Non-axisymmetric accretion on the classical TTS RW Aur A^*

¹ Astronomy Division, PO Box 3000, 90014 University of Oulu, Finland
² Stockholm Observatory, 133 36 Saltsjöbaden, Sweden
³ CAUP, Universidade do Porto, Rua das Estrelas, 4150 Porto, Portugal
⁴ Departamento de Matemática Aplicada, Rua das Taipas, 135, 4150 Porto, Portugal

Corresponding author: P. P. Petrov, Peter.Petrov@Oulu.Fi

Received: 7 September 2000
Accepted: 1 February 2001

Abstract

High-resolution spectroscopic monitoring of the exceptionally active classical 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 revealed a multicomponent structure of the spectrum, which includes: 1) a veiled photospheric spectrum of a K1-K4 star, 2) broad emission lines of neutrals and ions, 3) narrow emission lines of Hei and Heii, 4) red-shifted accretion features of many lines, 5) shell lines at about the stellar velocity, 6) 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 amplitude of km s^-1 and a period of about . The radial velocities of the narrow emission lines of He vary with the same period but in anti-phase to the photospheric lines. The equivalent widths of the narrow emissions vary with a phase-shift with respect to the velocity curve. The strength 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 mostly with the double period of about . One unexpected result is that no correlation was found between the veiling and the brightness, although both parameters varied in wide ranges. This is partly due to a contribution of the shell absorption to the photospheric line 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 is formed at low velocity at the beginning of the accretion column, and the other at high velocity near the stellar surface. The low velocity components are strong in low excitation lines of neutrals, while the high velocity components are strong in high excitation lines of ions, thus showing the gradients of temperature and density along the accretion column. Most of the observed features can be interpreted in the framework of non-axisymmetric magnetospheric accretion, but the origin of this asymmetry can be explained in different ways. We consider two possible models. The first model suggests that RW Aur A is a binary with a brown dwarf secondary in a nearly circular orbit with a period of . The orbiting secondary generates a moving stream of enhanced accretion from one side of the disk towards the primary. The other model assumes that RW Aur A is a single star with a rotational period of and with two footpoints of channeled accretion streams within a global magnetosphere which is tilted relative to the rotational axis or otherwise non-axisymmetric. Both models can explain qualitatively and quantitatively most of the observed variations, but there are some details which are less well accounted for.