Non-axisymmetric accretion on the classical TTS RW Aur A

High-resolution spectroscopic monitoring of the exceptionally active classi cal T Tauri star (CTTS) RW Aur A was carried out in three seasons of 1996, 1998 and 1999 with simultaneous B, V photometry. The high quality spectra r evealed a multicomponent structure of the spectrum, which includes: 1) a ve iled photospheric spectrum of a K1-K4 star, 2) broad emission lines of neut rals and ions, 3) narrow emission lines of HeI and HeII, a) red-shifted acc retion features of many lines, 5) shell lines at about the stellar velocity , Fl blue-shifted wind Features and 7) forbidden lines. Periodic modulations in many spectral features were found. The photospheric absorption lines show sinusoidal variations in radial velocity with an amp litude of +/- 6km s(-1) and a period of about 2.(d)77. The radial velocitie s of the narrow emission lines of He vary with the same period but ill anti -phase to the photospheric lines. The equivalent widths of the narrow emiss ions vary with a phase-shift with respect to the velocity curve. The streng th of the red-shifted accretion components of Na D and other lines is also modulated with the same period. The broad emission lines of metals vary mos tly with the double period of about 5.(d)5. One unexpected result is that no correlation was found between the veiling and the brightness, although both parameters varied in nide ranges. This is partly due to a contribution of the shell absorption to the photospheric l ine profiles, which make them vary in width and depth thus simulating lower veiling. The spectral lines of the accreting gas show two distinct components: one i s formed at low velocity at the beginning of the accretion column, and the other at high velocity near the stellar surface, The low velocity component s are strong in low excitation lines of neutrals, while the high velocity c omponents are strong in high excitation lines of ions, thus showing the gra dients of temperature and density along the accretion column. Most of the observed features can be interpreted in the framework of non-ax isymmetric magnetospheric accretion, but the origin of this asymmetry can b e explained in different wave. We consider two possible models. The first m odel suggests that RW Aur A is a binary with a brown dwarf secondary in a n early circular orbit with a period of 2.(d)77. The orbiting secondary gener ates a moving stream of enhanced accretion from one side of the disk toward s the primary. The other model assumes that RW Aur A is a single stat. with a rotational period of 5.(d)5 and with two footpoints of channeled accreti on streams within a global magnetosphere which is tilted relative to the ro tational axis or otherwise non-axisymmetric, Both models can explain qualit atively and quantitatively most of the observed variations, hut there are s ome details which are less well accounted for.