Low-temperature Extensions of the Virial Equation of State for Solar Modeling

Low-order density expansions cannot adequately describe recombination reactions. Therefore, the usefulness of an exact quantum virial expansion truncated at the order of ?5/2 is limited to the deeper interior of the Sun, where the plasma is nearly fully ionized. Here, intermediate steps towards full-fledged solar modeling are presented. They are (i) a smooth numerical representation of the quantum virial expansion and (ii) the construction of a smooth transition to the low-temperature regime (below 50,000 K), where the virial expansions breaks down due to H recombination. These technical steps are realized for a simplified H-only plasma; the extension to He and heavier elements will be dealt with later. Ultimately, the outcome will be that solar observations can be used to test the accuracy of the virial equation of state and to compare it with current more phenomenological formalisms ((c) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)