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Title:
Critical Tests of Magnetospheric Accretion Theory
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Authors:
Christopher M. Johns-Krull, Rice University
Jeff A. Valenti, Space Telescope Science Institute
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Abstract:
Magnetospheric accretion is the general model by which a newly formed, low mass star is believed to interact with its surrounding accretion disk. This interaction determines the rate of mass accretion onto the central star and possibly the rate at which material is ejected from the system in a bipolar jet. The interaction may also set an inner boundary for migrating planets in the disk. While this model can explain many of the features observed in classical T Tauri stars, relatively few attempts have been made to test the specific predictions of this model, and the attempts that have been made have generally not met with success. Magnetospheric accretion models make specific predictions relating the stellar mass, radius, dipole magnetic field strength, and rotation rate along with the disk accretion rate. Over the last few years we have carried out a program of magnetic field measurements on T Tauri stars, and we find that the observations do not agree with the theory. Specifically, the fields on T Tauri stars are not dipolar, and the mean field strength does not correlate with the predicted field strength: instead the field strength is very similar from one star to the next. Assuming a constant surface magnetic field strength, we show how magnetospheric accretion predictions can be modified to account for the non-dipole geometry of the magnetic field on T Tauri stars, yielding new equations relating the stellar and disk accretion parameters. We find that the data do indeed follow these new predictions.
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