EDP Sciences
Free access
Volume 454, Number 3, August II 2006
Page(s) 889 - 894
Section Stellar atmospheres
DOI http://dx.doi.org/10.1051/0004-6361:20054404

A&A 454, 889-894 (2006)
DOI: 10.1051/0004-6361:20054404

Opacity in the upper atmospheres of active stars

II. AD Leonis
D. J. Christian1, M. Mathioudakis1, D. S. Bloomfield1, J. Dupuis2, F. P. Keenan1, D. L. Pollacco1 and R. F. Malina3

1  Department of Pure and Applied Physics, The Queen's University Belfast, Belfast BT7 1NN, Northern Ireland, UK
    e-mail: d.christian@qub.ac.uk
2  Department of Physics and Astronomy, Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218, USA
3  Laboratoire d'Astrophysique de Marseille, BP 8, Traverse du Siphon, 13376 Marseille Cedex 12, France

(Received 24 October 2005 / Accepted 5 February 2006)

We present FUV and UV spectroscopic observations of AD Leonis, with the aim of investigating opacity effects in the transition regions of late-type stars. The $\ion{C}{iii}$ lines in FUSE spectra show significant opacity during both the quiescent and flaring states of AD Leonis, with up to 30% of the expected flux being lost during the latter. Other FUSE emission lines tested for opacity include those of  $\ion{O}{vi}$, while  $\ion{C}{iv}$, $\ion{Si}{iv}$ and $\ion{N}{v}$ transitions observed with STIS are also investigated. These lines only reveal modest amounts of opacity with losses during flaring of up to 20%. Optical depths have been calculated for homogeneous and inhomogeneous geometries, giving path lengths of $\approx$20-60 km and $\approx$10-30 km, respectively, under quiescent conditions. However path lengths derived during flaring are $\approx$2-3 times larger. These values are in excellent agreement with both estimates of the small-scale structure observed in the solar transition region, and path lengths derived previously for several other active late-type stars.

Key words: atomic data -- stars: activity -- stars: atmospheres -- stars: individual: AD Leonis -- stars: late-type -- ultraviolet: stars

© ESO 2006