Flares on dMe stars: IUE and optical observations of AT Mic, and comparison of far-ultraviolet stellar and solar flares

Open Access
Publisher Website
Google Scholar
Abstract
IUE observations of a large flare on the dMe star AT Mic are reported. Complementary IUE and visible-region spectrophotometry and UBV measurements are also described for the star in its quiescent state. The results are compared with calibrated solar UV spectra from Skylab. Far-UV data for two phases of the solar and the stellar flares are displayed at the same spectral resolution. Both continua and line spectra are compared and contrasted; data for Lyα were obtained after subtraction of the IUE geocoronal contamination using a new technique. The ratios of quiescent stellar-to-solar UV-line surface fluxes range up to 34, and show a slight trend with temperature. In the visible, the Balmer line intensities are close to theoretical recombination values. A composite quiescent AT Mic spectrum is presented, covering λλ 1200–5100 Å. In the flaring state, which was sampled with two consecutive IUEλλ 1150–1950 Å exposures, the UV-line fluxes are enhanced by up to a factor of 7, even when averaged over the exposure duration of 30min; peak values may well be much higher. The ratio AT Mic-to-solar flare-line power is 104-105. The chromospheric Lyα is less significant relative to transition-region lines than in the Sun. High densities (≈1012 cm−3) are suggested by the C III 1176-Å/Si IV 1398-Å ratio. The greater relative intensity of the He II 1640-Å line in AT Mic compared with the Sun may imply a large XUV flux in the flare, but other means of exciting the line are possible. Other reported IUE observations of flares have been re-extracted and re-analysed and in general show similar line ratios to those in the AT Mic flares. Far-UV continuum flares are presented for the AT Mic and other IUE flares. Such continua may arise from a heating of the star's photosphere by UV lines, a mechanism suggested for the presence of solar-flare continua.

This publication has 0 references indexed in Scilit: