Investigating the transport of angular momentum from young stellar objects

Aims. In this pilot study, we examine molecular jets from the embedded Class I sources, HH 26 and HH 72, to search, for the first time, for kinematic signatures of jet rotation from young embedded sources. Methods. High-resolution long-slit spectroscopy of the H2 1−0 S(1) transition was obtained using VLT/ISAAC. The slit was placed perpendicular to the flow direction about 2′′ from the sources. Position-velocity (PV) diagrams are constructed and intensity-weighted radial velocities transverse to the jet flow are measured. Results. Mean intensity-weighted velocities vary between vLSR ∼ −90 and −65 km s−1 for HH 26, and −60 and −10 km s−1 for HH 72; maxima occur close to the intensity peak and decrease toward the jet borders. Velocity dispersions are ∼45 and ∼80 km s−1 for HH 26 and HH 72, respectively, with gas motions as fast as −100 km s−1 present. Asymmetric PV diagrams are seen for both objects, which a simple empirical model of a cylindrical jet section shows could in principle be reproduced by jet rotation alone. Assuming magnetocentrifugal launching, the observed HH 26 flow may originate at a disk radius of 2−4 AU from the star with the toroidal component of the magnetic field dominant at the observed location, in agreement with magnetic collimation models. We estimate that the kinetic angular momentum transported by the HH 26 jet is ∼2 × 10−5 M yr−1 AU km s−1. This value (a lower limit to the total angular momentum transported by the flow) already amounts to 70% of the angular momentum that has to be extracted from the disk for the accretion to proceed at the observed rate. Conclusions. These results of this pilot study suggest that jet rotation may also be present at early evolutionary phases and support the hypothesis that they carry away excess angular momentum, thus allowing the central protostar to increase its mass.