A search for line intensity enhancements in the far-UV spectra of active late-type stars arising from opacity

摘要: Context. Radiative transfer calculations have predicted intensity enhancements for optically thick emission lines, as opposed to the normal reductions, astrophysical plasmas under certain conditions. In particular, results are be dependent both on geometry of emitting plasma and orientation observer. Hence in principle detection enhancement may provide a way determining an unresolved astronomical source. Aims. To investigate such we analysed sample active late-type stars observed far ultraviolet spectral region. Methods. Emission lines O vi FUSE satellite spectra ofEri, II Peg Prox Cen were searched due opacity. Results. We found strong evidence line opacity during or flare-like activity all three stars. The 1032/1038 ratios, value 2.0 thin case, up ∼30% larger several orbital phases. Conclusions. Our measurements, combined with radiative models, allow us constrain regions our stellar sources A spherical can ruled out, this would lead no enhancement. addition, theory tells that line-of-sight must close perpendicular its surface, observations at small angles surface either usual decrease over value. For future, outline laboratory experiment, could undertaken current facilities, which unequivocal test predictions opacity, particular dependence geometry.