The sharpest view on the high-mass star-forming region S255IR. Near-InfraRed Adaptive Optics Imaging on the Outbursting Source NIRS3

TL;DR

This study uses high-resolution near-infrared adaptive optics imaging to investigate an accretion outburst in the high-mass star-forming region S255IR, revealing jet ejection, flux variability, and evidence of episodic accretion in the protostar NIRS3.

Contribution

First near-infrared adaptive optics imaging of NIRS3's outburst, linking episodic accretion to jet ejection and flux changes in a high-mass protostar.

Findings

Discovery of a jet knot ejected during the outburst with a velocity of 450 km/s.

Observation of flux decrease at NIRS3, indicating accretion variability.

Evidence of a possible previous outburst in the late 1980s from historic photometry.

Abstract

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. 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$_{\rm s}$ broad-band and the H$_2$ and Br$\gamma$ narrow-band filters with an angular resolution of $\sim0\farcs06$, close to the diffraction limit. 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 $450\pm50\,\mathrm{km\,s^{-1}}$. We analyse the continuum-subtracted images from H$_2$ which traces jet shocked emission, and Br$\gamma$ 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. 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 $\sim2\,\mu{\rm m}$ 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.