Modulated temporal variability in DF Tauri: a bipolar BR gamma emission structure

Based on observations collected at the Observatorio Astronomico Nacional, San Pedro Martir, B.C., Mexico. We present day to day K-band spectroscopic and photometric observations of the classical T Tauri star DF Tau, obtained along a complete rotation period. The high S/N mean near infrared spectrum of DF Tau exhibits Br gamma in emission and numerous absorption lines, half of which are not yet identified. During the observing time, the equivalent width of the Br gamma emission line (EW) varied by a factor of 3 and the veiling by a factor of 2, while the 2.2 microns continuum flux changed by about 30%. The EW exhibits two clear maxima separated by about half the stellar rotation period. The EW, veiling and flux variations, find a natural explanation if we attribute these changes to a geometrical effect caused by the stellar rotation. We conclude that the physical phenomena that originates Br gamma emission is located in a bipolar structure whose axis is not aligned with the stellar rotation one. This bipolar structure projected onto the star might cover about 25% of its surface.