Radio outburst from a massive (proto)star

Abstract

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⊙), 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 ~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, ~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.

M17 MIR: A Massive Star Is Forming via Episodic Mass Accretion

“…Interestingly, we observe symmetrical EHV subknots very close to the central sources on both red-and blueshifted sides, suggesting symmetrical mass ejections on both sides likely triggered by the accretion outbursts reported in Chen et al (2021). Similar evidence of a connection between mass ejection and accretion outburst has also been observed for the low-mass protostar B335 (Kim et al 2024) and MYSO S255IR NIRS 3 (Cesaroni et al 2023;Fedriani et al 2023). It is reasonable to assume that the closest EHV subknots N1-1 and S1-1 represent the mass ejection caused by the ongoing accretion outburst, which started 9 yr before the ALMA observations in 2019 October.…”

“…We have independently estimated  M acc and  M ejec during the recent clustered accretion outbursts of M17 MIR. The value of   M M ejec acc is ∼6.2%, comparable to the ratio of the outbursting MYSO S255IR NIRS 3 estimated from its radio jet (Cesaroni et al 2023) and the ratios found for low-to intermediate-mass young stellar objects (YSOs) (Ellerbroek et al 2013;Bally 2016). The tight connection between accretion and ejection found for M17 MIR in the earliest forming stage of an MYSO strongly supports the scenario that episodic mass accretion and ejection might be the potential way for the growth of a high-mass protostar from a lower-mass protostar.…”

“…The SMA observations, conducted between 2008 and 2009 before the infrared outburst of S255IR NIRS 3 in 2015, reveal collimated bipolar CO outflow from the same source (Wang et al 2011). The recent high-resolution monitoring observations at multiple continuum bands by VLA and ALMA reveal the expansion of the radio jet triggered by the accretion outburst started in 2015 (Cesaroni et al 2018;Cesaroni et al 2023). Likewise, ALMA high-resolution observations reveal high-velocity north-south outflow traced by the CS J = 6-5, 18-17, and HDO 1 1,1 -0 0,0 transitions from NGC 6334I MM1 (Brogan et al 2018;McGuire et al 2018).…”