We present the results of the first detection of submillimeter water maser emission toward water-fountain nebulae. Using APEX we found emission at 321.226 GHz toward two sources: IRAS 18043−2116, and IRAS 18286−0959. The submillimeter H2O masers exhibit expansion velocities larger than those of the OH masers, suggesting that these masers, similarly to the 22 GHz masers, originate in fast bipolar outflows. The 321 GHz masers in IRAS 18043−2116 and IRAS 18286−0959, which figure among the sources with the fastest H2O masers, span a velocity range similar to that of the 22 GHz masers, indicating that they probably coexist. The intensity of the submillimeter masers is comparable to the 22 GHz masers, implying that the kinetic temperature of the region where the masers originate is Tk>1000 K. We propose a simple model invoking the passage of two shocks through the same gas that creates the conditions for explaining the strong high-velocity 321 GHz masers coexisting with the 22 GHz masers in the same region.

We report on the astrometric registration of VLBI images of the SiO and H2O masers in OH 231.8+4.2, the iconic Proto-Planetary Nebula also known as the Calabash nebula, using the KVN and Source/Frequency Phase Referencing. This, for the first time, robustly confirms the alignment of the SiO masers, close to the AGB star, which drives the bi-lobe structure with the water masers in the out-flow.

Only five planetary nebulae (PNe) have been confirmed to emit water masers. They seem to be very young PNe. The water emission in these objects preferentially traces circumstellar toroids, although in K 3-35 and IRAS 15103-5754, it may also trace collimated jets. We present water maser observations of these two sources at different epochs. The water maser distribution changes on timescales of months to a few years. We speculate that these changes may be due to the variation of the underlying radio continuum emission, which is amplified by the maser process in the foreground material.

Variable OH/IR stars are Asymptotic Giant Branch (AGB) stars with an optically thick circumstellar envelope that emit strong OH 1612 MHz emission. They are commonly observed throughout the Galaxy but also in the LMC and SMC. Hence, the precise inference of the distances of these stars will ultimately result in better constraints on their mass range in different metallicity environments. Through a multi-year long-term monitoring program at the Nancay Radio telescope (NRT) and a complementary high-sensitivity mapping campaign at the eMERLIN and JVLA to measure precisely the angular diameter of the envelopes, we have been re-exploring distance determination through the phase-lag method for a sample of stars, in order to refine the poorly-constrained distances of some and infer the currently unknown distances of others. We present here an update of this project.

The final stages of low-mass stellar evolution are characterized by significant mass loss due to stellar pulsations during the AGB phase, which lead to the development of planetary nebulae. Molecular masers of H2O, SiO, and ground state OH transitions are commonly detected in oxygen-rich late-type stars (OH/IR objects). In contrast, excited OH maser transitions are rare. We discuss our study of the carbon-rich pre-planetary nebula CRL618 (a prototypical post-AGB star). Observations conducted in May 2008 with the 305m Arecibo Telescope resulted in the first detection of a 4765MHz OH maser line in a late-type stellar object; the detection was confirmed a few months later also with Arecibo. Subsequent observations in 2015 and 2017 resulted in non-detection of the 4765MHz OH line. Our observations indicate that the 4765MHz OH maser in CRL 618 is highly variable, possibly tracing a short-lived phenomenon during the development of a pre-planetary nebula.

VLBI observations of SiO masers recover at most 40-50% of the total flux obtained by single dish observations at any spectral channel. Some previous studies seems to indicate that, at least, part of the lost flux is divided up into many weak components rather than in a large resolved emission area. Taking benefit of the high sensitivity and resolution of the HSA, we investigate the problem of the missing flux in VLBI observations of SiO maser emission at 7 mm in the AGB stars and obtain a high dynamic range map of IRC+10011. We conclude that the missing flux is mostly contained in many very weak maser components.

We are currently performing a monitoring program of the 1612 MHz OH maser emission of several dozen Galactic disk OH/IR stars with the Nancay Radio Telescope (NRT). They are complemented by several OH/IR stars toward the Galactic center, which were monitored with the Hartebeesthoek radio telescope. We use the maser variations to probe the underlying stellar variability. As early monitoring programs already have shown, some stars are large amplitude variables with periods up to 7 years, others show small or even no amplitude variations. This dichotomy in the variability behaviour is assumed to mark the border between the AGB and the post-AGB stages. With the current program, we wish to find objects in transition and to describe their variability properties. We consider the fading out of pulsations with steadily declining amplitudes as a viable process. Promising candidates in the disk are the small-amplitude variables OH 138.0+7.2 and OH 51.8−0.2. ’Non-variable’ OH/IR stars in the Galactic center region may be as frequent as in the disk.

