Ross A. Burns

Context. Class II 6.7 GHz CH ₃ OH (methanol) masers are powerful probes of the physical processes in high-mass young stellar objects. We selected three closely located sources from the Irbene single-dish methanol maser monitoring program for high-resolution imaging studies. Aims. Our goal is to investigate their milliarcsecond-scale structure and kinematics to improve our understanding of the physical processes driving maser activity and variability in these sources. Methods. We imaged three sources (G37.43+1.51, G37.479–0.105, and G37.55+0.20) with the European Very Long Baseline Interferometer Network (EVN), supplemented by archival data and single-dish light curves from the Irbene and Ibaraki radio telescopes. Maser emission parameters were derived for all identified cloudlets, and their spatial and kinematic distributions were analyzed. Results. We present the first milliarcsecond-resolution, high-sensitivity images of G37.479–0.105 and G37.55+0.20; other team already studied G37.43+1.51 studied with the EVN. In all three sources, most of the maser cloudlets form linear or arched structures with visible velocity gradients. In G37.43+1.51, two-epoch proper motion measurements reveal a lengthening of one of the maser groups (designated group A), with an expansion velocity of approximately 1.1 km s ⁻¹ . This is consistent with an overall expansion of the maser region. In G37.479–0.105, 6.7 GHz methanol maser cloudlets are distributed in parabolic structures, with the opening (i.e., the symmetry axis of the parabola) oriented roughly in the same direction as an outflow emanating from the region. This may indicate that the maser cloudlets trace the walls of the outflow cavity. In G37.55+0.20, masers form an elongated structure located at the center of a source of outflows and a formaldehyde maser. The light curve of the periodic maser components and their on-sky distribution suggest substantial differences in line-of-sight distances among cloudlets (assuming the maser pumping conditions are modulated at the speed of light) that appear close in projection. Combined with the position of the continuum maximum, this supports the colliding-wind binary model as the likely driver of maser variability. In all three high-mass protostars, the 6.7 GHz methanol masers appear to trace the innermost regions of the systems they reside in.

We present multiepoch, multiband Atacama Large Millimeter/submillimeter Array imaging of the new Class II millimeter methanol masers excited during the accretion outburst of the massive protostar G358.93−0.03 MM1. The highest angular resolution image (24 mas ≈160 au) reveals a nearly complete, circular ring of strong maser spots in the 217.2992 GHz ( v _t = 1) maser line that closely circumscribes the dust continuum emission from MM1. Weaker maser emission lies inside the eastern and southern halves of the maser ring, generally coincident with the centimeter masers excited during the outburst but avoiding the densest parts of the hot core gas traced by high excitation lines of CH ₃ CN. Using a variety of fitting techniques on the image cubes of the two strongest maser lines, each observed over three to four epochs, we find the diameter of the ring increased by ≳60% (from ≈1100 to ≈1800 au in the 217 GHz line) over 200 days, consistent with an average radial propagation rate of ≈0.01 c , while the maser intensity declined exponentially. Fitting the angular extent of the millimeter masers versus time yields a power law of index 0.39 ± 0.06, which also reproduces the observed extent of the 6.7 GHz masers in the first very long baseline interferometry epoch of R. A. Burns et al. 2020 This exponent is consistent with the prediction of radius versus time in the Taylor–von Neumann–Sedov self-similar solution for an intense spherical explosion from a point source ( R ∝ t ^2/5 ). These results demonstrate the explosive nature of accretion outbursts in massive protostars and their ability to generate subluminal heat waves traceable by centimeter and millimeter masers for several months as the energy traverses the surrounding molecular material.

The Irbene single-baseline radio interferometer (ISBI), operated by the Ventspils International Radio Astronomy Centre (VIRAC), offers a rare and versatile configuration in modern radio astronomy. Combining the 32-m and 16-m fully steerable parabolic radio telescopes separated by an 800-m baseline, this system possesses a unique capability for high-sensitivity, time-domain interferometric observations. Unlike large interferometric arrays optimized for sub-arcsecond resolution imaging, the Irbene system is tailored for studies that require high temporal resolution and a strong signal-to-noise ratio. This paper reviews key scientific applications of the Irbene interferometer, including simultaneous methanol maser and radio continuum variability studies, high-cadence monitoring of quasi-periodic pulsations (QPPs) in stellar flares, ionospheric diagnostics using GNSS signals, orbit determination of navigation satellites and forward scatter radar techniques for space object detection. These diverse applications demonstrate the scientific potential of compact interferometric systems in an era dominated by large-scale observatories.

Context. Maser flares are particularly significant in the study of massive star formation as they not only signal but also provide unique insights into transient phenomena such as accretion bursts. Aims. With this project, we aim to investigate the context of the ongoing 6.7 GHz methanol maser flare in the little-known massive star-forming region G11.497-1.485. Methods We carried out two epochs of the Karl G. Jansky Very Large Array (VLA) observation for 6.7 GHz and 12 GHz class II methanol, 22 GHz water masers, and continuum in the C, Ku, and K bands. Results. The VLA overview revealed the presence of five distinct radio-continuum sources (CM1-4 and N) in G11.497-1.485. The central source, CM1, is found to show signs of accretion disc fragmentation, highlighted by the centimetre-continuum-traced fragments, and is found to drive a high-energy jet, the ends of which are marked by non-thermal knots CM2 and CM3. CM1 showed a gradual flaring of methanol masers and a fading of a 22 GHz water maser, which might be signalling an accretion burst. The two remaining sources of the region, CM4 and N, make up one of the most compact jet and disc–jet systems found to date. Conclusions. The obtained data reveal, for the first time, the structure of the G11.497-1.485 region. The change in fluxes of the maser and the continuum emission confirm a transient event and reveal its impact on multiple sources in the region.

