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The aim of this work was to continue the monitoring of the H2O maser emission in IRAS 16293–2422 to detect superflares. We have been observing H2O maser emission at a wavelength of 1.35 cm towards the source IRAS 16293–2422 since 1999. The observations have been carried out with the 22-m radio telescope of the Pushchino Radio Astronomy Observatory (Russia). In 2007–2018, we also conducted several sessions of observations in OH lines at a wavelength of 18 cm in both circular polarizations with the Nançay Radio Telescope (France). Between 1997 and 2021, we observed three cycles of high activity of the H2O maser with a period of 8 yr. This variability could be related to the changing activity of the protostar in the tight binary system of IRAS 16293–2422 in the process of its formation. This variability could be related to an increase in the activity of the tight binary protostar IRAS 16293–2422 A, first seen in 2002. The H2O maser variation in flux density and radial velocity suggests that disturbances are propagating through organized structures in the form of chains ∼3.5 au long, with monotonic velocity gradients in the direction of propagation. The 18-cm OH emission in the main and satellite lines is thermal.

The monitoring results of the OH maser source in W75 N in the main line at 1665 MHz performed in 2007–2020 using the Nancay Radio Telescope (France) are presented. Generally, the maser in 2007–2009 is characterized by a high activity. During the monitoring, time variations of the Stokes parameters were found for all spectral features in the radial velocity range from –1 to 14 km/s. We found that its positional angle $$\chi $$ and the degree of linear polarization $${{m}_{{\text{L}}}}$$ , as well as the degree of circular polarization $${{m}_{{\text{C}}}}$$ of most features, change with time according to certain regularities, not chaotically. Or they can change very weakly and there is a correlation with the variability of the flux density. In this case, the radial velocities of the features change very little. It is shown that the main feature at 12 km/s during 50 years since the discovery of the OH maser in W75 N had two maxima of activity with an interval of about 30 years between them. The identification of spectral features in the 1665 MHz line with maser spots on VLBA maps has been carried out. We showed that a strong flare of the main features in 2007–2009 occurred in the VLA 2 source. During this flare, small changes in the angle $$\chi $$ and significant changes in the degree of both linear and circular polarization of features at 1.75, 2.1, and 3.05 km/s were observed. For the 0.65 km/s feature, significant changes in all emission parameters were observed. The emission in the 1612 MHz satellite line is broadband. The broadening of the line to 1.9 km/s indicates that, in addition to thermal motion of matter, there is also turbulent motion of matter in the medium.

We present the results of our observations of the star-forming region located in the Perseus arm and associated with the Sharpless nebula S231 in the lines of H2O at a wavelength of 1.35 cm and OH at a wavelength of 18 cm obtained with the Pushchino 22-m radio telescope (Russia) and the Nancay Large Radio Telescope (France), respectively. We provide a catalog of H2O maser spectra in the period from March 2003 to March 2020. The variability of the integrated flux that we have calculated based on the monitoring by Felli et al. (1987–2007) and our monitoring (2003–2020) has a quasi-cyclic pattern with a cycle duration from 4.3 to 7.7 yr. Flares of maser emission of both single and several features occurred in each activity cycle. The mean rise and decay times of the flare emission were $$0.30 \pm 0.02$$ and $$0.35 \pm 0.02$$ yr, respectively. The observed radial velocity drift and jumps of the H2O emission features can be a consequence of the impact of shocks emerging as the protostar activity rises on them. The complex pattern of variability shows that the H2O masing medium is fragmented and small-scale turbulent motions of matter can occur in it. We have observed the OH maser emission in the 1665 and 1667 MHz main lines and the 1720 MHz satellite line. A large number of OH emission features and their variability have been detected. We have found one Zeeman pair at ‒9.2 km/s with a small splitting, with the longitudinal magnetic field in the period 2008–2020 having decreased monotonically from 0.24 to 0.10 mG. For the three most intense features we have calculated the degrees of linear polarization and position angles. A correlation between the H2O and OH maser activities has been found.

