Laboratory measurement of opacity for stellar envelopes

We have measured the frequency dependent opacity of a low density iron plasma in Local Thermodynamic Equilibrium (LTE). The measured iron plasma conditions of 20 eV temperature and 10(-4) g/cc density, match those of stellar envelopes where iron dominates the radiative transport. Properties of the M-shell Delta n = 0 transition arrays in iron are measured in this experiment, providing the first direct test of opacity models used in stellar pulsation and evolution calculations. We describe new methods to obtain LTE opacity data for plasmas at 100 times lower density than previous measurements. Experimental requirements include: high spectral resolution, large homogenous plasma sources, and Planckian radiation fields lasting tens of nanoseconds. These conditions were achieved using the 500 kJ SATURN facility at Sandia National Laboratory. (C) 1997 Published by Elsevier Science Ltd. All rights reserved.