Opacity in the upper atmospheres of active stars - II. AD Leonis

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 ≈ 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.