We present the observation results of the star forming region NGC 2071 in the 1.35-cm H2O line and 18-cm OH lines obtained with the 22-m radio telescope in Pushchino (Russia) and the large radio telescope in Nancay (France), respectively. The catalog of H2O maser emission spectra from December 2010 to January 2020 is given. The total flux variability for the entire monitoring period (1979–2020) has two activity cycles of close duration (approximately 20 years), which were accompanied by strong flares at intervals of one to four years. It is shown that the medium for the generation of H2O maser emission is greatly fragmented and contains small-scale turbulent motions of matter. The observed drift and radial velocity jumps of the H2O emission features can be a consequence of the complex, inhomogeneous structure of the maser emission regions. OH emission in the 1665 and 1667 MHz main lines and in the 1612 MHz satellite line in the radial velocity range of 7–12 km/s is broadband. The ratio of intensities differs from equilibrium. The substantiation of the observed ratio between the intensities of this emission and absorption in the 1720 MHz line is given. OH maser emission was observed in 2007 in the left circular polarization in the 1667 MHz line at 13.02 and 13.55 km/s with a flux density of ~0.3 Jy and a width of ~0.25 km/s for each of the velocities.

The structure and evolution of powerful H2O maser flares in the source IRAS 18316–0602 are studied using the results of observations on the 22-m radio telescope of the Pushchino Radio Astronomy Observatory. The main origin of the last powerful, short-term flare in 2017 may be the superposition of two maser condensations with very similar radial velocities along the line of sight. All powerful flares occurring in IRAS 18316–0602 since 2002 can be associated with a cluster of maser condensations whose radial velocities are in the interval 41.0–43.5 km/s. It is suggested that this may be related to turbulent, possibly vortical, motions of material in this region.

Results of monitoring hydroxyl and water masers in the star-forming region S128 are presented. A large number of emission features in the 1665 MHz OH line have been detected in both circular polarizations. In spite of the strong variability of the flux density in the main 1665 MHz line, the radial velocities of the features remained constant. Zeeman splitting of the 1720MHz line equal to 0.86 km/s was detected, corresponding to a longitudinal magnetic field of 3.6 mG. The variability of the H2O emission has a cyclic character with a quasi-period of 4–14 yrs. The evolution of individual features confirms that the H2O sources A and B are associated with an ionization front between two colliding CO clouds, and shows that the activity was transferred from maser B to maser A in 1999–2001.

The results of monitoring the H2O maser observed toward the region GH2O 092.67+03.07 (IRAS 21078+5211) located in the Giant Molecular Cloud Cygnus OB7 are presented. The observations were carried out with the 22-m radio telescope of the Pushchino Radio Astronomy Observatory in 2006–2017. Strong flares of the H2O maser emission with flux densities up to 19 800 Jy were detected. The flares exhibited both global (over the source) and local characters. All the flares were accompanied by strong variations in the H2O spectra within the corresponding radial-velocity ranges. Individual H2O components form both compact clusters and chains 1–2-AU long. Analysis of the variations of the fluxes, radial velocities, and line shapes of features during the flares showed that the medium may be strongly fragmented, with small-scale turbulent motions taking place in the H2Omaser region.

The results of a study of the maser source IRAS 18316−0602 in the H2O line at λ = 1.35 cm are reported. The observations were carried out on the 22-m radio telescope of the Pushchino Radio Astronomy Observatory (Russia) from June 2002 until March 2017. Three superflares were detected, in 2002, 2010, and 2016, with peak flux densities of >3400, 19 000, and 46 000 Jy, respectively. An analysis of these superflares is presented. The flares took place during periods of high maser activity in a narrow interval of radial velocities (40.5–42.5 km/s), and could be associated with the passage of a strong shock. The emission of three groups of features at radial velocities of about 41, 42, and 43 km/s dominated during themonitoring. The flare in 2016 was accompanied by a considerable increase in the flux densities of several features with velocities of 35–56 km/s.

We report long-term observations of H2O and OH maser emission sources at wavelengths of 1.35 and 18 cm associated with star-forming regions. Strong quasi-periodic flares of maser emission have been observed. Several sources (in particular, G25.65+1.05, IRAS 16293−2422, Cep A) have displayed strong flares in the H2O line, when their peak flux density raised by a few orders of magnitude above the quiet state. Possible causes of this are discussed.

Results of observations of the star-forming region S252A in the 1.35-cm H2O and 18-cm OH lines obtained using the 22-m Pushchino (Russia) and Nancay (France) radio telescopes are presented. A catalog of H2O maser spectra for 1995-2019 is presented. The variability of the integrated flux has two components: a cyclic component with a time interval between cycles ~30-35 yrs and a short-period component with a mean period of about 2.6 yrs. This may reflect non-stationary formation of a protostar. It is shown that the medium where the H2O maser emission and thermal OH emission are generated is strongly fragmented, and contains small-scale turbulent motions comparable to the thermal motions of the matter. The observed drift and jumps in the radial velocity of the H2O emission features could be a consequence of complex, non-uniform structure of the maser condensations.

