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The Classical T Tauri Star Sz 68: Doppler Imaging and Evidence for Magnetospheric Accretion

We have Doppler imaged the surface of the classical T Tauri star Sz 68, both under the assumption of solid-body rotation as well as allowing for differential rotation. Assuming solid-body rotation, we find evidence for a large, cool, polar spot similar to those found on previously imaged naked T Tauri stars. Allowing for substantial levels of antisolar differential rotation in the image reconstruction causes the bulk of the polar spot to disappear, although several small spots remain at latitude +60°. Checks between the various lines marginally favor the solid-body rotation case. We examine the potential influence of accretion hot spots on the line profiles in Sz 68, concluding that we are unable to resolve these structures in our Doppler image of the star. We also examine the Hα and Na I line profiles in this star and compare their variations to the stellar surface image. We conclude that the Hα line is formed primarily in an accretion flow, and we find strong evidence that this accretion flow is modulated by the stellar rotation. These results are most easily interpreted within the framework of magnetospheric accretion models for classical T Tauri stars. The phasing of accretion as traced by redshifted absorption in the Hα line profile compared with the stellar surface image strongly suggests a dipole magnetic field inclined by approximately 15° with the rotation axis in this star.