STELLAR JET ROTATION · Do Jets from Young Stars Rotate?

Do Jets from Young Stars Rotate?

Current star formation theory proposes that protostellar jets exist as a mechanism for transporting excess angular momentum away from the disk allowing it to spin more slowly and thus further accrete onto the newly forming star. The most widely accepted theory proposes magneto-centrifugal ejection as a mechanism for launching observed bipolar jets, but this class of models lacks observational confirmation. Long standing observational difficulties lie in the fact that young stars are often heavily embedded, infall and outflow kinematics are complex and confused close to the source, and the spatial scales (in angular terms) are relatively small. Goal The goal of this project is to search for conclusive observational evidence that jets from young stars rotate, in order to test widely supported models that bipolar jets extract angular momentum. Objective The objective of the project is to establish that measured differences in Doppler shift across the jet width are justifiably interpreted as a rotation of the flow. Main results We conducted a multi-wavelength analysis of the radial velocity profile across jets from a sample of protostars of various ages to determine if there was a difference in radial velocity between the borders of the jet, which may be interpreted as a jet rotation signature. 1. Where the jet width was spatially resolved, we were able to measure differences in the radial velocity across the jet width in four of five cases of sufficient signal-to-noise, via data from different telescopes / instruments / wavelenghts including HST/STIS in the near-ultraviolet (UV), and GEMINI/GNIRS and VLT/ISAAC in the near-infrared (IR). Signatures are consistently detected, as expected where the origin of the signature is intrinsic to the system itself. 2. Detections of radial velocity differences were reported in jets from protostars at earlier evolutionary stages, as expected where jets are the means of extracting angular momentum throughout the accretion phase. However, there is difficulty ruling out alternative explanations to jet rotation, such as asymmetric shocking, since observations are necessarily conducted at a large distance from the source to avoid extinction. 3. We have found agreement in the sense of rotation of the near-UV jet and disk in RW Aur (the only 'problem' case to-date). However, this result may contradict an earlier one at optical wavelengths and we are currently investigating how the two results can be reconciled. Note that RW Aur is a highly variable and complex multiple system in which other factors may be coming into play and confuse the intrinsic signature. Conclusions These results illustrate how this study is very observationally demanding. Further investigations into jet rotation close to the disk-plane and at high angular resolution are clearly warranted. Indeed, our Gemini/NIFS+AO data, which is currently under analysis, should reveal some criticalresults in terms of jet-disk rotation agreement. Impact Determining whether we are observing rotation in protostellar jets is a crucial step in assessing the role of jets in extracting angular momentum from disks which heavily impacts on conditions in the terrestrial planet forming zone.

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