature and density conditions equivalent to those found in stars can now be produced in laboratory experiments for various atomic species. Recently the photo-absorption spectra of nickel and iron plasmas have been measured during the LULI 2010 campaign, for temperatures between 15 and 40 eV and densities of ~3 mg/cm3. A large theoretical collaboration, the "OPAC", has been formed to prepare these experiments. We present here the set of opacity calculations performed by eight different groups for conditions relevant to the LULI 2010 experiment and to astrophysical stellar envelope conditions., Comment: 15 pages, 10 figures

Seismology of stars is strongly developing. To address this question we have formed an international collaboration OPAC to perform specific experimental measurements, compare opacity calculations and improve the opacity calculations in the stellar codes [1]. We consider the following opacity codes: SCO, CASSANDRA, STA, OPAS, LEDCOP, OP, SCO-RCG. Their comparison has shown large differences for Fe and Ni in equivalent conditions of envelopes of type II supernova precursors, temperatures between 15 and 40 eV and densities of a few mg/cm3 [2, 3, 4]. LEDCOP, OPAS, SCO-RCG structure codes and STA give similar results and differ from OP ones for the lower temperatures and for spectral interval values [3]. In this work we discuss the role of Configuration Interaction (CI) and the influence of the number of used configurations. We present and include in the opacity code comparisons new HULLAC-v9 calculations [5, 6] that include full CI. To illustrate the importance of this effect we compare different CI approximations (modes) available in HULLAC-v9 [7]. These results are compared to previous predictions and to experimental data. Differences with OP results are discussed., Comment: 4 pages, 3 figures, conference Inertial Fusion Sciences and Applications, Bordeaux, 12th to 16th September 2011; EPJ web of Conferences 2012

The lifetime of solar-like stars, the envelope structure of more massive stars, and stellar acoustic frequencies largely depend on the radiative properties of the stellar plasma. Up to now, these complex quantities have been estimated only theoretically. The development of the powerful tools of helio- and astero- seismology has made it possible to gain insights on the interiors of stars. Consequently, increased emphasis is now placed on knowledge of the monochromatic opacity coefficients. Here we review how these radiative properties play a role, and where they are most important. We then concentrate specifically on the envelopes of $\beta$ Cephei variable stars. We discuss the dispersion of eight different theoretical estimates of the monochromatic opacity spectrum and the challenges we need to face to check these calculations experimentally., Comment: 6 pages, 5 figures, in press (conference HEDLA 2010)

Opacity calculations are basic ingredients of stellar modelling. They play a crucial role in the interpretation of acoustic modes detected by SoHO, COROT and KEPLER. In this review we present our activities on both theoretical and experimental sides. We show new calculations of opacity spectra and comparisons between eight groups who produce opacity spectra calculations in the domain where experiments are scheduled. Real differences are noticed with real astrophysical consequences when one extends helioseismology to cluster studies of different compositions. Two cases are considered presently: (1) the solar radiative zone and (2) the beta Cephei envelops. We describe how our experiments are performed and new preliminary results on nickel obtained in the campaign 2010 at LULI 2000 at Polytechnique., Comment: 6 pages, 4 figures, invited talk at SOHO24