The results of observations of OH maser emission in the star-forming region G43.8–0.1 are presented. In spite of strong flux-density variations in the main lines at 1665 and 1667 MHz, the radial velocities of the spectral features varied only slightly. The main spectral features are identified with maser spots in previously published maps for epochs 1993 and 2001. It is suggested that the regions of OH maser emission may be elongated, nonuniform structures with weak radial velocity gradients (larger-scale analogs of water-maser filaments). The line-of-sight magnetic fields are determined for two Zeeman pairs, which remained essentially constant over at least 17 years.

In 2017-18 the source of outstanding H2O maser bursts G25.65+1.05 have been intensively studied with a wide range of baselines - from compact array (JVLA) and ground VLBI (EVN, VLBA, KaVA) to space VLBI (RadioAstron mission supported by VLBA and EVN telescopes) - the report gives a brief summary of these observations.

Observations of H2O maser sources at 1.35 cm associated with extended regions of 4.5-µm emission (indicated as “green” on Spitzer survey maps—so-called Extended Green Objects, EGOs) are reported. EGOs are considered as characteristic signposts of regions of formation of massive stars, which host high-velocity outflows, as well as methanol, water, and hydroxyl masers. The observations were carried out in January–May 2015 on the 22-meter radio telescope of the Pushchino Radio Astronomy Observatory. The sample studied includes 24 EGOs north of declination -29° taken from the Spitzer GLIMPSE survey, together with one of the brightest Class I methanol masers G6.05-1.45 (M8E) and the Class I methanol maser in an IRDC G359.94+0.17. H2O maser emission was detected toward 11 of the EGOs: G11.94-0.62, G14.33-0.64, G16.59-0.06, G23.01-0.41, G24.943+0.074, G28.83-0.25, G34.3+0.2, G34.403+0.233, G35.20-0.74, G45.47+0.07, and G49.267-0.337. These including the well known H2O maser in the W44 region, G34.3+0.2. H2O emission from G28.83-0.25 was detected for the first time, at 77.6 km/s, with a flux density of 19 Jy in January and 16 Jy in February 2015. The source was probably caught at an early stage of the propagation of a shock wave. The Class I methanol masers G359.94+0.17 and G6.05-1.45 (M8E) and 13 of the EGOs were not detected in the H2O line, with 3s upper limits of ~6-7 Jy. Spectra and maser-emission parameters are given for the detected H2Omasers, for some of which strong variability of the H2O maser emission was observed. The detected H2Omasers, together with the Class I methanol masers and extended 4.5-µm emission, are associated with a very early stage in the development of young stellar objects in the regions of the EGOs. However, this sample of EGOs is not uniform. The presence of 44-GHz Class I methanol masers together with EGOs cannot be considered the only sign of early stages of star formation.

The results of a study of H2O and OH maser emission in the complex region of active star formation W75 N are presented. Observations were obtained using the 22-m radio telescope of the Pushchino Radio Astronomy Observatory (Russia) and the Nan3ay radio telescope (France). Flaring H2O maser features may be identified with maser spots associated with the sources VLA 1 and VLA 2. Themain H2O flares occurred in VLA 1. The flare emission was associated with either maser clusters having closely spaced radial velocities and sizes up to ~2 AU or individual features. The maser emission is generated in a medium where turbulence on various scales is present. Analysis of the line shapes during flare maxima does not indicate the presence of the simplest structures—homogeneous maser condensations. Strong variability of the OH maser emission was observed. Zeeman splitting of the 1665-MHz line was detected for several features of the same cluster at a radial velocity of +5.5 km/s. The mean line-of-sight magnetic field in this cluster is ~0.5 mG, directed away from the observer. Flares of the OH masers may be due to gas compression at a shock or MHD wave front.

We present the results of our investigation of the H2O maser emission from individual features in IRAS 20126+4104, which is a cold infrared source. The observations were performed at the 22-m radio telescope of the Pushchino Radio Astronomy Observatory. A radial-velocity drift of the features caused by their deceleration at the interface between between an accelerated jet and the surrounding molecular environment has been detected. The observed tendency for the H2O emission as a whole to drift is associated with the successive excitation of maser features or their clusters by the front of a shock (or magnetohydrodynamic) wave. An estimate of the jet rotation period is provided (∼150 years).

