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Context. We exploit the increased sensitivity of the recently installed adaptive optics SOUL at the LBT to obtain new high-spatialresolution near-infrared images of the massive young stellar object IRAS20126+4104 and its outflow. Aims. We aim to derive the jet proper motions and kinematics, as well as to study its photometric variability by combining the novel performances of SOUL together with previous near-infrared images. Methods. We used both broad-band (Ks, K') and narrow-band (Br H2) observations from a number of near-infrared cameras (UKIRT/UFTI, SUBARU/CIAO, TNG/NICS, LBT/PISCES, and LBT/LUCI1) to derive maps of the continuum and the H2 emission in the 2.12 µm line. Three sets of images, obtained with adaptive optics (AO) systems (CIAO, in 2003; FLAO, in 2012; SOUL, in 2020), allowed us to derive the proper motions of a large number of H2 knots along the jet. Photometry from all images was used to study the jet variability. Results. We derived knot proper motions in the range of 1.7-20.3 mas yr-1 (i.e. 13-158 km s-1 at 1.64 kpc), implying an average outflow tangential velocity of ~80 km s-1. The derived knot dynamical age spans a ~200-4000 yr interval. A ring-like H2 feature near the protostar location exhibits peculiar kinematics and may represent the outcome of a wide-angle wind impinging on the outflow cavity. Both H2 geometry and velocities agree with those inferred from proper motions of the H2O masers, located at a smaller distance from the protostar. Although the total H2 line emission from the knots does not exhibit time variations at a >~0.3 mag level, we have found a clear continuum flux variation (radiation scattered by the dust in the cavity opened by the jet) which is anti-correlated between the blue-shifted and red-shifted lobes and may be periodic (with a period of ~12-18 yr). We suggest that the continuum variability might be related to inner-disc oscillations which have also caused the jet precession. Conclusions. Our analysis shows that multi-epoch high-spatial-resolution imaging in the near-infrared is a powerful tool to unveil the physical properties of highly embedded massive protostars. [ABSTRACT FROM AUTHOR]

In 2019 September, a sudden flare of the 6.7 GHz methanol maser was observed toward the high-mass young stellar object (HMYSO) G24.33+0.14. This may represent the fourth detection of a transient mass accretion event in an HMYSO after S255IR NIRS3, NGC 6334I-MM1, and G358.93−0.03-MM1. G24.33+0.14 is unique among these sources as it clearly shows a repeating flare with an 8 yr interval. Using the Atacama Large Millimeter/submillimeter Array (ALMA), we observed the millimeter continuum and molecular lines toward G24.33+0.14 in the pre-flare phase in 2016 August (ALMA Cycle 3) and the mid-flare phase in 2019 September (ALMA Cycle 6). We identified three continuum sources in G24.33+0.14, and the brightest source, C1, which is closely associated with the 6.7 GHz maser emission, shows only a marginal increase in flux density with a flux ratio (Cycle 6 |$/$| Cycle 3) of 1.16 ± 0.01, considering an additional absolute flux calibration uncertainty of |$10\%$|⁠. We identified 26 transitions from 13 molecular species other than methanol, and they exhibit similar levels of flux differences with an average flux ratio of 1.12 ± 0.15. In contrast, eight methanol lines observed in Cycle 6 are brighter than those in Cycle 3 with an average flux ratio of 1.23 ± 0.13, and the higher excitation lines tend to show a larger flux increase. If this systematic increasing trend is real, it would suggest radiative heating close to the central HMYSO due to an accretion event which could expand the size of the emission region and/or change the excitation conditions. Given the low brightness temperatures and small flux changes, most of the methanol emission is likely to be predominantly thermal, except for the 229.759 GHz (8−1–70 E) line known as a class I methanol maser. The flux change in the millimeter continuum of G24.33+0.14 is smaller than in S255IR NIRS3 and NGC 6334I-MM1 but is comparable with that in G358.93−0.03-MM1, suggesting different amounts of accreted mass in these events. [ABSTRACT FROM AUTHOR]

Aims. Recently, substantial flaring in the 6.7 GHz methanol maser line has been observed toward the high-mass young stellar object (YSO) S255 NIRS 3, where an accretion burst was also detected in the IR. Our goal is to study the change in the properties of the 6.7 GHz masers between the pre- and outburst phases, and investigate the connection between the maser and the accretion burst. Methods. With the Karl G. Jansky Very Large Array (JVLA) and the European VLBI Network (EVN), we performed observations of the 6.7 GHz masers (covering a range in angular resolution from a few milliarcseconds to ≈1") during the burst phase and compared these observations with pre-burst measurements at similar spatial scales. Results. The accretion burst and the subsequent increase in IR luminosity are very likely the origin of the 6.7 GHz maser flare. Since most maser centers operate in the unsaturated regime, a change by a relatively small factor (≈-5) in the flux of pumping photons has produced an exponential growth in the maser intensity. The main pre-burst maser cluster is no longer detected during the burst. Compared to the pre-burst phase, flaring 6.7 GHz masers emit across a different VLSR range that is more strongly redshifted, and the emission extends over a larger area at larger separation from the high-mass YSO. In particular, the outburst peak emission originates from a remarkably extended (0".02-0".03) maser plateau at a radial distance of 500-1000 AU from the source. Conclusions. Both the maser flare and the extraordinarily large extent of the maser structure can be a natural consequence of the burst in the accretion luminosity of the high-mass YSO. Our results strongly support models that predict IR radiative pumping for the 6.7 GHz CH3OH masers. [ABSTRACT FROM AUTHOR]