One of the key problems in the theory of star formation is how the star can be formed from a rotating cloud without accreting the angular momentum together with the matter, since otherwise the star would have to spin up more and more, finally reaching the breakup velocity and stopping the process of star formation. There must, therefore, be a mechanism that carries angular momentum away from the star without significantly reducing the stellar mass.
It is now widely accepted, that magnetic fields play an important role in extracting angular momentum from the star and thus braking its rotation (cf. Bodenheimer 1995). Two mechanisms are known. First, a wind coupled to the star by the stellar magnetic field can exert a strong torque due to the large radius of the magnetosphere. Second, the magnetic field can thread a circumstellar accretion disk, disrupting the disk inside the corotation radius and leaving those parts of the disk that rotate slower than the star. It will then accelerate the rotation of the disk and brake the star, fixing its rotation rate at certain equilibrium value (Ghosh & Lamb 1979a,b, Camenzind 1990, Königl 1991, Cameron & Campbell 1993, Yi 1994, Cameron et al. 1995, Ghosh 1995, Lovelace et al. 1995, Armitage & Clarke 1996).
There is some evidence that the latter braking mechanism is indeed at work in classical T Tauri stars (CTTS). Stars of this type are surrounded by by disks, while weak line T Tauri stars (WTTS) are not (cf. Bertout 1989). The rotation rates of CTTS are found to be considerably smaller than those of WTTS, in agreement with the assumption that disks brake the stellar rotation (Bouvier et al. 1993,Edwards et al. 1993). The present model of the pre-main sequence (PMS) evolution of stellar rotation includes both braking by disks and winds. During the T Tauri phase, the star is prevented from spinning up as long as the disk exists. The star begins to spin up when the disk disappears, finally being spun down again by the wind (Bouvier 1994).
In any case the geometry as well as the strength of the magnetic field are decisive for the efficiency of the braking process. Since pre-main sequence stars are fully convective, the decay time of a fossil field would be some years only and thus a dynamo process is necessary to generate the stellar magnetic field. The field structure depends on the type of dynamo, especially the relative magnitude of the two field generating processes, namely the rotational shear and the mean helicity ( -effect) of the convective motions. In the solar dynamo the shear dominates and the resulting magnetic field is axisymmetric and has dipole geometry. It is, however, not clear at all if stellar magnetic fields are always generated by -dynamos. Observations show that for lower main sequence stars the normalized surface differential rotation,
decreases with increasing rotation rate (Hall 1991, Donahue et al. 1996), a behaviour also found from theoretical work by Rüdiger et al. (1997). Johns-Krull (1996) derived the differential rotation of two CTTS and one WTTS from the profiles of photospheric absorption lines. He found that the data are consistent with polar acceleration or rigid rotation but not with equatorial acceleration. Doppler imaging of the WTTS V410 Tauri shows a non-axisymmetric distribution of stellar spots and that differential rotation, if present at all, must be very small and solar-type (Rice & Strassmeier 1996). We must therefore expect a rotational shear much smaller than in the Sun, probably with opposite sign.
© European Southern Observatory (ESO) 1997
Online publication: March 24, 1998