Opacity in the upper atmospheres of active stars

D. J. Christian; M. Mathioudakis; D. S. Bloomfield; J. Dupuis; F. P. Keenan; D. L. Pollacco; R. F. Malina

doi:doi:10.1051/0004-6361:20054404

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

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)

Abstract
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