We report on magnetic field measurements associated with the well-known extreme red supergiant (RSG), VY Canis Majoris (VY CMa). We measured both linear and circular polarization of the SiO v = 0, J = 1 − 0 transition using a sensitive radio interferometer. The measured magnetic field strengths are surprisingly high. A lower limit for the field strength is expected to be at least ~ 10 Gauss based on the high degree of linear polarization. Since the field strengths are very high, the magnetic field must be a key element in understanding the stellar evolution of VY CMa as well as the dynamical and chemical evolution of the complex circumstellar envelope of the star.

For several decades (1987-2015) we have been carrying out observations of water masers in the circumstellar envelopes (CSE’s) of Mira variables, Red Supergiants (RSG’s) and Semi-Regular Variables (SRV’s) with the Medicina 32-m and Effelsberg 100-m antennas. The single-dish monitoring observations provide evidence for strong H2O maser profile variations, which likely are connected to structural changes in the maser shells. Such variations include strong flares in intensity lasting several (tens of) months and systemic velocity gradients of maser components developing over years, as well as other secular variations which are superimposed on periodic variations following the stellar light variations.

When complemented with interferometric observations, it is possible to derive the 3-D distribution of the maser spots, and their lifetime, as we have done for RX Boo (Winnberg et al. 2008) and U Her (Winnberg et al. 2011; Brand et al. in prep.).

We introduce the newly developed database of circumstellar maser sources. Until now, the compilations comprehensively including the three major maser species in evolved stars (i.e., SiO, H2O, OH) has been practically limited only to the Benson’s catalog (Benson et al. 1990), which was published more than a quarter of a century ago. For OH masers alone, there exists the University of Hamburg (UH) database, but there is no updated compilation work for H2O and SiO masers. In order to utilize the information of masers in actual studies, it is highly desirable to have a database containing all the three masers. We are currently constructing a database covering SiO, H2O and OH masers. This database consists of a web-service, which accesses compiled maser observations in available archives and combines them with the data we newly collected and IR databases. The archives currently used are the OH maser archive from Engels & Bunzel (2015), and H2O and SiO archives, which are currently under construction. So far, the information of about 27,000 observations (about 10,000 objects) has been implemented. We also have a plan to extend the database by including higher transitions and other types of objects, such as young stellar objects, in future. In this paper, we briefly summarize, (1) outline of the data collected, and (2) future development plans of the eDAMS system. The URL of the database is as follows: http://maserdb.ins.urfu.ru/

We present polarimetric observations of the 4 ground-state transitions of OH, toward a sample of maser-emitting planetary nebulae (PNe) using the Australia Telescope Compact Array. This sample includes confirmed OH-emitting PNe, confirmed and candidate H2O-maser-emitting PNe. Polarimetric observations provide information related to the magnetic field of these sources. Maser-emitting PNe are very young PNe and magnetic fields are a key ingredient in the early evolution and shaping process of PNe. Our preliminary results suggest that magnetic field strengths may change very rapidly in young PNe.

Using quasi-simultaneous observations of 86 stars with known SiO maser emission, we searched for systematic differences between the strengths of the 43 and 86 GHz v=1 maser lines. Although for individual stars there is wide scatter between the line strengths spanning nearly an order of magnitude, there is no evidence of a systematic difference between these line strengths for the entire sample.

Masers have been well-known phenomena for decades, but water masers at 183, 321, 325 and 658 GHz have only been detected since the 1990s. Early detections came from single-dish telescopes with follow-up observations from the PdBI and the Submillimeter Array. Detecting them at these short wavelengths has been very difficult due to water in our atmosphere, meaning that even in very good weather, one can only detect very bright masers, such as those in stellar atmospheres. In the last 7 years, a new window on submillimeter water masers, both Galactic and now extragalactic, has opened. Located at high altitude, above a large fraction of the Earth’s atmosphere, ALMA sits on the edge of the driest desert on the planet, meaning that the air that does remain above the telescope is frequently extremely low in water vapor content. Combine this with sensitive, stable receivers covering a number of masing transitions from 183-658 GHz and you have an excellent machine for detecting and characterizing submillimeter water masers. In addition, other molecules also exhibit maser emission in the ALMA observing bands, such as SiO and HCN.

Very Long Baseline Interferometry (VLBI) is providing key information to the study of maser processes in the Universe, from star formation regions or circumstellar envelopes around evolved stars, to Galactic structure and cosmology, through precise astrometry. VLBI networks offer various capabilities and, most importantly, support to users, to ensure that these infrastructures are fully accesible and that the best science can emerge. In this paper we describe the advances in VLBI that enable exciting maser studies.

