A magnetic accretion disk model for the infrared excesses of T Tauri stars

We describe a magnetic accretion disk model for the infrared colors of T Tauri stars in the Taurus-Auriga molecular cloud. In this model, the stellar magnetic field truncates the disk several stellar radii above the stellar photosphere; material then flows along magnetic field lines and forms a bright ring at which the accretion stream impacts the star. The model successfully reproduces the observations for reasonable values of the magnetic field strength, 100-500 G; the stellar rotational period, 4-10 days; and the mass accretion rate, 10(-8) to 10(-6) M. yr(-1). The truncation radius, R(0), lies well inside the corotation radius, R(c). We estimate R(0)/R(c) approximate to 0.6-0.8 for classical T Tauri stars in our sample. This result constrains models for the rotational evolution and bipolar outflows of pre-main-sequence stars. Magnetic disk models make several testable predictions. The near-IR colors should correlate with the stellar magnetic field and the rotational period. The magnitude of the near-IR veiling should correlate strongly with the stellar rotational period. Strong CO emission or absorption features should be present only in stars with high accretion rates. Observations also discriminate between various types of magnetic disk geometries if intrinsic stellar parameters, such as the stellar radius and magnetic field strength, are well known.