Astronomy & Astrophysics

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Issue		A&A Volume 377, Number 2, October II 2001
Page(s)		557 - 565
Section		Formation, structure and evolution of stars
DOI		10.1051/0004-6361:20011108

A&A 377, 557-565 (2001)
DOI: 10.1051/0004-6361:20011108

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

L. Errico¹, S. A. Lamzin² and A. A. Vittone¹

¹ Osservatorio Astronomico di Capodimonte, Via Moiariello 16, 80131 Napoli, Italy
² Sternberg Astronomical Institute, Moscow V-234, 119899, Russia

(Received 9 April 2001 / Accepted 19 June 2001 )

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 $f \simeq 0.25,$ accretion rate $\dot M_\mathrm{ac} \simeq 3.6\times 10^$ $M_\odot$ yr^-1, accretion energy flux ${\cal F}=2.3\times 10^$ erg s^-1 cm^-2 and accretion luminosity $L_\mathrm{ac} \simeq 0.37$ $L_\odot$ . 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 Ålines 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.

Key words: stars: pre-main sequence -- stars: individual: BP Tau -- ultraviolet: stars -- X-rays: stars

Offprint request: L. Errico, errico@na.astro.it

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