Ambruster and collaborator A. Brown (U. Colorado) observed the flare star EV Lac (dM4.5e; d=5 pc; Pd) with the EUVE and IUE spacecraft during 1993 Sep 9-13. The observations were timed to coincide with an international ground-based campaign organized by collaborator R. Gershberg (Crimean Ap. Obs., Ukr.). The observations looked for factors in the upper atmospheric temperature and energy structures that could shed light on the star's propensity for unusually energetic flares at both optical and X-ray wavelengths. Large amounts of data were obtained, both from the two NASA spacecraft and from more than half a dozen ground-based sites. Some of the EUVE results were reported last year.
This year, analysis focused on the flares. The EUVE Deep Survey light curve contains four flares, including one unusually large one, Flare 1 (at 1993 Sep 9, 0610 UT), with a factor 20 flux increase and a total energy ergs. The other 3 flares showed typical factor 2-3 flux increases.
In order to study the effects of the flares on various levels of the star's atmosphere, Ambruster and students Brian Abbott and Brian Pomerance reduced and analyzed the extensive IUE data (23 SWP-Lo and 18 LWP-Lo images) obtained during the EUVE observation, presenting a paper at the June 1995 AAS meeting. The only lines measurable on all the IUE spectra were Mg II (2800 Å) and C IV (1550 Å). Fortunately, these lines are well correlated with total chromospheric and transition region (TR) fluxes, respectively.
The first two EUVE DS flares, Flares 1 and 2, were both accompanied by significant (factor 2-5) chromospheric (Mg II 2800) and transition region (C IV 1550) flares in the IUE data. (There was no IUE coverage during Flares 3 and 4.) There was no ground-based coverage during the large flare, Flare 1. However, an e-folding decay time, = 18.6 min, was determined from the EUVE DS light curve, which is similar to some other coronal flares on M dwarfs, e.g., Prox Cen, and suggestive of events similar to compact flares on the Sun. The decay time, and the fact that the lines visible in the EUVE short wavelength (sw) spectrum originate at soft X-ray temperatures (5.8 log T 7.2), establish the DS flare as primarily a gradual phase event. Flare 1 was strong enough (barely) in the EUVE data for a separate spectral analysis: interestingly, the high temperature (10 K) plasma in the flare is consistent with normal, or low density (n cm). This might merely reflect expansion of the flare plasma, and not necessarily low density conditions at the site of the initial instability.
For Flare 2, however, ground-based coverage revealed considerable activity immediately preceding the EUVE DS flare: two weak decametric radio bursts were observed within 20 min of a U = 3.1 mag flare which, in turn, occurred during an EUVE data gap min before Flare 2. The EUVE DS light curve around flare peak, although incomplete, implies a decay time similar to Flare 1, i.e., min. The chromospheric Mg II flares, since they occur in exposures begun after the DS flare peak in both cases, are unambiguously gradual phase events, which is consistent both with solar and other stellar flare observations. Both C IV flares come from exposures begun hr before the associated DS flare peak. This emission, though it probably represents gradual phase cooling (on analogy with C IV flares on other cool stars), could also contain a contribution from flare peak, or even from the initial impulsive phase. Volume Emission Measure modelling, supplemented by the C IV ( 1550) data, is underway with collaborator Brown (U. Colorado), and a final paper is in preparation. This research was supported by NASA grants NAG5-980 and 2279.