Multi-epoch very long baseline interferometry (VLBI) observations measure three-dimensional water maser motions in protostellar outflows, enabling analysis of inclination and velocity. However, these analyses assume that water masers and shock surfaces within outflows are co-propagating. We compare VLBI data on maser-traced bow shocks in the high-mass protostar AFGL 5142-MM1, from seven epochs of archival data from the VLBI Exploration of Radio Astrometry (VERA), obtained from 2014 April to 2015 May, and our newly conducted data from the KVN and VERA Array (KaVA), obtained in 2016 March. We find an inconsistency between the expected displacement of the bow shocks and the motions of individual masers. The separation between two opposing bow shocks in AFGL 5142-MM1 was determined to be 337.17 ± 0.07 mas in the KaVA data, which is less than an expected value of 342.1 ± 0.7 mas based on extrapolation of the proper motions of individual maser features measured by VERA. Our measurements imply that the bow shock propagates at a velocity of 24 ± 3 km s⁻¹, while the individual masing gas clumps move at an average velocity of 55 ± 5 km s⁻¹; that is ,the water masers are moving in the outflow direction at double the speed at which the bow shocks are propagating. Our results emphasize that investigations of individual maser features are best approached using short-term high-cadence VLBI monitoring, while long-term monitoring on timescales comparable to the lifetimes of maser features is better suited to tracing the overall evolution of shock surfaces. Observers should be aware that masers and shock surfaces can move relative to each other, and that this can affect the interpretation of protostellar outflows.

We present the results of a 5-yr monitoring program of 42 sources targeted at 6.7-GHz methanol masers, conducted from 2017 March to 2022 October using the Irbene 32- and 16-m radio telescopes. Sources were observed with irregular intervals where time between two consecutive observations ranged from 24 h to 35 d. We found that more than 55 per cent of the sources showed significant variability, but often only one or a few spectral features were varying significantly. Numerous type of variability were found in our sample: low-variable, periodic, irregular, synchronized, and anticorrelated between features and steadily raising or falling flux. Our analysis techniques also uncover new variability trends for several sources. The maser monitoring program is one of the first single-dish science initiatives at the Irbene radio telescope complex, initiated shortly after the instrument’s reconstruction and upgrades. Our findings unequivocally demonstrate its suitability for maser research purposes.

This paper describes an overview of the FLASHING (Finest Legacy Acquisitions of SiO-/H₂O-maser Ignitions by the Nobeyama Generation) project promoted using the 45 m telescope of Nobeyama Radio Observatory, which aims to intensively monitor H₂O (at 22 GHz) and SiO (at 43 GHz) masers associated with so-called “water fountain” sources. Here we show scientific results based on the data taken in the first five seasons of FLASHING, from 2018 December to 2023 April. We show the evolution of the H₂O maser spectra, discovering new spectral components that break the record of the jet’s top speed and/or systematic velocity drifts in the spectrum indicating acceleration or deceleration of the maser gas clumps. For the 43 GHz SiO maser emission, we newly detect it in one source and find that it is no longer present in another source. This implies that the jets from these water fountains can be accelerated or decelerated, and show how circumstellar envelopes are destroyed.

Context. The G25.65+1.04 source is one of the few known ‘super-flare’ water masers in the Milky Way, but in contrast to other super-flare sources, it remains the least studied. In fact, even the nature of the source driving the water maser is still unclear. Aims. With this project, we aim to clarify the composition and properties of the G25.65+1.04 region by means of examining the parameters of the continuum sources and establishing their association with masers of different types. Methods. Our previous VLA (Karl G. Jansky Very Large Array) observation detected four continuum peaks (VLA 1-4), three of which (VLA 1-3) were closely spaced and presented a linear orientation. However, the observation, which used the VLA B configuration, lacked the spatial resolution to resolve individual sources. A higher-resolution (A configuration) VLA observation of the continuum and spectral lines was conducted in 2019 using the L, S, C, and Ku bands. Results. For the first time, the continuum source VLA 1 – associated with the flaring water maser – is resolved into two components: VLA 1A and 1B. The component VLA 1A and the water maser are found to spatially coincide and are thought to be powered by the same source, a protostar at an early stage of evolution showing active ejection. We argue that VLA 2 pinpoints an actively ejecting high-mass protostar, as it is associated with a 6.7 GHz methanol maser and a magnetised jet traced by a 22 GHz H₂O maser. Highly polarised OH maser emission is detected in the vicinity of VLA 1-2, with the brightest OH maser found in VLA 2. The magnetic field, identified from the OH maser emission, ranges from ∼ + 0.4 mG in VLA 1A to ∼ − 8 mG in VLA 2. Conclusions. The G25.65+1.04 region is found to consist of at least two young stellar objects: VLA 1A and VLA 2. Both sources are found to be at an active accretion and ejection stage of evolution.