Optical and X-ray studies of newly discovered flare stars

Abstract

Results are presented demonstrating that selection of dMe flare stars from their EU V/optical flux ratio revealed some of the most active members from the ROSAT Wide Field Camera (WFC) and EUVE all-sky surveys. During a total photometric monitoring time of 121 hrs on 6 new stars, there were 54 separate optical flares recorded, including a dramatic AU = 3.8 mag flare, emitting an energy in the U-band of Eu lx 10 erg. Three other flares had AU > 2.2 mag.
The flaring rate of two of these new stars is shown to be comparable to that of famous flare stars.
The high time-resolution spectra (AA3600-4200A) obtained simultaneously during the large flare were analyzed and it was found that line symmetry changed significantly within the first minute and even changed from a blue asymmetry to
• red one on timescales of 10 secs. These are the first such observations in
• dMe star flare. Subsequent analysis, in the context of the thick target flare model, revealed strong evidence for the existence of '-.s (1-3) MeV particle beams.
Although this phenomenon has previously been reported on the Sun for Ha and X-ray lines of CaXIX and FeXXV, and symmetric and asymmetric broadening of the Balmer and Ca II H & K lines has also been observed at low time resolution (> 1 mm) before on dMe stars, but until this work, interpretation of these observations has been hampered by poor time resolution, resulting in incorrect conclusions about the plasma dynamics.
Firm evidence for coronal activity in three of the newly-discovered flare stars is presented by ROSJ4T HRI X-ray (0.2 keV-2.0 keV) data. A large flare on one of these stars emitted energy, Ex 2 x 101 erg, with a loop length 0.4 R. and lasting 7 hrs. This flare was modeled and found to be not inconsistent with the solar 2-ribbon scenario.