Maser emission provides an unique window into astronomical sources across vast spatial scales - from tens to hundreds of astronomical units around protostars and evolved stars up to kiloparsecs in distant galaxies. These natural microwave amplifiers penetrate dust shells in star-forming regions, revealing the dynamics of accretion disks and outflows, trace envelopes and winds of evolved stars, map Galactic structure, while also allowing us to follow the evolution of all these systems. Owing to their compactness and brightness masers provide precise astrometry as cosmic rulers: measuring positions, structures and kinematics in dense regions, not easily accessible at other wavelengths. SKA-Mid will observe hydroxyl, methanol, and formaldehyde masers using bands 2, 5a and 5b, and later methylidyne radical masers in band 4. SKA-Mid's sensitivity and broad frequency coverage will support discoveries of new maser types and allow for simultaneous multi-transition and multi-species maser observations. In addition, bright masers can serve in the science verification of the SKA-Mid array during the deployment phase.

This paper describes the identification of Gaia counterparts of a sample of oxygen-rich AGB stars with OH maser emission as a first step towards the compilation of a general Gaia Catalogue of Galactic AGB stars. With this catalogue, tests of evolutionary models for the AGB star population in the solar neighbourhood will become feasible. We cross-matched AGB star candidates showing OH maser emission with Gaia DR3 using a cross-match with AllWISE and 2MASS as intermediate steps to avoid ambiguities. With the help of the Virtual Observatory, we retrieved photometric data from the near-ultraviolet to the far-infrared and built spectral energy distributions (SEDs) of the sources. The SEDs were fitted with theoretical models. The fit results, together with information from the literature, allowed us to clean the sample from non-AGB stars and to obtain bolometric fluxes for the AGB stars. Distances based on Gaia parallaxes were used to derive the stellar luminosities. We identified unique Gaia counterparts for 1487 OH masers. Of these, 1172 had an unambiguous classification as AGB stars; they make up the Gaia OH/IR star sample. Parallaxes with relative errors <20% and astrometric excess noise <1.5 mas were available for 222 OH/IR stars. We conclude that the study of the AGB population in the solar neighbourhood is limited by the obscuration by circumstellar dust, as DR3 only provides parallaxes for a few of our candidates. The location of the OH/IR stars matches that of LPV discovered by in the (BP-RP; G_abs) diagram, but the OH/IR star sample is biased towards redder colours (BP-RP>4) mag and larger amplitudes (>1 mag in the G-band), which are typical for periodic large-amplitude Mira variables.

Between 1987 and 2023 we carried out single-dish monitoring campaigns of water-maser emission at 22 GHz in the circumstellar envelopes (CSEs) of four stars: RCas, oCet, RLeo and ChiCyg. The variability in integrated flux in RCas and oCet followed that in the optical with the same period, but with a lag of about one-third in phase. RLeo was too often below our sensitivity threshold for us to determine a radio period. No maser was detected in ChiCyg. The variability in the masers in RCas has a distinctive pattern that confirms the existence of a zone in the CSE with favourable conditions for maser excitation. The total flux, modulated by the pulsations of the star, gradually increases to a maximum, which is followed by a similar decrease. This takes about 20 years. The pattern is repeated after an interval of quiescence of several years. During its decline, the variation in the flux resembles a damped harmonic oscillator. There are two dominant emission components that move almost tangentially on either side of the star with respect to the observer. The redshifted one is consistent with an origin in a single cloud that lived for at least about eight years, falling back towards the star. The blue component, moving in the CSE hemisphere nearest to us, has no drift in the line-of-sight velocity and appears to originate in a time series of short-living clouds with line-of-sight velocities within ca. 1 km/s of each other. No systematic drifts are found in RLeo and oCet. A few bursts of emission were detected in RCas and RLeo that lasted about a year and caused an increase in the flux density by 1-2 orders of magnitude. The velocity range of the maser emission is < 10 km/s and is clearly correlated with the bolometric luminosity of the stars.

We present our past and current long-term monitoring program of water masers in the circumstellar envelopes of evolved stars, augmented by occasional interferometric observations. Using as example the Mira-variable U Her, we identify three types of variability: periodic (following the optical variation), long-term (years-decades) and short-term irregular (weeks-months). We show there are regions in the maser shell where excitation conditions are favourable, which remain stable for many years. Lifetimes of maser clouds in the wind-acceleration zone are of the order of up to a few years. Much longer lifetimes are found for the peculiar case of a maser cloud outside that zone (as in RT Vir), or in some cases where the motion of spectral features can be followed for the entire 2 decade monitoring period (as in red supergiant VX Sgr).

