ASCA detection of a superhot 100 million K X-ray flare on the weak-lined TTauri star V773 Tauri

We present results of a approximate to 40 ks ASCA observation of the active weak-lined T Tauri star V773 Tau (HD 283447) and the surrounding Barnard 2 09 dark cloud, obtained in 1995 February. During this observation, V773 Tau exhibited a dramatic X-ray flare, with the X-ray count rate increasing rap idly by a factor of similar to 20, then decreasing exponentially with an e- folding timescale of approximate to 2.3 hr. The peak flare luminosity was a t least similar to 10(33) ergs s(-1) (0.7-10 keV; distance = 150 pc), which is among the highest X-ray luminosities observed to date for T Tauri stars . The total energy release was similar to 10(37) ergs. However, the most sp ectacular aspect of this hare was its temperature, which reached a maximum value of at least 100 million K. Spectral fits near flare maximum give a te mperature of similar to 10 keV, which slowly declined to a value similar to 6 keV at the end of the observation. These temperature measurements are ba sed on high signal-to-noise ratio spectra, and provide the first unambiguou s evidence for superhot flaring plasma at temperatures of similar to 10(8) K in T Tauri stars. A simple cooling-loop model gives electron densities th at are similar to those of solar flares, but requires loop sizes that are c omparable to or larger than the star itself. The flare showed other interes ting behavior, including a high (and possibly variable) absorption column d ensity, N-H = 4 x 10(22) cm(-2), and an apparent increase in the global met al abundance during the flare.