Milliarcsecond structure and variability of methanol maser emission in three high-mass protostars

{The variability study of 6.7\,GHz methanol masers has become a useful way to improve our understanding of the physical conditions in high-mass star-forming regions.} {Based on the single-dish monitoring using the Irbene telescopes, we selected three sources with close sky positions.} {We imaged them using the European Very Long Baseline Interferometer Network and searched available data on VLBI archives to follow detailed changes in their structures and single maser spot variability.} {All three targets show a few groups of maser cloudlets of a typical size of 3.5\,mas and the majority of them show linear or arched structures with velocity gradients of order 0.22\kms\,mas$^{-1}$. The cloudlets and overall source morphologies are remarkably stable on time scales of 7-15\,yr supporting a scenario of variability due to changes in the maser pumping rate.}Comment: 20 page

A Keplerian disk with a four-arm spiral birthing an episodically accreting high-mass protostar

High-mass protostars (M⋆ > 8M⊙) are thought to gain the majority of their mass via short, intense bursts of growth. This episodic accretion is thought to be facilitated by gravitationally unstable and subsequently inhomogeneous accretion disks. Limitations of observational capabilities, paired with a lack of observed accretion burst events, have withheld affirmative confirmation of the association between disk accretion, instability and the accretion burst phenomenon in high-mass protostars. Following its 2019 accretion burst, a heatwave driven by a burst of radiation propagated outward from the high-mass protostar G358.93-0.03-MM1. Six very long baseline interferometry observations of the radiatively pumped 6.7 GHz methanol maser were conducted during this period, tracing ever increasing disk radii as the heatwave propagated outward. Concatenating the very long baseline interferometry maps provided a sparsely sampled, milliarcsecond view of the spatio-kinematics of the accretion disk covering a physical range of ~50–900 AU. We term this observational approach ‘heatwave mapping’. We report the discovery of a Keplerian accretion disk with a spatially resolved four-arm spiral pattern around G358.93-0.03-MM1. This result positively implicates disk accretion and spiral arm instabilities into the episodic accretion high-mass star formation paradigm

Recent updates on the Maser Monitoring Organisation

The Maser Monitoring Organisation (M2O) is a research community of telescope operators, astronomy researchers and maser theoreticians pursuing a joint goal of reaching a deeper understanding of maser emission and exploring its variety of uses as tracers of astrophysical events. These proceedings detail the origin, motivations and current status of the M2O, as was introduced at the 2021 EVN symposium

Discovery of recurrent flares of 6.7 GHz methanol maser emission in Cepheus A HW2

Context. Class II methanol masers at 6.7 GHz originate close to high-mass young stellar objects (HMYSOs). Their high sensitivity to local condition variations makes them a useful marker of the activity of the emerging massive stars. Aims. We aim to closely examine the variability of the 6.7 GHz CH3OH masers in Cep A HW2 using the new and archival single-dish and high-resolution Very-Long-Baseline Interferometry (VLBI) datasets. Methods. We monitored 6.7 GHz methanol masers towards the target between 2009 and 2021 using the Torun 32m radio telescope, and analysed nine datasets of VLBI observations taken over 16 yr. Results. Faint, extremely redshifted maser emission located close to the presumed central star position and coincident with the radio jet shows flaring activity with a period of ~5 yr. Flares have an asymmetric profile with a rise-to-decay time ratio of 0.18 and relative amplitude higher than 10. The velocity of the flaring cloudlets drifts at a rate of 3–4 × 10−5 km s−1 d−1 for about 11.5 yr of the monitoring. The time-lag between the peaks of the two flaring features implies a propagation speed of the exciting factor of ~1000 km s−1. Synchronised and anticorrelated variations of the flux density of blue- and redshifted features begin ~2.5 yr after flares of the extremely redshifted emission. Conclusions. Our observations suggest that the recurrent flares are the response of low-gain unsaturated maser regions to a relatively low increase in luminosity, which has little effect on the behaviour of most maser regions of higher gain

Detection of periodic flares in 6.7 GHz methanol masers G45.804−0.356 and G49.043−1.079

Context. Periodicity in 6.7 GHz methanol maser sources is a rare phenomenon that was discovered during long-term monitoring programmes. Understanding the underlying processes that lead to periodic variability might provide insights into the physical processes in high-mass star-forming regions. Aims. We aim to identify and describe new periodic methanol masers. Methods. The observations were obtained with the Torun 32 m antenna. Time series analysis was conducted using well-proven statistical methods. Additionally, NEOWISE data were used to search for a correlation between infrared and maser fluxes. Results. We found two new periodic sources, G45.804−0.356 and G49.043−1.079, with periods of 416.9 and 469.3 days, respectively. For G49.043−1.079, infrared variability is simultaneous with methanol flares. Conclusions. A most likely cause of the periodicity in G49.043−1.079 is modulated accretion. For G45.804-0.356, the periodicity cannot be explained with the available data, and further research is needed

Observations of 12.2 GHz methanol masers towards northern high-mass protostellar objects

