MULTIFREQUENCY MONITORING OF RU LUPI .2. THE MODEL

The data obtained during multifrequency monitoring of extreme T Tauri star RU Lupi and presented in Giovannelli et al. (1995), are explained by a model in which a young star with a moderately strong global magn etic field is actively accreting mass from a circumstellar disk. We ar gue that the magnetic field pressure stops the accretion disk at the h eight similar to R above the stellar surface. A part of the matter fr om the innermost regions of the accretion disk freezes into the global stellar magnetic field lines and then slides along them, accelerating via gravity up to velocity V-0 similar to 300 km s(-1). Finally the m atter is stopped near the stellar surface via shock wave mechanism. Th e temperature and density of the gas immediately after the shock front are of the order of 10(6) K and 3 x 10(13) cm(-3), respectively. Ther mal energy of the shock is carried away via radiation, which cools the decelerating post-shock gas almost to the effective temperature of th e star. A half of liberated thermal energy radiates away from the star , so that extreme UV and soft X-ray quanta heat and ionize inflowing g as of pre-shock zone. Thus we suppose that there are at least two regi ons with different physical conditions where the observed emission lin es are formed: before and after shock front. The second half of radiat ion flux from post-shock zone moves inward and is absorbed in upper, s till dense layers of stellar atmosphere (N > 10(15) cm(-3)), forming a kind of transition region. A ring-like accretion belt around the magn etic pole is formed in each hemisphere of the star at gas temperature near 6500 K. We argue that in the case of RU Lupi these two ''spots'' occupy similar to 30% of the stellar surface, and that these are the r egions where observed optical and UV ''veiling'' continuum form. The m odel explains the continuum emission at lambda > 0.9 mu m as a result of the radiation of the accretion disk heated by viscous dissipation a nd the absorption of radiation from the stellar surface. The main para meters of RU Lupi as a star were found from a comparison of observed a nd theoretical continuum energy distribution: T-eff = 3800 K, L simil ar or equal to 0.5L., R similar or equal to 1.6R. The mass of the sta r M < 1M. and its age t similar to 10(6) yr were estimated from RU Lup i's position on theoretical H-R diagram. An accretion rate M over dot similar to 3 X 10(-7) M./yr is derived, but during the so called flare events, it can be at least 2 times larger. An important peculiarity o f the model is that the luminosity of the central young star is only 1 0% of the observed luminosity of RU Lupi, while the remaining 90% is c onditioned by accretion, Evolutionary status and stellar wind problems of RU Lupi are also discussed on.