News from the opacity consortium OPAC

Abstract

Abstract. The international OPAC consortium (see list below) was formed three years ago. It is composed of astrophysicists, plasma physicists and experimentalists from different laboratories b . This consortium examines specific opacity calculations used in stellar physics. They contribute to solve the problems suggested by the astrophysical community in performing new calculations and new experiments with laser installation. We show here the specific example of the iron opacity peak that plays an important role in the envelope of intermediate-mass and massive stars and we present our first conclusions on iron and nickel. OPEN PROBLEMS COMING FROM SEISMIC OBSERVATIONSThe stellar community hopes to progress on the internal dynamics of stars thanks to the space-borne seismic missions SoHO, CoRoT and Kepler. But good identification and interpretation of acoustic frequencies are based on a proper microscopic physics. Along the last decade, some questions on energy transfer, linked to plasma opacities, have arisen and two cases have retained our attention. 1. The extraction of the solar internal sound speed and density has revealed clear discrepancies with the Standard Solar Model predictions [1][2][3]. These discrepancies are not explained by the transport of momentum by rotation during the main sequence evolution [4]. So the energy balance and (or) energy transfer stay clearly under suspicion. In parallel, the recent OPAS calculations [5] have revealed large differences in individual element spectra with previous calculations and in particular with OP [6,7]. OPAS calculations also show some compensation effects between low Z nuclei and the iron group contributions in the Rosseland mean opacity values. All these facts encourage further works and experimental verifications. 2. The second case concerns massive stars. SPB and Cephei stars are interesting pulsators that might put important constraints on rotation and magnetic fields, two key actors for massive stars. Presently, the 40 stars observed by Kepler show acoustic and gravity modes probably up to = 20 (deformed stars) that are difficult to identify [8]. The first difficulty comes from the iron group opacities, which drive their pulsations. The observed modes do not agree with the stability of modes deduced from the OP or OPAL tables [9], which suggests that those opacities are underestimated [10,11]. Up to now, our effort is dedicated to the second case. The first analysis of the OPAC consortium confirms an underestimate of these opacities through comparison with more recent opacity calculations based on old and new measurements [12][13][14][15]. We shall here present our first conclusions and new orientations for the coming years.

The IACOB project

“…Moreover, with the progress in detection capabilities, B pulsating stars were also discovered in the Small Magellanic Cloud (e.g., Kourniotis et al 2014, and references therein) while, adopting the typical metallicity of this galaxy (Z ≈ 0.003, e.g., Buchler 2008), no modes could be excited in a theoretical standard β Cephei model (Salmon et al 2009). The solution to these discrepancies between theoretical predictions and observations may still be related to the considered opacities (see, e.g., Salmon et al 2012;Turck-Chièze & Gilles 2013;Moravveji 2016), or to the existence of other types of driving mechanisms of stellar oscillations not accounted for in the computations.…”

Radiative properties of matter based on quantum statistical method

“…We have also compared the calculations of mean opacities with some other state-of-the-art opacity codes being frequently used in the field of stellar physics: STA, OP, CAS-SANDRA, OPAS, SCO-RCG, and LEDCOP. [59] Comparison is shown in Table 7. The results of OPAQS are tested by employing two different approaches for the broadening phenomena: Stark broadening is included in the calculations of OPAQS1, but omitted in the case study of OPAQS2.…”

“…Planck and Rosseland mean opacities computed for iron plasma at density of 3.4 mg/cc different temperatures. Values are taken from Ref [59]. for comparison.…”

The IACOB project: III. New observational clues to understand macroturbulent broadening in massive O- and B-type stars

“…In addition, it is important to remark that the use of different input parameters 14 (e.g. opacities, metal mixture, metallicity, overshooting parameter) or even evolutionary codes may alter the results (see, e.g., Miglio et al 2007a,b;Zdravkov & Pamyatnykh 2008;Salmon et al 2012;Turck-Chièze & Gilles 2013;Martins & Palacios 2013;Castro et al 2014, for recent illustrations of these effects).…”