Magnetospheric accretion-ejection processes in the classical T Tauri star AA Tauri ^*,**

¹ Laboratoire d'Astrophysique de Grenoble, Université Joseph-Fourier, BP 53, 38041 Grenoble Cedex 9, France e-mail: jbouvier@obs.ujf-grenoble.fr
² Departamento de Física, ICEx-UFMG, CP. 702, Belo Horizonte, MG, 30123-970, Brazil
³ Steward Observatory, University of Arizona, 933 N. Cherry Av., Tucson AZ 85721, USA
⁴ Department of Optics and Quantum Electronics, University of Szeged, Dóm tér 9, 6720 Szeged, Hungary
⁵ Ulugh Beg Astronomical Institute of the Uzbek Academy of Sciences, Astronomicheskaya 33, 700052 Tashkent, Uzbekistan
⁶ Institute of Astronomy, Madingley Road, Cambridge CB3 0HA, UK
⁷ Konkoly Observatory, 1525 Budapest PO Box 67, Hungary
⁸ V.A. Ambartsumyan Byurakan Astrophysical Observatory, Armenia

Received: 12 July 2006
Accepted: 28 November 2006

Abstract

Context.Accretion and ejection are complex and related processes that vary on various timescales in young stars.

Aims.We intend to investigate the accretion and outflow dynamics and their interaction from observations of the classical T Tauri star AA Tau.

Methods.From a long time series of high resolution () HARPS spectra and simultaneous broad-band photometry, we report new evidence for magnetospheric accretion as well as ejection processes in the nearly edge-on classical T Tauri star AA Tau.

Results.AA Tau's light curve is modulated with a period of 8.22 d. The recurrent luminosity dips are due to the periodic occultation of the central star by the magnetically-warped inner disk edge located at about 9 . Balmer line profiles exhibit a clear rotational modulation of high-velocity redshifted absorption components with a period of 8.22 days as well, with a maximum strength when the main accretion funnel flow passes through the line of sight. At the same time, the luminosity of the system decreases by about 1 mag, indicative of circumstellar absorption of the stellar photosphere by the magnetically-warped, corotating inner disk edge. The photospheric and He I radial velocities also exhibit periodic variations, and the veiling is modulated by the appearance of the accretion shock at the bottom of the accretion funnel. Diagnostics of hot winds and their temporal behaviour are also presented.

Conclusions.The peculiar geometry of the young AA Tau system (nearly edge-on) allows us to uniquely probe the acretion-ejection region close to the star. We find that most spectral and photometric diagnostics vary as expected from models of magnetically-channelled accretion in young stars, with a large scale magnetosphere tilted by 20° onto the star's spin axis. We also find evidence for time variability of the magnetospheric accretion flow on a timescale of a few rotational periods.