Are Protostellar Jets Spinning? Chin-Fei Lee, Paul P.T. Ho, Aina Palau, Naomi Hirano, Tyler L. Bourke, Hsien Shang, & Qizhou Zhang.
Protostellar jets are seen associated with protostars, representing one of the most intriguing signposts of star formation. They were not originally anticipated in any theory of star formation but turn out to be fundamental phenomena of star formation. They are believed to be launched from accretion disks around the protostars, carrying away extra angular momenta from the disks and thus allowing material to fall onto the protostars. The amount of angular momenta carried by the jets is predicted to be different in different models, allowing us to determine the launching mechanism. The Submillimeter Array (SMA) with the capability to probe warm and dense molecular gas at high angular resolution, can be used to extract this crucial information from the jets.
Herbig-Haro (HH) 211 is a well-defined bipolar (two-sided) jet located at only 1000 light-year away in the constellation Perseus. Lying close to plane of the sky, it is one of the best candidates for our study. The jet has a velocity of 100-200 km/s. Sitting at the center of the jet is a Class 0 protostar deeply embedded in the cloud. It is only about 20,000 years old with a mass of only 6 percent of the mass of our Sun. It will eventually grow into a star like our Sun in tens of million years.
Top) The two-sided (red for receding and blue for approaching sides) SiO (J=8-7) jet observed with the SMA (Lee et al. 2007). Gray image shows the shocked H2 emission produced by the jet (Hirano et al. 2006). The jet is knotty, consisting of a chain of knots on each side of the protostar, with an interknot spacing of 600 - 900 AU. The innermost pair of knots are only at 500 AU away from the protostar. These knots likely trace the internal shocks in the jet produced by a periodical variation in the jet velocity. The period of the variation is estimated to be ~ 30 yrs. This periodic variation may be due to a solar-type magnetic cycle or a periodic disturbance of an unseen stellar companion orbiting the protostar at ~ 4 AU away with a period of 30 yrs. The averaged two-sided mass-loss rate of the jet is estimated to be (1-3) x10-6M⊙yr-1.
Right) Position-velocity diagrams cut across the first four brightest SiO knots closest to the protostar. A velocity gradient is consistently seen across the jet axis, with the same sense as that seen in the circumstellar envelope, suggesting that the velocity comes from jet rotation. If this is the case, the implied angular momentum of the jet is < 50 AU km/s, indicating that the jet must be launched very close to the protostar, as predicted in the X-wind model by Shu et al 1995. Further observations have been proposed to check it.
References:1) Hirano, N., Liu, S.-Y., Shang, H., et al. 2006, ApJL, 634, L141
2) Lee, C.-F., Ho, P.T.P., Aina Palau, et al., 2007, ApJ, 670, 1188
3) Shu, F.H., Narita, J., Ostriker, E.C., & Shang, H., 1995, ApJL, 455, L155