AbstractThe OH/IR stars evolving on the Asymptotic Giant Branch are large-amplitude variable stars with periods in the range of ~400 to 2500 days, significantly longer than those of the related Mira variables. We use preliminary results from a monitoring program of the 1612-MHz OH maser variations of a sample of > 70 OH/IR stars to study a possible extension of Mira Period-Luminosity Relations to longer periods. The period distribution of the sample is split around P ~ 1100 days. Using WISE W3 absolute magnitudes as proxies for the luminosity and the best available distances, we found no convincing relation between periods and absolute magnitudes. A cause could be rapid evolution of the ‘extreme OH/IR stars’ (the group with P >1100 days) close to the tip of the AGB, where no increase of luminosity is expected on the short time-scales involved.

As intermediate-mass stars, precursors of planetary nebulae, head towards their final fate, they pass through the red-giant stage where they experience an increase of mass loss. This induces the creation of a circumstellar envelope of dust and gas. By the very end of this evolutionary stage, for those objects exhibiting the highest mass-loss rate, the amount of dust in the circumstellar envelope is such that it blocks optical radiation, turning them into so-called OH/IR stars. These stars are commonly observed throughout the Galaxy and are also observed in the Magellanic Clouds. Since optically obscured, the measurement of their distances using optical parallaxes as e.g. delivered by Gaia, is not possible. This issue can be circumvented thanks to maser emission. As their name gives it away, the physical conditions turn out to be ideal for a strong (1612-MHz) OH maser emission to be produced in the outer layers of the radially-expanding spherical circumstellar envelope. Combining single-dish monitoring and interferometric mapping of this OH maser emission, the “phase-lag” method provides a way to measure their distance. We have been revisiting this method through a project called “NRT phase-lag distance”. Here we present the method itself and the modus operandi of our project. We also present an analysis of the faint emission in the outer OH maser shell of OH 83.4-0.9 and use this analysis to discuss the limitations of the “phase-lag” method. Finally we compare the distances obtained from this method with those obtained from optical and radio astrometry.

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.

Ross A. Burns,∗ Agnieszka Kobak, Alessio Caratti o Garatti, Alexander Tolmachev, Alexandr Volvach, Alexei Alakoz, Alwyn Wootten, Anastasia Bisyarina, Andrews Dzodzomenyo, Andrey Sobolev, Anna Bartkiewicz, Artis Aberfelds, Bringfried Stecklum, Busaba Kramer, Callum Macdonald, Claudia Cyganowski, Fransisco Colomer, Cristina Garcia Miro, Crystal Brogan, Dalei Li, Derck Smits, Dieter Engels, Dmitry Ladeyschikov, Doug Johnstone, Elena Popova, Emmanuel Proven-Adzri, Fanie van den Heever, Gabor Orosz, Gabriele Surcis, Gang Wu, Gordon MacLeod, Hendrik Linz, Hiroshi Imai, Huib van Langevelde, Irina Valtts, Ivar Shmeld, James O. Chibueze, Jan Brand, Jayender Kumar, Jimi Green, Job Vorster, Jochen Eislöffel, Jungha Kim, Koichiro Sugiyama, Karl Menten, Katharina Immer, Kazuyoshi Sunada, Kee-Tae Kim, Larisa Volvach, Luca Moscadelli, Lucas Jordan, Lucero Uscanga, Malcolm Gray, Marian Szymczak, Mateusz Olech, Melvin Hoare, Michał Durjasz, Mizuho Uchiyama, Nadya Shakhvorostova, Olga Bayandina, Pawel Wolak, Sergei Gulyaev, Sergey Khaibrakhmanov, Shari Breen, Sharmila Goedhart, Silvia Casu, Simon Ellingsen, Stan Kurtz, Stuart Weston, Tanabe Yoshihiro, Tim Natusch, Todd Hunter, Tomoya Hirota, Willem BAAN, Wouter Vlemmings, Xi Chen, Yan Gong, Yoshinori Yonekura, Zsófia Marianna Szabó and Zulema Abraham The Maser Monitoring Organisation E-mail: ross.burns@nao.ac.jp

AbstractThe database of circumstellar OH masers by Engels & Bunzel (2015) was updated to include new 1612, 1665, and 1667 MHz OH maser observations published between 2015 and 2022. A cross-correlation of the database was made with infrared catalogues (AllWISE and 2MASS) and with GAIA DR3. This led frequently to significantly improved coordinates and identified contaminations with non-stellar sources. About 40% of all OH maser-detected stars were not detected by GAIA. These are mostly representatives of the population of highly obscured stars at the end of AGB or at the beginning of post-AGB evolution.

