Far‐Ultraviolet Observations of Flares on the dM0e Star AU Microscopii

Abstract

We report on far-ultraviolet (FUV) observations of Ñares on the dM0e star AU Microscopii obtained on 1998 September 6 with the Space Telescope Imaging Spectrograph onboard the Hubble Space T elescope. The data consist of medium-resolution echelle spectra covering the wavelength region from 1170 to 1730 with a spectral resolution of j/*j \ 45,800. The observations were obtained using the A TIME-TAG observing mode in which the time and position of each detected photon is recorded. This allows a study of variability that is constrained only by counting statistics. During a total on-source time of 10,105 s, we observed numerous microÑare bursts as well as four well-deÐned Ñare events. These Ñares lasted between 10 s and 3 minutes and were most easily detected in the FUV continuum and the Si IV and C IV resonance lines. Variations in both the cooler (e.g., Lya, C II, O I) and hotter (O V, N V, Fe XXI) emission lines were much less pronounced. We examined the physical characteristics of the Ñare events, including the time history of the wavelength-integrated Ñuxes in the continuum and the various emission lines. In particular, we searched for, but did not Ðnd, evidence for increased emission in the red wing of the Lya line, which would indicate the presence of moderately energetic proton beams. We integrated the emission over the entire time of major Ñare activity to investigate the average line and continuum properties of the time-averaged spectrum, including Ñows and turbulence as a function of temperature. We also considered the shape of the FUV continuum, which may actually be caused by the enhancement of numerous weak emission lines. Unfortunately, none of the events were strong enough to allow a detailed examination of line proÐles as a function of time.

Opacity in the upper atmosphere of AU Mic

“…For these reasons, this lower limit to the density was verified with the C  1175.99 Å (2s2p 3 P 1 -2p 2 3 P 0 ) and 1176.37 Å (2s2p 3 P 2 -2p 2 3 P 1 ) line ratio, as both of these lines have relatively low optical depths and fall in the same channel. Our density estimate is in agreement with densities determined for AU Mic of 6.3 × 10 10 cm −3 during quiescence (Pagano et al 2000) and 1.7 × 10 12 cm −3 during flare activity (Robinson et al 2001).…”

“…The higher resolution observations of AU Mic by Robinson et al (2001) revealed that the 10 5 K slow moving plasma (<60 km s −1 ) is basically optically thin, but suggests that the faster moving plasma (100 km s −1 ) may show opacity effects.…”

Search for indications of stellar mass ejections using FUV spectra

“…9. Del Zanna et al (2002) analyzed the same data set and termed the increase in ct/s of exposure 9 of the AU Mic data a transition-region explosive event similar to events found in Robinson et al (2001) and Ayres et al (2001). The profiles of AU Mic show only small asymmetries.…”

X-raying the AU Microscopii debris disk

“…AU Mic is one of the brightest nearby X-ray emitters (log L X ≈ 29.3) and shows strong flaring activity, making AU Mic a valuable target for flare studies as shown by e.g. UV observations (Robinson et al 2001).…”