THE ULTRAVIOLET AND VISIBLE-LIGHT VARIABILITY OF BP TAURI - POSSIBLE CLUES FOR THE ORIGIN OF T-TAURI STAR ACTIVITY

BP Tau is a moderate-strength T Tauri star, which has previously been studied photometrically by Vrba et al. and shown to have a 7.6-day light cycle. We have re-observed BP Tau in order to monitor this star with the IUE satellite. In conjunction with the UV spectroscopy, we have also obtained nearly simultaneous ground-based optical photometry. At optical wavelengths, we initially found the star to be varying with a period of 7.6 days. This variability suddenly vanished and was followed by a quiescent state that lasted about 11 days. The star then resumed its variability with a period of about 6.1 days. The UV continuum and most of the UV emission lines appeared to vary in phase with the optical flux. No variations were observed on the very short timescales ranging from several hours to half a day expected on the basis of axisymmetric, quasi-steady-state accretion disk models of T Tauri stars. However, the nearly week-long variations we have observed may be consistent with a magnetically buffered accretion model, which has been proposed by Uchida and Shibata.