AbstractRecently, 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.

AbstractBased on the results of an on-going monitoring program of 1612 MHz OH masers in OH/IR stars, we determined a lifetime encompassing late AGB and early post-AGB evolution of at least 4500 years. Fading of the OH masers observed with the Nançay Radio Telescope is detected in several post-AGB OH/IR stars on timescales of decades, while AllWISE/NEOWISE light curves taken almost in parallel show diverse behaviours.

AbstractWater fountains (WFs) are thought to represent an early stage in the morphological evolution of circumstellar envelopes surrounding low- and intermediate-mass evolved stars. These objects are considered to transition from spherical to asymmetric shapes. Despite their potential importance in this transformation process of evolved stars, there are only a few known examples. To identify new WF candidates, we used databases of circumstellar OH (1612 MHz) and H2O (22.235 GHz) maser sources, and compared the velocity ranges of the two maser lines. Finally, 41 sources were found to have a velocity range for the H2O maser line that exceeded that of the OH maser line. Excluding known planetary nebulae and after reviewing the maser spectra in the original literature, we found for 11 sources the exceedance as significant, qualifying them as new WF candidates.

AbstractThe 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.

In the version of this Letter originally published, the caption of Fig. 2 incorrectly said J = 3–2; it should have said J = 2–1. This has now been corrected.

Citation for published version (APA): Decin, L., Homan, W., Danilovich, T., de Koter, A., Engels, D., Waters, L. B. F. M., Muller, S., Gielen, C., GarcaHernandez, D. A., Stancliffe, R., Van de Sande, M., Molenberghs, G., Kerschbaum, F., Zijlstra, A., & El Mellah, I. (2019). Reduced maximum mass-loss rate of OH/IR stars due to overlooked binary interaction. Nature Astronomy. https://doi.org/10.1038/s41550-019-0703-5

Abstract We present ALMA band 7 data of the extreme OH/IR star, OH 26.5+0.6. In addition to lines of CO and its isotopologues, the circumstellar envelope also exhibits a number of emission lines due to metal-containing molecules, e.g., NaCl and KCl. A lack of C18O is expected, but a non-detection of C17O is puzzling given the strengths of H217O in Herschel spectra of the star. However, a line associated with Si17O is detected. We also report a tentative detection of a gas-phase emission line of MgS. The ALMA spectrum of this object reveals intriguing features which may be used to investigate chemical processes and dust formation during a high mass-loss phase.

AbstractTIGvival is a spectroscopic monitoring program of long-period variables (LPV) using our robotic telescope TIGRE. Since 2013, we obtain low-noise, high-resolution spectra (R= 20 000) that cover the optical regime (3800 Å to 8800 Å). We are now continuously monitoring 7 LPVs with different periods and chemical properties. Our 350+ spectra evenly sample the target cycles, as far as ground-based observations allow. Analyzing the TIGvival spectra of Mira as a sample case, our measurements indicate that the strength of the TiO-absorption is phase-shifted with respect to the visual light curve.

Since 2013 we are performing with the Nancay Radio Telescope (NRT) a monitoring program of >100 Galactic disk OH/IR stars, having bright 1612-MHz OH maser emission. The variations of the maser emission are used to probe the underlying stellar variability. We wish to understand how the large-amplitude variations are lost during the AGB - post-AGB transition. The fading out of pulsations with steadily declining amplitudes seems to be a viable process.

Observations of H2O isotopologues of the extreme OH/IR star OH 26.5+0.6 show that the star has an initial mass consistent with it being an intermediate-mass star. The HerschelHIFI spectra show clear detection of H16 2 O and H 17 2 O while H 18 2 O is missing, consistent with the prediction of hot bottom burning which occurs in stars with an initial mass ≥ 5 M . The star is currently losing mass at a high rate of a few 10−4 M yr−1 which is thought to commence in the past ∼ 200 years. We present new ALMA CO J=3-2 image which show that this high mass loss (superwind) region is compact, surrounded by extended shells of lower mass loss.