Radio outburst from a massive (proto)star II. A portrait in space and time of the expanding radio jet from S255IR NIRS 3

Context. Growing observational evidence indicates that the accretion process leading to star formation may occur in an episodic way, through accretion outbursts revealed in various tracers. This phenomenon has also now been detected in association with a few young massive (proto)stars (>8 M-circle dot), where an increase in the emission has been observed from the IR to the centimetre domain. In particular, the recent outburst at radio wavelengths of S255IR NIRS 3 has been interpreted as due to the expansion of a thermal jet, fed by part of the infalling material, a fraction of which has been converted into an outflow. Aims. We wish to follow up on our previous study of the centimetre and millimetre continuum emission from the outbursting massive (proto)star S255IR NIRS 3 and confirm our interpretation of the radio outburst, based on an expanding thermal jet. Methods. The source was monitored for more than 1 yr in six bands from 1.5 GHz to 45.5 GHz with the Karl G. Jansky Very Large Array, and, after an interval of similar to 1.5 yr, it was imaged with the Atacama Large Millimeter/submillimeter Array at two epochs, which made it possible to detect the proper motions of the jet lobes. Results. The prediction of our previous study is confirmed by the new results. The radio jet is found to expand, while the flux, after an initial exponential increase, appears to stabilise and eventually decline, albeit very slowly. The radio flux measured during our monitoring is attributed to a single lobe, expanding towards the NE. However, starting from 2019, a second lobe has been emerging in the opposite direction, probably powered by the same accretion outburst as the NE lobe, although with a delay of at least a couple of years. Flux densities measured at frequencies higher than 6 GHz were satisfactorily fitted with a jet model, whereas those below 6 GHz are clearly underestimated by the model. This indicates that non-thermal emission becomes dominant at long wavelengths. Conclusions. Our results suggest that thermal jets can be a direct consequence of accretion events, when yearly flux variations are detected. The formation of a jet lobe and its early expansion appear to have been triggered by the accretion event that started in 2015. The end of the accretion outburst is also mirrored in the radio jet. In fact, similar to 1 yr after the onset of the radio outburst, the inner radius of the jet began to increase, at the same time the jet mass stopped growing, as expected if the powering mechanism of the jet is quenched. We conclude that our findings strongly support a tight connection between accretion and ejection in massive stars, consistent with a formation process involving a disk-jet system similar to that of low-mass stars.