Opacity in the upper atmosphere of AU Mic

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 C  multiplet and O  resonance lines, hence yielding information at two different levels within the atmosphere. Significant deviations from the optically thin fluxes are found for C  in both quiescent and flare spectra, where only 60% of the flux is actually observed. This could explain the apparent deviation of C  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 C  1175.71 Å component of the multiplet. Using a lower limit to the electron density (1011 cm−3) we derive an effective 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.