A higher-than-predicted measurement of iron opacity at solar interior temperatures

• Published in: Nature (London) Vol. 517; no. 7532; pp. 56 - 59
• Main Authors: Bailey, J. E., Nagayama, T., Loisel, G. P., Rochau, G. A., Blancard, C., Colgan, J., Cosse, Ph, Faussurier, G., Fontes, C. J., Gilleron, F., Golovkin, I., Hansen, S. B., Iglesias, C. A., Kilcrease, D. P., MacFarlane, J. J., Mancini, R. C., Nahar, S. N., Orban, C., Pain, J.-C., Pradhan, A. K., Sherrill, M., Wilson, B. G.
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 01.01.2015; Nature Publishing Group
• Subjects: 639/766/1960/1134; 639/766/33/34/867; 639/766/36/1121; Astrophysics; Convection; Experiments; Humanities and Social Sciences; Hypotheses; Iron; letter; Methods; multidisciplinary; Opacity; Optical properties; Properties; Science; Solar physics; Solar radiation; Temperature; United States
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/nature14048

Laboratory measurements of iron opacity made under conditions similar to those inside the Sun reveal much higher opacity than predicted, helping to resolve inconsistencies within stellar models of the internal temperatures of stars. A new view of stellar interiors Internal temperature profiles of the Sun and other stars are controlled in large part by the rate at which radiation is absorbed by stellar matter. Until now it has not been possible to determine the opacity of matter in star-like conditions in the laboratory, but James Bailey et al . have now achieved that feat using the Sandia National Laboratories' Z facility, the world's most powerful X-ray generator. The experiments reveal a wavelength-resolved iron opacity that is 30 to 400 times greater than predicted in conditions very similar to those at the radiation/convection zone boundary in the Sun. Previous measurements of stellar interiors have been based on observations of surface waves, and there were serious discrepancies between theoretical predictions and observations. The new measurements account for about half of adjustment in opacity figures required to restore agreement between standard solar models and observations. Nearly a century ago it was recognized 1 that radiation absorption by stellar matter controls the internal temperature profiles within stars. Laboratory opacity measurements, however, have never been performed at stellar interior conditions, introducing uncertainties in stellar models 2 , 3 , 4 , 5 . A particular problem arose 2 , 3 , 6 , 7 , 8 when refined photosphere spectral analysis 9 , 10 led to reductions of 30–50 per cent in the inferred amounts of carbon, nitrogen and oxygen in the Sun. Standard solar models 11 using the revised element abundances disagree with helioseismic observations that determine the internal solar structure using acoustic oscillations. This could be resolved if the true mean opacity for the solar interior matter were roughly 15 per cent higher than predicted 2 , 3 , 6 , 7 , 8 , because increased opacity compensates for the decreased element abundances. Iron accounts for a quarter of the total opacity 2 , 12 at the solar radiation/convection zone boundary. Here we report measurements of wavelength-resolved iron opacity at electron temperatures of 1.9–2.3 million kelvin and electron densities of (0.7–4.0) × 10 22 per cubic centimetre, conditions very similar to those in the solar region that affects the discrepancy the most: the radiation/convection zone boundary. The measured wavelength-dependent opacity is 30–400 per cent higher than predicted. This represents roughly half the change in the mean opacity needed to resolve the solar discrepancy, even though iron is only one of many elements that contribute to opacity.