We present the results of our study of the H2O maser emission in the intricate complex of active star formationW75 N.Our observations have been carried out at the 22-m radio telescope of the Pushchino Radio AstronomyObservatory. A possible identification of its flares with the sources VLA 1 and VLA 2 has been made. Two cycles of maser activity have been detected: 2006–2012 and 2012–2015. The main H2O maser flares occurred in VLA 1. Analysis of the evolution of the flares has shown that the flare emission is associated with a cluster of maser condensations with close radial velocities up to ~2 AU in extent. The medium in which the maser emission is generated is turbulent with various turbulence scales. Our investigation of the line shape at the epochs of flare maxima has also shown the absence of structures of the simplest form, homogeneous maser condensations. The emission in the velocity range from −10.5 to −12 km s−1 has been identified with a structure like an elongated filament with an extent of ~4 AU.

We present the results of our monitoring of four maser sources associated with cold infrared sources. The observations were performed in the water-vapor line at 1.35 cm with the 22-m radio telescope at the Pushchino Radio Astronomy Observatory and in the hydroxyl lines at 18 cm with the Nancay radio telescope. Cyclic variability of the H2O maser emission was detected in all four sources. The durations of the cycles are different and lie within the range 1.8–5.5 yr. In each source, the cycles differed by a factor from 1.5 to 2.25. A fairly periodic pattern of variability was observed in IRAS 20126+4104 on a time scale from 3.6 to 5.3 yr. Models of the maser sources are discussed. In IRAS 20126+4104, we detected strong variability of the maser emission in the main OH 1665 and 1667 MHz lines. We calculated the magnetic field strength for two maser features from the Zeeman splitting of the OH line: in IRAS 18265-1517 (1.3 mG at epoch 2008) and IRAS 20126+4104 (10 mG at epoch 2014). Variability of the magnetic field was detected in the second feature. In the same sources, we detected maser emission in the 1667 MHz OH line. The presence of emission in both main OH 1665 and 1667 MHz lines is consistent with the existing model calculations and will allow the gas density in the masing regions to be refined.

The results of a study of the maser source G 10.623-0.383 in the λ = 1.35 cm H2O line using the 22-mradio telescope of the Pushchino Radio AstronomyObservatory (Russia) and in the main hydroxyl lines (λ = 18 cm) using the Nanзay Radio Telescope (France) are presented. Uncorrelated long-term variations of the integrated intensities and the velocity centroids with characteristic times of 11 yrs (mean value) and 32 yrs, respectively, are studied. The drift of the velocity centroid may be associated with maser condensations whose material is collapsing onto the OB cluster. It is shown that the H2O maser source contains maser condensation configurations on various scales over a long time, which evolve with time. OH maser emission was only detected in the main lines at 1665 and 1667 MHz. The flux densities of the strongest emission components were variable, but their radial velocities did not change. A Zeeman pair was found at 1667 MHz with a splitting of about 1.44 km/s, corresponding to a line-of-sight magnetic field of 4.1 mG, which was preserved over at least 25 years. The characteristics of the H2O andOHmaser variability suggests that the masers are located in different parts of G 10.623–0.383.

The results of observations of OH (λ = 18 cm) and H2O (λ = 1.35 cm) masers toward AS 501 obtained with the Nancay Observatory Radio Telescope (France) and the 22-m radio telescope of the Pushchino Radio Astronomy Observatory (Russia), respectively, are presented. Nine cycles of H2O maser activity ranging from 2.8 to 6.0 years were detected, identifying AS 501 as an irregular variable star. Zeeman splitting was found only in the 1612-MHz satellite line at −59.2 km/s. The splitting is 0.11 km/s, corresponding to a line-of-sight magnetic field strength of 0.48 mG. The field is directed toward the observer. The detected radial-velocity drift of the H2O emission features can be explained in a model with elongated filaments with radial-velocity gradients.

We present the results of our long-term monitoring of the 1.35-cm water-vapor maser source ON 1 performed at the 22-m radio telescope of the Pushchino Radio Astronomy Observatory from 1981 to 2013. Maser emissionwas observed in a wide range of radial velocities, from −60 to +60 km s−1. Variability of the integrated flux with a period of ∼9 years was detected. We show that the stable emission at radial velocities of 10.3, 14.7, and 16.5 km s−1 belongs to compact structures that are composed of maser spots with close radial velocities and that are members of two water-maser clusters, WMC 1 and WMC 2. The detected short-lived emission features in the velocity ranges from −30 to 0 and from 35 to 40 km s−1 as well as the high-velocity ones are most likely associated with a bipolar molecular outflow observed in the CO line.

Results of long-term studies of circumstellar molecular maser emission of late-type giant and supergiant variable stars are reported. In the 1.35-cm H2O line, the peak flux density correlates with the optical brightness lagging behind it by 0.3–0.4 P (P is the stellar period). “Superperiods” of 10 to 15 P are visible in several stars, demonstrated as high maxima in the visible light curve and associated flares in the H2O maser line. In the 18-cm OH lines, full polarization of the maser emission has been measured. Variable Zeeman patterns suggesting a changing magnetic field of a few milligauss have been detected.

