EDP Sciences
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Volume 390, Number 1, July IV 2002
Page(s) 219 - 224
Section Stellar atmospheres
DOI http://dx.doi.org/10.1051/0004-6361:20020690

A&A 390, 219-224 (2002)
DOI: 10.1051/0004-6361:20020690

Opacity in the upper atmosphere of AU Mic

D. S. Bloomfield1, M. Mathioudakis1, D. J. Christian2, F. P. Keenan1 and J. L. Linsky3

1  Department of Pure and Applied Physics, The Queen's University of Belfast, Belfast, BT7 1NN, Northern Ireland, UK
2  Department of Physics and Astronomy, Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218, USA
3  JILA, University of Colorado, Boulder, CO 80309; and National Institute of Standards and Technology, USA

(Received 30 January 2002 / Accepted 6 May 2002)

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 $\ion{C}{iii}$ multiplet and $\ion{O}{vi}$ resonance lines, hence yielding information at two different levels within the atmosphere. Significant deviations from the optically thin fluxes are found for $\ion{C}{iii}$ in both quiescent and flare spectra, where only 60% of the flux is actually observed. This could explain the apparent deviation of $\ion{C}{iii}$ 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 $\ion{C}{iii}$ 1175.71 Å component of the multiplet. Using a lower limit to the electron density (10 11 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.

Key words: atomic data -- stars: activity -- stars: atmospheres -- stars: individual: AU Mic -- stars: late-type -- ultraviolet: stars

Offprint request: D. S. Bloomfield, s.bloomfield@qub.ac.uk

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