An extraordinary accretion event detected on DF Tauri

We report on the detection of an extreme flarelike event on DF Tauri during a spectroscopic monitoring program of some classical T Tauri stars known to show hot spots on their surfaces. These observations were performed on a nightly basis within roughly one rotational period of DF Tau between 2000 January 7 and 14. The estimated amplitude of variation during this time reached up to 6 mag in the B band. Significant inverse P Cygni profiles in high Balmer lines - evidence of mass accretion - dominated the spectrum obtained on January 8, when the brightness of DF Tau was approaching its maximum. The flux in the ultraviolet dropped dramatically on the following day. Ca II H and K emission lines, the indicator of chromospheric activity, turned into deep, wide absorption, and all other emission lines in the blue end were also absent (filled in or weak absorption), suggesting severe absorption in the line of sight possibly caused by a transient envelope produced in the process of a violent accretion shock. The effect of veiling decreased rapidly from January 10, and normal P Cygni profiles, indicating the presence of strong wind, began to appear. Based on the unique features displayed, we propose that the spectroscopic record of this flarelike brightening on DF Tau, believed to be due to episodic mass accretion accompanied by the release of gravitational energy, might actually be taken as an isolated, snapshot view of the YY Orionis phenomenon.