Observations of the masers in the course of RadioAstron mission yielded detections of fringes for a number of sources in both water and hydroxyl maser transitions. Several sources display numerous ultra-compact details. This proves that implementation of the space VLBI technique for maser studies is possible technically and is not always prevented by the interstellar scattering, maser beaming and other effects related to formation, transfer, and detection of the cosmic maser emission. For the first time, cosmic water maser emission was detected with projected baselines exceeding Earth Diameter. It was detected in a number of star-forming regions in the Galaxy and two megamaser galaxies NGC 4258 and NGC 3079. RadioAstron observations provided the absolute record of the angular resolution in astronomy. Fringes from the NGC 4258 megamaser were detected on baseline exceeding 25 Earth Diameters. This means that the angular resolution sufficient to measure the parallax of the water maser source in the nearby galaxy LMC was directly achieved in the cosmic maser observations. Very compact features with angular sizes about 20\muas\, have been detected in star-forming regions of our Galaxy. Corresponding linear sizes are about 5-10 million kilometers. So, the major step from milli- to micro-arcsecond resolution in maser studies is achieved by the RadioAstron mission. The existence of the features with extremely small angular sizes is established. Further implementations of the space–VLBI maser instrument for studies of the nature of cosmic objects, studies of the interaction of extremely high radiation field with molecular material and studies of the matter on the line of sight are planned.

The RadioAstron space-VLBI mission has successfully detected extragalactic H2O MegaMaser emission regions at very long Earth to space baselines ranging between 1.4 and 26.7 Earth Diameters (ED). The preliminary results for two galaxies, NGC 3079 and NGC 4258, at baselines longer than one ED indicate masering environments and excitation conditions in these galaxies that are distinctly different. Further observations of NGC 4258 at even longer baselines are expected to reveal more of the physics of individual emission regions.

In this proceeding, we summarize the key science goals and reference design for a next-generation Very Large Array (ngVLA) that is envisaged to operate in the 2030s. The ngVLA is an interferometric array with more than 10 times the sensitivity and spatial resolution of the current VLA and ALMA, that will operate at frequencies spanning ~1.2 – 116 GHz, thus lending itself to be highly complementary to ALMA and the SKA1. As such, the ngVLA will tackle a broad range of outstanding questions in modern astronomy by simultaneously delivering the capability to: unveil the formation of Solar System analogues; probe the initial conditions for planetary systems and life with astrochemistry; characterize the assembly, structure, and evolution of galaxies from the first billion years to the present; use pulsars in the Galactic center as fundamental tests of gravity; and understand the formation and evolution of stellar and supermassive blackholes in the era of multi-messenger astronomy.

The Square Kilometre Array (SKA), reaching a collecting area of one square kilometre, will be the world’s largest radio telescope. Even in its first stage of deployment (SKA1, whose construction will be completed in 2026) it will enable transformational science on a very broad range of scientific objectives. Amongst them, there is the investigation of several Galactic and extra-galactic Masers. In this paper I will present the status of the SKA project and I will describe the capabilities of the SKA, with a focus on those that are more relevant for Maser science.

Observations at low frequencies (<8GHz) are dominated by distinct direction dependent ionospheric propagation errors, which place a very tight limit on the angular separation of a suitable phase referencing calibrator and astrometry. To increase the capability for high precision astrometric measurements an effective calibration strategy of the systematic ionospheric propagation effects that is widely applicable is required. The MultiView technique holds the key to the compensation of atmospheric spatial-structure errors, by using observations of multiple calibrators and two dimensional interpolation. In this paper we present the first demonstration of the power of MultiView using three calibrators, several degrees from the target, along with a comparative study of the astrometric accuracy between MultiView and phase-referencing techniques. MultiView calibration provides an order of magnitude improvement in astrometry with respect to conventional phase referencing, achieving ~100micro-arcseconds astrometry errors in a single epoch of observations, effectively reaching the thermal noise limit.

We discuss specific aspects of space-ground VLBI (SVLBI) data processing of spectral line experiments (H2O & OH masers) in Radioastron project. In order to meet all technical requirements of the Radioastron mission a new software FX correlator (ASCFX) and the unique data archive which stores raw data from all VLBI stations for all experiments of the project were developed in Astro Space Center. Currently all maser observations conducted in Radioastron project were correlated using the ASCFX correlator. Positive detections on the space-ground baselines were found in 38 sessions out of 144 (detection rate of about 27%). Finally, we presented upper limits on the angular size of the most compact spots observed in two galactic H2O masers, W3OH(H2O) and OH043.8-0.1.