The sharpest view on the high-mass star-forming region S255IR

Abstract

Context. Massive stars have an impact on their surroundings from early in their formation until the end of their lives. However, very little is known about their formation. Episodic accretion may play a crucial role, but observations of these events have only been reported towards a handful of massive protostars. Aims. We aim to investigate the outburst event from the high-mass star-forming region S255IR where recently the protostar NIRS3 underwent an accretion outburst. We follow the evolution of this source both in photometry and morphology of its surroundings. Methods. We perform near-infrared adaptive optics observations on the S255IR central region using the Large Binocular Telescope in the K s broad-band and the H 2 and Brγ narrow-band filters with an angular resolution of ∼ 0 ′′ . 06, close to the diffraction limit.Results. We discover a new near-infrared knot north-east from NIRS3 that we interpret as a jet knot that was ejected during the last accretion outburst and observed in the radio regime as part of a follow-up after the outburst. We measure a mean tangential velocity for this knot of 450 ± 50 km s −1 . We analyse the continuum-subtracted images from H 2 which traces jet shocked emission, and Brγ which traces scattered light from a combination of accretion activity and UV radiation from the central massive protostar. We observe a significant decrease in flux at the location of NIRS3, with K=13.48 mag being the absolute minimum in the historic series. Conclusions. Our observations strongly suggest a scenario where the episodic accretion is followed by an episodic ejection response in the near-infrared, as it was seen in the earlier radio follow-up. The 30 years of ∼ 2 µm photometry suggests that NIRS3 might have undergone another outburst in the late 1980s, being the first massive protostar with such evidence observed in the near-infrared.

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.…”

“…The radio jet, launched by the recent accretion event of the outbursting MYSO S255IR NIRS3, was observed to expand at a speed of ∼285 km s −1 (Cesaroni et al 2024). In comparison, jet velocities of other infrared bright MYSOs, measured from shock gas tracers in the near-infrared, range from ∼80 to 450 km s −1 (Fedriani et al 2018(Fedriani et al , 2023Massi et al 2023). Currently, neither radio continuum nor near-infrared high-resolution data from multiple epochs are available for M17 MIR.…”

Radio outburst from a massive (proto)star

“…This can be appreciated in Fig. 4 by comparing the radio light curves with the IR light curve of the accretion outburst obtained from Table B.1 of Fedriani et al (2023). From this comparison, the time lag of ∼600 days between the peaks of the two curves clearly stands out, and is roughly consistent with the delay of ∼400 days (see Paper I) between the onset of the NIR burst and that of the radio burst.…”

“…One of the results obtained in our studies is that although the jet initially consists of only the NE lobe (see also Fedriani et al 2023), a SW lobe appears between approximately 22 and 35 months after the onset of the radio outburst, in July 2016. We propose that this new lobe is powered by the same accretion outburst (which began in mid-June 2015; see Caratti o Garatti et al 2017) as the NE lobe, despite the significant time lag between the two.…”