Experimental constraint of carbon and oxygen equation of state and ionization under the conditions in the envelopes of white dwarf stars

Accurate modeling of the interiors of white dwarf stars is important for constraining age and star formation history of our galaxy, determining the composition of exoplanets, and informing models for progenitors of supernovae. The equation of state of white dwarf materials (primarily carbon and oxygen) is most uncertain and current in-use models have largest discrepancies at the Gbar-keV conditions in the envelopes, where the atoms are partially ionized. Using the National Ignition Facility to drive materials containing carbon and oxygen in a convergent geometry, we can for the first time reach the relevant pressure-temperature conditions where the theoretical models can be tested. We will report on a series of experiments underway to use streaked x-ray radiography to measure the principal Hugoniot of carbon, deuterated water, and a compound of mixed carbon and oxygen (cellulose acetate). These experiments will provide critical data to test predictions of the conditions at which L- and K-shell ionization modify the compressibility along the principal Hugoniot.