The protostellar environment where young stars form has physical conditions suitable to excite a number of molecular maser lines that have traditionally provided an unique probe of star formation kinematics, at the highest angular resolution of radio very long baseline interferometry (VLBI) observations. In the following, we will discuss a number of recent results on our understanding of the gas dynamics traced by masers in the vicinity of young forming stars. These findings provide direct clues on how our community can substantially contribute to the field of star formation in the next decade.

The Maser Monitoring Organisation is a collection of researchers exploring the use of time-variable maser emission in the investigation of astrophysical phenomena. The forward directed aspects of research primarily involve using maser emission as a tool to investigate star formation. Simultaneously, these activities have deepened knowledge of maser emission itself in addition to uncovering previously unknown maser transitions. Thus a feedback loop is created where both the knowledge of astrophysical phenomena and the utilised tools of investigation themselves are iteratively sharpened. The project goals are open-ended and constantly evolving, however, the reliance on radio observatory maser monitoring campaigns persists as the fundamental enabler of research activities within the group.

Recently, remarkable progress has been made in understanding the formation of high mass stars. Observations provided direct evidence that massive young stellar objects (MYSOs), analogously to low-mass ones, form via disk-mediated accretion accompanied by episodic accretion bursts, possibly caused by disk fragmentation. In the case of MYSOs, the mechanism theoretically provides a means to overcome radiation pressure, but in practice it is poorly studied - only three accretion bursts in MYSOs have been caught in action to date. A significant contribution to the development of the theory has been made with the study of masers, which have proven to be a powerful tool for locating “bursting” MYSOs. This overview focuses on the exceptional role that masers play in the search and study of accretion bursts in massive protostars.

Disk-jet systems are common in astrophysical sources of different nature, from black holes to gaseous giant planets. The disk drives the mass accretion onto a central compact object and the jet ejects material along the disk rotation axis. Magnetohydrodynamic disk winds can provide the link between mass accretion and ejection, which is essential to ensure that the excess angular momentum is removed and accretion can proceed. However, up to now, we have been lacking direct observational proof of disk winds. This work presents a direct view of the velocity field of a disk wind around a forming massive star. Achieving a very high spatial resolution of 0.05 au, our water maser observations trace the velocities of individual streamlines emerging from the disk orbiting the forming star. We find that, at low elevation above the disk midplane, the flow co-rotates with its launch point in the disk, in agreement with magneto-centrifugal acceleration. Beyond the co-rotation point, the flow rises spiraling around the disk rotation axis along a helical magnetic field. We have performed (resistive-radiative-gravito-) magnetohydrodynamic simulations of the formation of a massive star and record the development of a magneto-centrifugally launched jet presenting many properties in agreement with our observations.

Methanol and water masers indicate young stellar objects. They often exhibit flares, and a fraction shows periodic activity. Several mechanisms might explain this behavior but the lack of concurrent infrared (IR) data complicates the identification of its cause. Recently, 6.7 GHz methanol maser flares were observed, triggered by accretion bursts of high-mass YSOs which confirmed the IR-pumping of these masers. This suggests that regular IR changes might lead to maser periodicity. Hence, we scrutinized space-based IR imaging of YSOs associated with periodic methanol masers. We succeeded to extract the IR light curve from NEOWISE data for the intermediate mass YSO G107.298+5.639. Thus, for the first time a relationship between the maser and IR variability could be established. While the IR light curve shows the same period of ~34.6 days as the masers, its shape is distinct from that of the maser flares. Possible reasons for the IR periodicity are discussed.

Imaging the inner few 1000 AU around massive forming stars, at typical distances of several kpc, requires angular resolutions of better than 0″.1. Very Long Baseline Interferometry (VLBI) observations of interstellar molecular masers probe scales as small as a few AU, whereas (new-generation) centimeter and millimeter interferometers allow us to map scales of the order of a few 100 AU. Combining these informations all together, it presently provides the most powerful technique to trace the complex gas motions in the proto-stellar environment. In this work, we review a few compelling examples of this technique and summarize our findings.

Ring−like sources of 6.7 GHz methanol maser emission were discovered a decade ago with the European VLBI Network. In the past years we have been incessantly working to understand the nature of these rings. In general, the methanol rings do not coincide with H II regions nor they show 22 GHz water maser emission. Here, we present a proper motion study over a time baseline up to 10.5 years for the first sub-sample of methanol maser rings. Our findings suggest that in three targets G23.207−00.377, G23.389+00.185, and G23.657−00.127, such rings form in outflows or even in winds close to the central sources, and the masers trace slow proper motions of a few km s−1 typically.

Whether high-mass stars (M > 7M⊙) emerge from a scaled-up version of the low-mass star formation scenario, i. e. through disk-mediated accretion, is still debated. We present the first results of an observational programme aimed to map the innermost regions of high-mass stellar objects by combining together high-spatial resolution maser and radio continuum observations, and near-infrared imaging.

Very Long Baseline Interferometry studies of different maser species observed at multiple epochs allow complementary measurements of the 3-dimensional velocity field of gas close (≲ 103 AU) to massive young stellar objects. Here, we review our recent results toward the high-mass star-forming region G23.01–0.41, where all the strongest molecular maser transitions known to date cluster within 2000 AU from the center of an hot molecular core and are associated with a so called extended green object. The overall maser kinematics reveals a common outflowing motion from a central object; the details of the spatial distribution and velocity field of each maser species hint at the presence of different dynamical structures: a collimated jet, a wide-angle wind, and a flattened rotating core. We further compare the simultaneous presence of maser emission from different molecular species with a recent evolutionary sequence for masers associated with massive young stellar objects.

6.7 and 12.2 GHz CH$_{3}$OH (methanol) and 22.2 GHz H$_{2}$O masers are believed to be good tracers of the earliest phases of high-mass star formation. Interferometric and VLBI (Very Long Baseline Interferometry) observations have shown that water masers are predominantly associated with the innermost portions of the jets/outflows emerging from (proto-)stellar objects. On the other hand, the astrophysical environment traced by the 6.7 GHz (and the associated 12.2 GHz) CH$_{3}$OH masers is still to be more precisely determined. So far, most high-resolution studies have focused either on CH$_{3}$OH or on H$_{2}$O masers and little is known on their connection, wehereas it would be important to study both types of maser emission in the same object. The goal of our long-term project is to perform interferometric and VLBI observations of H$_{2}$O and CH$_3$OH masers towards a selected sample of high-mass YSOs where both maser types have been detected. This work presents preliminary results obtained for a few objects of our sample, and discusses possible implications.