Fig. 5.9 (a) Image of the object HH 30 taken by the HST; (b) A schematic interpretation of the disk and of the jet (After Bertout, 2003)
5.2.3 The Rotation of T-Tauri Stars
The rotation periods of T-Tauri stars may be measured by spectroscopy from the broadening of the line profiles, or by photometry by estimates from their light-curves.
The general conclusion that arises from this study is that T-Tauri stars are slow rotators: their periods (which are generally several tens of days) are about ten times the periods that would be expected as a result of the collapse of a cloud, if there had not been any loss of angular momentum. They are also significantly higher than those of stars of comparable age and spectral type that do not have a disk (where the periods are about 8 days).
This results strongly confirms the theory of the loss of angular momentum to the disk through magnetic coupling (see Sect. 5.1.4). Coupling between stellar accretion and magnetic activity is equally revealed by other observations: intense X-ray emission, non-thermal radio emission, and photometric observation of large starspots. Direct detection of a magnetic field in a T-Tauri star has been obtained by measurement of the Zeeman broadening observed in an atomic transition (Fig. 5.10).
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