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, th e stellar magnetic field truncates the disk several stellar radii abov e the stellar photosphere; material then flows along magnetic field li nes and forms a bright ring at which the accretion stream impacts the star. The model successfully reproduces the observations for reasonabl e values of the magnetic field strength, 100-500 G; the stellar rotati onal 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 corotatio n radius, R(c). We estimate R(0)/R(c) approximate to 0.6-0.8 for class ical T Tauri stars in our sample. This result constrains models for th e 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 rotati onal period. The magnitude of the near-IR veiling should correlate str ongly with the stellar rotational period. Strong CO emission or absorp tion features should be present only in stars with high accretion rate s. Observations also discriminate between various types of magnetic di sk geometries if intrinsic stellar parameters, such as the stellar rad ius and magnetic field strength, are well known.