Detection of magnetic dipole lines of Fe XII in the ultraviolet spectrum of the dwarf star Epsilon Eri

Abstract

We report observations of the dwarf star Epsilon Eri (K2 V) made with the Space Telescope Imaging Spectrograph (STIS) on the Hubble Space Telescope (HST). The high sensitivity of the STIS instrument has allowed us to detect the magnetic dipole transitions of Fe XII at 1242.00A and 1349.38A for the first time in a star other than the Sun. The width of the stronger line at 1242.00A has also been measured; such measurements are not possible for the permitted lines of Fe XII in the extreme ultraviolet. To within the accurcy of the measurements, the N V and the Fe XII lines occur at their rest wavelengths. Electron densities and line widths 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 region, which is useful in constraining possible heating processes. The Fe XII lines are also present in archival STIS spectra of other G/K-type dwarfs.