Radio continuum observations using the Australia telescope compact array at 5. 5, 9.0, 17.0 and 22.8 GHz have detected free-free emission associated with 45 of 49 massive young stellar objects and HII regions. Of these, 26 sources are classified as ionized jets (12 of which are candidates), 2 as ambiguous jets or disc winds, 1 as a disc-wind, 14 as HII regions and 2 were unable to be categorised. Classification as ionized jets is based upon morphology, radio flux and spectral index, in conjunction with previous observational results at other wavelengths. Radioluminosity and momentum are found to scale with bolometric luminosity in the same way as low-mass jets, indicating a common mechanism for jet production across all masses. In 13 of the jets, we see associated non-thermal/optically-thin lobes resulting from shocks either internal to the jet and/or at working surfaces. Ten jets display non-thermal (synchrotron emission) spectra in their lobes, with an average spectral index of α = −0.55 consistent with Fermi acceleration in shocks. This shows that magnetic fields are present, in agreement with models of jet formation incorporating magnetic fields. Since the production of collimated radio jets is associated with accretion processes, the results presented in this paper support the picture of disc-mediated accretion for the formation of massive stars with an upper-limit on the jet phase lasting approximately 6.5 × 10 4 yr. Typical mass loss rates in the jet are found to be 1.4×10 −5 M yr −1 with associated momentum rates of the order (1 − 2) × 10 −2 M km s −1 yr −1 .
It is important to determine if massive stars form via disc accretion, like their low-mass counterparts. Theory and observation indicate that protostellar jets are a natural consequence of accretion discs and are likely to be crucial for removing angular momentum during the collapse. However, massive protostars are typically rarer, more distant and more dust enshrouded, making observational studies of their jets more challenging. A fundamental question is whether the degree of ionisation in jets is similar across the mass spectrum. Here we determine an ionisation fraction of ~5–12% in the jet from the massive protostar G35.20-0.74N, based on spatially coincident infrared and radio emission. This is similar to the values found in jets from lower-mass young stars, implying a unified mechanism of shock ionisation applies in jets across most of the protostellar mass spectrum, up to at least ~10 solar masses.
In conjunction with a previous southern-hemisphere work, we present the largest radio survey of jets from massive protostars to date with high-resolution, ($\sim 0{_{.}^{\prime\prime}}04$) Jansky Very Large Array (VLA) observations towards two subsamples of massive star-forming regions of different evolutionary statuses: 48 infrared-bright, massive, young, stellar objects (MYSOs) and 8 infrared dark clouds (IRDCs) containing 16 luminous (${\, L_{\rm {bol}}}>10^3{{\rm \, L_{\odot }}}$) cores. For $94{{\ \rm per\ cent}}$ of the MYSO sample we detect thermal radio (α ≥ −0.1 whereby Sν∝να) sources coincident with the protostar, of which $84{{\ \rm per\ cent}}$ (13 jets and 25 candidates) are jet-like. Radio luminosity is found to scale with ${\, L_{\rm {bol}}}$ similarly to the low-mass case supporting a common mechanism for jet production across all masses. Associated radio lobes tracing shocks are seen towards $52{{\ \rm per\ cent}}$ of jet-like objects and are preferentially detected towards jets of higher radio and bolometric luminosities, resulting from our sensitivity limitations. We find jet mass loss rate scales with bolometric luminosity as ${\dot{m}_{\rm jet}}\propto {\, L_{\rm {bol}}}^{0.9\pm 0.2}$, thereby discarding radiative, line-driving mechanisms as the dominant jet-launching process. Calculated momenta show that the majority of jets are mechanically capable of driving the massive, molecular outflow phenomena since pjet > poutflow. Finally, from their physical extent we show that the radio emission can not originate from small, optically-thick Hii regions. Towards the IRDC cores, we observe increasing incidence rates/radio fluxes with age using the proxy of increasing luminosity-to-mass (L/M) and decreasing infrared flux ratios (S70 μm/S24 μm). Cores with $L/M\, <\, 40\, L_\odot \, M_\odot ^{-1}$ are not detected above (5.8 GHz) radio luminosities of ∼1 mJy kpc2.
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