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Wavelength (|m)

Fig. 5.10 Measurement of the Zeeman broadening of the Ti I transition at 2.2 |m, obtained for the T-Tauri type star BP Tau. Crosses: observations; solid line: model in the absence of a magnetic field (After Johns-Krull et al., 1999)

How do low-mass stars acquire their mass and angular momentum? Both originate in the initial molecular cloud, whose collapse phase ends with the formation of a disk. In broad outline, the process is understood: transfer of the disk's angular momentum to the star implies the transfer of mass, which falls from the disk to the star with a free-fall time that is shorter than the rotation period. The star thus acquires a rapid rotation which results in the generation of a strong magnetic field within its convective interior. The star's magnetic field couples with the disk, at a distance of several stellar radii. This slows down the star's rotation to that of the disk at that distance. Strong stellar winds result, the terminal velocities of which and the associated mass-losses may be measured, with good agreement with predictions.

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