C. Sauty, Z. Meliani, J. J. G. Lima, K. Tsinganos, V. Cayatte, N. Globus

Abstract
Context. A large sample of T Tauri stars exhibits optical jets and approximately half of them rotate slowly, only at ten per cent of their breakup velocity. The disk locking mechanism has been shown to be inefficient to explain this observational fact.
Aims. We show that low mass accreting T Tauri stars may have a strong stellar jet component which can effectively brake the star to the observed rotation speed.
Methods. By means of a nonlinear separation of the variables in the full set of the MHD equations we construct semi analytical solutions describing the dynamics and topology of the stellar component of the jet which emerges from the corona of the star.
Results. We analyze two typical solutions with the same mass loss rate but different magnetic lever arms and jet radii. The first solution with large lever arm and jet radius effectively brakes the star and can be applied to the visible jets of T Tauri stars, such as RY Tau. The second solution with a smaller lever arm and a very narrow jet radius may explain why similar stars, either Weak line T Tauri Stars (WTTS) or Classical T Tauri Stars (CTTS) do not all have visible jets. For instance, RY Tau itself seems to have different phases depending probably on the activity of the star.
Conclusions. First, stellar jets seem to be able to brake pre-main sequence stars with a low mass accreting rate. Second, jets may be visible only part time due to changes in their boundary conditions. We also suggest a possible scenario for explaining the dichotomy between CTTS and WTTS which rotate faster and do not have visible jets.

Keywords
magnetohydrodynamics (MHD) – stars: pre-main sequence – stars: winds, outflows – ISM: jets and outflows – stars: mass-loss – stars: rotation

Astronomy & Astrophysics
Volume 533, Number of pages A46_1
2011 September

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