UV spectra of T Tauri stars from the HST and IUE satellites: BP Tau

Abstract

Ultraviolet spectra of BP Tau observed with HST/GHRS and IUE satellites were analysed. We found that BP Tau activity can be explained in the frame of a disk accretion model if we assume that the stellar magnetic axis is strongly inclined to the disk plane. The following set of accretion process parameters were derived: relative surface area of the accretion zone f ≃ 0.25, accretion rate Mac ≃ 3.6 × 10-8 M⊙yr-1, accretion energy flux F = 2.3 × 1010 erg s-1 cm-2 and accretion luminosity Lac ≃ 0.37 L⊙. The relevance of these parameters is discussed. We argue that the Calvet & Gullbring (1998) accretion shock model is too crude to believe that the accretion spot surface area is indeed proportional to the square of the accretion rate, as Ardila & Basri (2000) found through this model. A strong flare in the FeII 2811.8, 2812.1 Å lines was detected, it was probably produced by an increase of the accretion rate. During the flare, the accretion luminosity was comparable to or even larger than the stellar bolometric luminosity.