Detection of magnetic dipole lines of Fexii in the ultraviolet spectrum of the dwarf star ɛ Eri

Abstract

We report observations of the dwarf star ɛ Eri (K2V) made with the Space Telescope Imaging Spectrograph (STIS) on the Hubble Space Telescope. The high sensitivity of the STIS instrument has allowed us to detect the magnetic dipole transitions of Fexii at 1242.00 and 1349.38Å for the first time in a star other than the Sun. The width of the stronger line at 1242.00Å has also been measured; such measurements are not possible for the permitted lines of Fexii in the extreme-ultraviolet. To within the accuracy of the measurements, the Nv and the Fexii lines occur at their rest wavelengths. Electron densities and linewidths have been measured from other transition region lines. Together, these can be used to investigate the non-thermal energy flux in the lower and upper transition regions, which is useful in constraining possible heating processes. The Fexii lines are also present in archival STIS spectra of other G/K-type dwarfs.