Opacity in the upper atmosphere of AU Mic

Details Opacity in the upper atmosphere of AU Mic Opacity in the upper atmosphere of AU Mic

Jul 2002

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002a%26a...390..219b&link_type=abstract

Astronomy and Astrophysics, v.390, p.219-224 (2002)

Astronomy and Astrophysics

Astrophysics

9

Atomic Data, Stars: Activity, Stars: Atmospheres, Stars: Individual: Au Mic, Stars: Late-Type, Ultraviolet: Stars

Scientific paper

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 Iii multiplet and O Vi resonance lines, hence yielding information at two different levels within the atmosphere. Significant deviations from the optically thin fluxes are found for C Iii in both quiescent and flare spectra, where only 60% of the flux is actually observed. This could explain the apparent deviation of C Iii 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 Iii 1175.71 Åcomponent 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.

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