Abstract
Context. 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 β Cep and slowly pulsating B stars, whose oscillations are triggered by the κ-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. Aims. 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. Methods. Monochromatic cross sections for Ni were obtained with the SCO-RCG code. The Toulouse-Geneva evolution code was used to build the stellar models. Results. 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.
Highlights
Opacities describe the interactions between radiation and matter
In most parts of main-sequence stars, which are in the hydrogen-burning phase, the most important elements that contribute to the Rosseland mean opacity (RMO) are hydrogen and helium
SCO-RCG is an opacity computation code consisting of two parts: the building of the atomic structure that consistently takes plasma effects into account is devoted to the super-configuration code for opacity (SCO) part (Blenski et al 2000), avoiding the isolatedatom approximation
Summary
Opacities describe the interactions between radiation and matter. In stellar physics, they are used to evaluate the amount of energy that is carried by the radiation field. In addition to structural effects, the domination of the RMO by the Fe-peak elements enables them to trigger oscillations through the κ-mechanism for stars in the upper part of the main sequence. Nickel is of prime importance in the excitation of the pulsations, especially for high-overtone g-modes in upper main-sequence stars (e.g., Daszynska-Daszkiewicz et al 2017) or in sdBs (Jeffery & Saio 2006). Hui-Bon-Hoa & Vauclair (2018a) found that the radiative accelerations for nickel in the Z-bump are of similar strength between OP and OPAL, which poses the question whether the OPAL opacities might be underestimated there Accurate data for this element are needed to build realistic models. The number of detailed calculations in SCO-RCG is largely dominant, and subsequently the computed spectrum is less sensitive to the modelling of the remaining statistical contributions (UTA, SOSA, and STA)
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