Context. Class II methanol masers at 6.7 and 12.2 GHz occur close to high-mass young stellar objects (HMYSOs). When they are observed simultaneously, such studies may contribute to refining the characterisation of local physical conditions. Aims. We aim to search for the 12.2 GHz methanol emission in 6.7 GHz methanol masers that might have gone undetected in previous surveys of northern sky HMYSOs, mainly due to their variability. Contemporaneous observations of both transitions are used to refine the flux density ratio and examine the physical parameters. Methods. We observed a sample of 153 sites of 6.7 GHz methanol maser emission in the 12.2 GHz methanol line with the Torun 32 m radio telescope, using the newly built X-band receiver. Results. The 12.2 GHz methanol maser emission was detected in 36 HMYSOs, with 4 of them detected for the first time. The 6.7–12.2 GHz flux density ratio for spectral features of the contemporaneously observed sources has a median value of 5.1, which is in agreement with earlier reports. The ratio differs significantly among the sources and for the periodic source G107.298+5.639 specifically, the ratio is weakly recurrent from cycle to cycle, but it generally reaches a minimum around the flare peak. This is consistent with the stochastic maser process, where small variations in the physical parameters along the maser path can significantly affect the ratio. A comparison of our data with historical results (from about ten years ago) implies significant (>50%) variability for about 47 and 14% at 12.2 and 6.7 GHz, respectively. This difference can be explained via the standard model of methanol masers

Ground Penetrating Radar Investigations of Landslides: A Case Study in a Landslide in Radziszów

The article presents the results of research of the activated landslide in Radziszów in 2010. The study focused on the evaluation of engineering geology conditions, preceded by geophysical surveys. It mainly focused on the GPR method using the IDS georadar equipped with antennas in the frequency range of 100 MHz. Antenna selection was based on archival research which showed that the depth of occurrence of clays, where the roof can be a potential slip surface, ranges between 3-10 m below the surface

Ground penetrating radar investigations of landslides: a case study in a landslide in Radziszów

The article presents the results of research of the activated landslide in Radziszów in 2010. The study focused on the evaluation of engineering geology conditions, preceded by geophysical surveys. It mainly focused on the GPR method using the IDS georadar equipped with antennas in the frequency range of 100 MHz. Antenna selection was based on archival research which showed that the depth of occurrence of clays, where the roof can be a potential slip surface, ranges between 3–10 m below the surface. GPR method applied allowed the course of the potential slip surface to be determined and the results obtained significantly correlated with the results of engineering geology tests carried out

A search for the OH 6035 MHz line in high-mass star-forming regions

Context. The excited states of OH masers detected in the environment of high-mass young stellar objects (HMYSOs) are important for improving our understanding of the physical conditions of these objects and also provide information about their magnetic fields. Aims. We aim to search for excited-state OH 6035 MHz maser emission in HMYSOs which might have escaped detection in previous surveys or were never searched for. Methods. A sample of HMYSOs derived from untargeted surveys of the 6668 MHz methanol maser line was observed at 6035 MHz OH transition with the Torun 32 m radio telescope. The 6035 MHz detections were observed in the OH 6031 MHz line. Two-thirds of the detections were observed at least three times over a two-year period. Results. Out of 445 targets, 37 were detected at 6035 MHz, including seven new discoveries. The 6031 MHz line was detected towards ten 6035 MHz sources, one of which was not previously reported. All the newly detected sources are faint with the peak flux density lower than 4 Jy and show significant or high variability on timescales of 4 to 20 months. Zeeman pair candidates identified in three new sources imply a magnetic field intensity of 2–11 mG. Comparison of our spectra with those obtained ~10 yr ago indicates different degrees of variability but there is a general increase in the variability index on an ~25 yr timescale, usually accompanied by significant changes in the profile shape

Multi-frequency VLBI observations of maser lines during the 6.7 GHz maser flare in the high-mass young stellar object G24.33+0.14

Context. Recent studies have shown that 6.7 GHz methanol maser flares can be a powerful tool for verifying the mechanisms of maser production and even the specific signatures of accretion rate changes in the early stages of high-mass star formation. Aims. We characterize the spatial structure and evolution of methanol and water masers during a flare of methanol maser emission at 6.7 GHz in the high-mass young stellar object (HMYSO) G24.33+0.14. Methods. Very Long Baseline Array (VLBA) was used to image the 6.7 and 12.2 GHz methanol and 22.2 GHz water vapor masers at three epochs guided by monitoring the methanol line with the Torun 32m telescope. The 6.7 GHz maser maps were also obtained with the European VLBI Network (EVN) and Long Baseline Array (LBA) during the flare. The Wide-field Infrared Survey Explorer (WISE) data were used to find correlations between the 6.7 GHz maser and infrared (IR) fluxes. Results. The 6.7 GHz methanol maser cloudlets are distributed over ~3500 au, and the morphology of most of them is stable although their brightness varies following the course of the total flux density on a timescale of two months. The 12.2 GHz methanol maser cloudlets cover an area an order of magnitude smaller than that of 6.7 GHz emission, and both transitions emerge from the same masing gas. The 22.2 GHz maser cloudlets lie in the central region and show a systematic increase in brightness and moderate changes in size and orientation, together with the velocity drift of the strongest cloudlet during two months of the Very Long Baseline Interferometry (VLBI) observing period. Time lag estimates imply the propagation of changes in the physical conditions of the masing region with a subluminal speed (~0.3c). A tight correlation of IR (4.6 μm) and 6.7 GHz flux densities is found, supporting the radiative pumping model. Proper motion analysis does not reveal any signs of expansion or inflow of the methanol cloudlets within ~6 mas over ~10 yr. Comparison with the 230 GHz Atacama Large Millimeter Array (ALMA) data indicates that the methanol masers are distributed in the inner part of the rotating disk, whereas the 22.2 GHz emission traces the compact inner component of the bipolar outflow or a jet structure. Conclusions. The maser morphology in the target is remarkably stable over the course of the flare and is similar to the quiescent state, possibly due to less energetic accretion events that can repeat on a timescale of ~8 yr