The Bright Accretion Rings on Magnetic T Tauri Stars

Abstract

We model the properties of bright rings on accreting classical T Tauri stars whose magnetospheres are tilted with respect to the axis of rotation. These rings have interesting geometries and azimuthal intensity profiles depending on β, the inclination angle of the dipole field, ξ, the ratio of the star's radius to the accretion radius, and , the ratio of the star's luminosity to the total ring luminosity. As the star rotates, the rings may be occulted periodically, and the apparent brightness of the star may vary as a function of time. With knowledge of i, the observer's inclination, it is possible to predict Δm, the amplitude of magnitude variations due to the occultation. Conversely, measurements of Δm and either i or can constrain the values of the other variables. To observe any variation i must be greater than zero. Stars viewed nearly pole-on (i ≈ 10°) must have β 50° to allow any significant variation. On the other hand, with i 60°, even a very small β ≈ 5° can produce a significant Δm. We find that stars that are as bright as their accretion rings ( ≈ 1) cannot produce variations greater than ≈ 0.8 mag; to observe 2 mag variations ≈ 0.1 is required, independently of i or β. We predict a distribution of Δm that is consistent with observed populations of T Tauri stars in the Orion Nebula and in the Taurus-Auriga Molecular Cloud.