Observations of star-forming regions show that several T Tauri stars are associated with well collimated jets (Fig. 1). These jets are made of strongly accelerated plasma that travels at a few hundred km/s and is magnetically collimated at large distances. The jets are usually associated with Classical T Tauri Stars (CTTS), which are low mass stars in their late stages of pre-main sequence evolution, showing strong evidence of the presence of a surrounding accretion disk. The outflow is usually thought to be ejected from the disk. However, a significant part of the jet, if not all, may be ejected by the star itself, at least for the lower mass accreting T Tauri stars. Observations also reveal that approximately half of T Tauri stars rotate slowly, at about 10% or less of their breakup speed. This indicates that a very efficient mechanism is at work to remove the angular momentum in these stars ; the nature of this mechanism is still controversial. The low rotational velocities of T Tauri stars can be understood by considering the effect of stellar jets to remove their angular momentum. In particular, the team modeled stellar jets from CTTS with a mass loss rate consistent with observations. The group analyzed two interesting self-similar MHD solutions (Fig. 2). The first corresponds to a gradually cylindrically collimated wide wind without oscillations in its width. The second is a narrower outflow that refocuses towards the jet axis and also oscillates in width.
From the analysis of the first solution, it has been shown that jets can efficiently brake the central star and remove the bulk of its angular momentum over a timescale of a million years, at least for low mass accreting stars. This corresponds to the typical lifetime of a star in the CTTS phase. Furthermore, with this mechanism the star can even slow down within 0.6 million years, if the disk does not transport angular momentum onto the star. This fairly short slow-down time shows the efficiency with which a magnetized outflow can remove angular momentum from a rapidly rotating young stellar object. This result strongly suggests that stellar jets may explain the low rotational speeds observed in those objects, independently of any disk locking. This mechanism could be complementary to other angular momentum removal mechanisms occurring at the wind emerging from the interaction zone between the disk and the magnetosphere (the so called X-wind). The solution fairly well reproduces the mass loss rate, terminal speed and rotation rate of the jet. The two solutions can be used to explain the different behavior between CTTS and WTTS, which may both actually have jets, but these jets are visible only in CTTS. The connection of CTTS with their disks through their magnetospheres may increase the width of their jets such as to make them visible. The mass loss rate measured in CTTS is thought to decrease as the star evolves towards the WTTS stage of WTTS. Weak T Tauri stars seem to have lost the gas component of their accretion disk or at least gas falling onto the star. The second MHD solution suggests that even with the same mass loss rate, the jets of WTTS may be invisible with the present angular resolution because of their small width . The lack of accretion disk prevents the formation of disk winds and the possibility of higher mass loss rates. The lower jet width may be caused either by a lower stellar magnetic activity or by a lack of magnetic connection into the disk. The magnetic braking time in this case would be of the order of 10 million years, which is indeed the time a T Tauri star is expected to spend in the weak-line stage.
The two solutions can also explain the different stages of the jet of a single star, e.g. RY Tau. Recent observations indicate that the plasma outflow from the rotating and magnetized star may have an intermittency exhibiting at least two phases. During one phase the cylindrical wind is relatively wide and non-oscillating. During the other phase the outflow is narrow and oscillates. Oscillations may produce radiative shocks seen in various emission lines, such as the observed UV lines of RY Tau. As a conclusion, this modeling suggests that CTTS may produce intermittent visible jets in their active phase that efficiently brake the star. Indeed, in the non-oscillating solution the magnetic field dominates the dynamics in the jet. Near the star, the magnetic field has a dipolar structure and the wind is more efficient in this case in extracting stellar angular momentum. On the other hand, WTTS may have jets as well, which are always invisible because they are too narrow. These jets would be weaker because of the lack of direct connection with the accretion disk through a large dead zone.