PROFESSOR ANIL K. PRADHAN

A surprising conundrum faces astronomy. The composition of the Sun – the benchmark for astronomical objects – has been called into question. Abundances of common elements in the Sun are uncertain by up to about 50%. The new abundances of volatile elements such as carbon, nitrogen and oxygen, derived from state-of-the-art spectroscopy and elaborate 3-D radiative transfer models yield abundances that are much lower than the canonical "standard" solar abundances, by as much as 45% for oxygen. That, in turn, has important implications for a host of fields in astrophysics such as helioseismology, primordial helium abundance, cosmological distances via the Cepheid period-luminosity relation, and asteroseismology of the host stars of extra-solar planets. The problem, and potential solution, appears to lie in the inverse relationship between radiative opacities and elemental abundances. Extensive attempts to calculate stellar opacities over the past three decades still fall short of including all necessary atomic processes that contribute to plasma opacities. That uncertainty is compounded by recent experiments at the Sandia National Laboratory on the Z-pinch inertial confinement fusion device that, for the first time, re-creates conditions existing at the solar radiative-convection zone boundary. Measured monochromatic opacities disagree with all known theoretical opacities models, including those obtained from the most sophisticated quantum mechanical R-matrix calculations under the International Opacity Project. This talk will describe the various interdisciplinary aspects of astrophysics, plasma physics, and atomic physics that need to be addressed in order to finally understand the Sun as well as we might.