Abstract
In this paper we investigate the validity of the optically thin assumption in the transition region of the late-type star AU Mic. We use Far-Ultraviolet Spectroscopic Explorer (FUSE) observations of the Ciii multiplet and Ovi resonance lines, hence yielding information at two dierent levels within the atmosphere. Significant deviations from the optically thin fluxes are found for Ciii in both quiescent and flare spectra, where only 60% of the flux is actually observed. This could explain the apparent deviation of Ciii observed in emission measure distributions. We utilize escape probabilities for both homogeneous and inhomogeneous geometries and calculate optical depths as high as 10 for the Ciii 1175.71 A component of the multiplet. Using a lower limit to the electron density (10 11 cm 3 )w e derive an eective thickness of<100 km for the scattering layer. The emission originates from very small and compact regions, consistent with a filling factor of 10 5 derived for the flare plasma.
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