Locating the Launching Region of T Tauri Winds: The Case of DG Tauri

It is widely believed that T Tauri winds are driven magnetocentrifugally fromaccretion disks close to the central stars. The exact launching conditions areuncertain. We show that a general relation exists between the poloidal andtoroidal velocity components of a magnetocentrifugal wind at large distancesand the rotation rate of the launching surface, independent of the uncertainlaunching conditions. We discuss the physical basis of this relation and verifyit using a set of numerically-determined large-scale wind solutions. Bothvelocity components are in principle measurable from spatially resolvedspectra, as has been done for the extended low-velocity component (LVC) of theDG Tau wind by Bacciotti et al. For this particular source, we infer that thespatially resolved LVC originates from a region on the disk extending from$\sim 0.3$ to $\sim 4.0\AU$ from the star, which is consistent with, and arefinement over, the previous rough estimate of Bacciotti et al.Comment: 11 pages, 1 figure, submitted to Astrophysical Journal Letter