ASCA Detection of a Superhot 100 Million K X-Ray Flare on the Weak-lined T Tauri Star V773 Tauri

We present results of a ≈ 40 ks ASCA observation of the active weak-lined T Tauri star V773 Tau (HD 283447) and the surrounding Barnard 209 dark cloud, obtained in 1995 February. During this observation, V773 Tau exhibited a dramatic X-ray flare, with the X-ray count rate increasing rapidly by a factor of ~20, then decreasing exponentially with an e-folding timescale of ≈ 2.3 hr. The peak flare luminosity was at least ~10³³ 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 ~10³⁷ ergs. However, the most spectacular aspect of this flare was its temperature, which reached a maximum value of at least 100 million K. Spectral fits near flare maximum give a temperature of ~10 keV, which slowly declined to a value ~6 keV at the end of the observation. These temperature measurements are based on high signal-to-noise ratio spectra, and provide the first unambiguous evidence for superhot flaring plasma at temperatures of ~10⁸ K in T Tauri stars. A simple cooling-loop model gives electron densities that are similar to those of solar flares, but requires loop sizes that are comparable to or larger than the star itself. The flare showed other interesting behavior, including a high (and possibly variable) absorption column density, N_H = 4 × 10²² cm^-2, and an apparent increase in the global metal abundance during the flare.