Accretion and UV variability in BP Tauri

Abstract

BP Tau is one of the few classical T Tauri stars for which the presence of a hot spot in the surface has been reported without ambiguity. The most likely source of heating is gravitational energy released by the accreting material as it shocks with the stellar surface. This energy is expected to be radiated mainly at UV wavelengths. In this work we report the variations of the UV spectrum of BP Tau for 1992 January 5-19, when the star was monitored with IUE during two rotation periods. Our data indicate that lines that can be excited by recombination processes, such as those from O I and He II, have periodic-like light curves, whereas lines that are only collisionally excited do not follow a periodic-like trend. These results agree with the expectations of the magnetically channeled accretion models. The kinetic energy released in the accretion shocks is expected to heat the gas to temperatures of similar to 10(6) K, which henceforth produces ionizing radiation. The UV (Balmer) continuum and the O I and He II lines are direct outputs of the recombination process. However, the C IV, Si II, and Mg II lines are collisionally excited not only in the shock region but also in inhomogeneous accretion events and in the active (and flaring) magnetosphere, and therefore their light curves are expected to be blurred by these irregular processes. We also report the detection of warm infalling gas from the presence of redshifted (81 km s(-1)) absorption components in some of the high-resolution Mg II profiles available in the IUE and Hubble Space Telescope archives.