Opacity in the upper atmospheres of active stars

II. AD Leonis

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  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 , while , and  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 ≈20–60 km and ≈10–30 km, respectively, under quiescent conditions. However path lengths derived during flaring are ≈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.