Radio outburst from a massive (proto)star. II. A portrait in space and time of the expanding radio jet from S255 NIRS3

Abstract

Observations indicate that the accretion process in star formation may occur through accretion outbursts. This phenomenon has also now been detected in a few young massive (proto)stars (>8 Msun). The recent outburst at radio wavelengths of the massive (proto)star S255 NIRS3 has been interpreted by us as expansion of a thermal jet, fed by the infalling material. To follow up on our previous study and confirm our interpretation, we monitored the source for more than 1 yr in six bands from 1.5 GHz to 45.5 GHz and, after ~1.5 yr, with the Atacama Large Millimeter/submillimeter Array at two epochs, which made it possible to detect the proper motions of the jet lobes. 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. The radio flux measured during our monitoring is attributed to a single NE lobe, However, from 2019 a second lobe has been emerging to the SW, probably powered by the same accretion outburst, although with a delay of at least a couple of years. Flux densities at >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. Our results suggest that thermal jets can be a direct consequence of accretion events, when yearly flux variations are detected. The end of the accretion outburst is mirrored in the radio jet, as ~1 yr after the onset of the radio outburst, the inner radius of the jet began to increase while the jet mass stopped growing, as expected if the powering mechanism of the jet is quenched. Our findings 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.