1996A&A...306..877L

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 magnetic field is actively accreting mass from a circumstellar disk. We argue that the magnetic field pressure stops the accretion disk at the height ∼R_* above the stellar surface. A part of the matter from 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 V₀∼300km/s. Finally the matter is stopped near the stellar surface via shock wave mechanism. The temperature and density of the gas immediately after the shock front are of the order of 10⁶K and 3x10¹³cm^–3, respectively. Thermal energy of the shock is carried away via radiation, which cools the decelerating post-shock gas almost to the effective temperature of the 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 gas of pre-shock zone. Thus we suppose that there are at least two regions with different physical conditions where the observed emission lines are formed: before and after shock front. The second half of radiation flux from post-shock zone moves inward and is absorbed in upper, still dense layers of stellar atmosphere (N>10¹⁵cm^–3), forming a kind of transition region. A ring-like accretion belt around the magnetic pole is formed in each hemisphere of the star at gas temperature near 6500K. We argue that in the case of RU Lupi these two "spots" occupy ∼30% of the stellar surface, and that these are the regions where observed optical and UV "veiling" continuum form. The model explains the continuum emission at λ>0.9µm as a result of the radiation of the accretion disk heated by viscous dissipation and the absorption of radiation from the stellar surface. The main parameters of RU Lupi as a star were found from a comparison of observed and theoretical continuum energy distribution: T_eff=3800K, L_*≃0.5L_☉, R_*≃1.6R_☉. The mass of the star M<1M_☉ and its age t∼10⁶yr were estimated from RU Lupi's position on theoretical H-R diagram. An accretion rate {dot}(M)∼3x10^–7M_☉/yr is derived, but during the so called flare events, it can be at least 2 times larger. An important peculiarity of the model is that the luminosity of the central young star is only 10% of the observed luminosity of RU Lupi, while the remaining 90% is conditioned by accretion. Evolutionary status and stellar wind problems of RU Lupi are also discussed on.