English (United Kingdom)

https://curated-unify.zendy.io/wp-json/zendy-region/v1/featured_content/oa?rat=en

https://curated-unify.zendy.io/wp-json/zendy-region/v1/highlighted_journal/

Global: Zendy Plus annual

Presents the access of premium content as premium feature

Premium Content

Presents the keyphrase highlighting as premium feature

Keyphrase Highlighting

Presents the summarisation as premium feature

Summarisation

Insights

Presents the pdf analysis as premium feature

PDF Analysis

Presents the zaia usage as premium feature

ZAIA

Global: Zendy Tools annual

Global: Zendy Free Plan

Global: Zendy Tools monthly

Global: Zendy Plus monthly

Radiative transfer calculations have predicted intensity enhancements foroptically thick emission lines, as opposed to the normal intensity reductions,for astrophysical plasmas under certain conditions. In particular, the resultsare predicted to be dependent both on the geometry of the emitting plasma andthe orientation of the observer. Hence in principle the detection of intensityenhancement may provide a way of determining the geometry of an unresolvedastronomical source. To investigate such enhancements we have analyzed a sampleof active late-type stars observed in the far ultraviolet spectral region.Emission lines of O VI in the FUSE satellite spectra of epsilon Eri, II Peg andProx Cen were searched for intensity enhancements due to opacity. We have foundstrong evidence for line intensity enhancements due to opacity during active orflare-like activity for all three stars. The O VI 1032/1038 line intensityratios, predicted to have a value of 2.0 in the optically thin case, are foundto be up to ~30% larger during several orbital phases. Our measurements,combined with radiative transfer models, allow us to constrain both thegeometry of the O VI emitting regions in our stellar sources and theorientation of the observer. A spherical emitting plasma can be ruled out, asthis would lead to no intensity enhancement. In addition, the theory tells usthat the line-of-sight to the plasma must be close to perpendicular to itssurface, as observations at small angles to the surface lead to either nointensity enhancement or the usual line intensity decrease over the opticallythin value. For the future, we outline a laboratory experiment, that could beundertaken with current facilities, which would provide an unequivocal test ofpredictions of line intensity enhancement due to opacity, in particular thedependence on plasma geometry.

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