Measurement of the Free-Free Absorption of Hydrogen at Stellar Interior Conditions

The transport of energy from the burning core to the surface is an essential component of any modeling of stellar bodies. Due to the extreme conditions inside stars, we rely, however, on untested simplistic theories to describe both major mechanisms of heat transport: conduction and radiation transport. In this contribution, we give the first experimental results for the optical properties of hydrogen, the main ingredient of stars, under conditions similar to those of red dwarf stars at ~1000x solid density and pressures of several gigabars. These extreme densities and temperature were created by indirect laser-compression on the National Ignition Facility. The transmission of several staggered X-ray pulses allows us to track the density evolution in the target and obtain the opacity of the hydrogen. The results show great deviations from often-used models.