We present observations of H2O maser emission from the complex region of active star formation Sgr B2 performed in 2005–2012. The observations were carried out with the 22-m radio telescope of the Pushchino Radio Astronomy Observatory. Seven flares with flux densities higher than 1000 Jy were detected. The flares occurred in all three main sites of star formation in Sgr B2, N,M, and S. The highest peak flux densities were 3200 Jy (60.9 km/s), 2350 Jy (69.4 km/s), and 7300 Jy (69.3 km/s) in N, M, and S, respectively. This last flare was the strongest during our monitoring campaign from 1982 to 2012, both in S and in the entire Sgr B2 complex. Possible associations of the flares were determined. High-velocity, short-lived emission was detected at 124–128 km/s. Emission at 127 km/s with a flux density of 23 Jy is associated with region M. Emission at 80.6 and 84.6 km/s, at radial velocities higher than those observed previously, was detected in region S.

We present the results of our observations of the maser radio emission source G188.946+0.886 in hydroxyl (OH) molecular lines with the radio telescope of the Nancay Observatory (France) and in the H2O line at λ = 1.35 cm with the RT-22 radio telescope at the Pushchino Observatory (Russia). An emission feature in the 1720-MHz satellite line of the OH ground state has been detected for the first time. The radial velocity of the feature, V LSR = 3.6 km s−1, has a “blue” shift relative to the range of emission velocities in the main 1665- and 1667-MHz OH lines, which is 8–11 km s−1. This suggests a probable connection of the observed feature in the 1720-MHz line with the “blue” wing of the bipolar outflow observed in this region in the CO line. We have estimated the magnetic field strength for three features (0.90 and 0.8 mG for 1665 MHz and 0.25 mG for 1720 MHz) from the Zeeman splitting in the 1665- and 1720-MHz lines. No emission and (or) absorption has been detected in the other 1612-MHz satellite OH line. Three cycles of H2O maser activity have been revealed. The variability is quasi-periodic in pattern. There is a general tendency for the maser activity to decrease. Some clusters of H2O maser spots can form organized structures, for example, chains and other forms.

The results of a study of the H2O and OH maser emission from the cool IR source IRAS 16293−2422 are presented. The observations analyzed were obtained in H2O lines with the 22-m telescope of the Pushchino Radio Astronomy Observatory during 1999–2015 and in OH lines with the Nanc¸ ay radio telescope (France). A large number of very strong flares of the H2O maser were detected, reaching fluxes of tens of thousands of Jansky. Individual features can form organized structures resembling chains ∼2 AU in length with a radial-velocity gradient along them. The observed drift of the H2O emission (2003–2004) in space and velocity (from 4.3 to 5.3 km/s) is not due solely to proper motion of the features. The other origin of the drift is a drift of the emission maximum during a flare as the shock consecutively excites spatially separated features in the structure in the form of a chain. The OH-line observations at 18 cm show that the emission remains unpolarized and thermal, with a line width of 0.7 km/s, which corresponds to a cloud temperature of ∼30 K.

Results of monitoring the H2O and OH masers in W44C, located near the cometary HII region G34.3+0.15, are reported. Observations in the water-vapor line at λ = 1.35 cm were carried out on the 22-meter radio telescope of the Pushchino Radio Astronomy Observatory (Russia) from November 1979 to March 2011, and in the hydroxyl lines at λ = 18 cm on the large Nancay radio telescope (France). Activity maxima and minima of the water maser alternated. The average period of the activity is ∼ 14 years, consistent with results obtained earlier for a number of other sources associated with regions of active star formation. In periods of enhanced maser activity, two series of strong H2O maser flares were observed, which were related to two different clusters of maser spots located at the front of a shock at the periphery of the ultracompact region UH II. These series of flares may be associated with cyclic activity of the protostellar object in UH II. In the remaining time intervals, there were mainly short-lived flares of single features. The Stokes parameters for the observations in the hydroxyl lines were determined. Zeeman splitting was observed in the profile of the 1667 MHz OH main line at a velocity of 58.5 km/s, and was used to estimate the intensity of the line-of-sight component of the magnetic field (1.2 mG).

A catalog of maser spectra in the 1.35-cm water-vapor line towards the maser source NGC 2071 in a region of massive star formation is presented for 1994–2010. The observations were carried out using the 22-m antenna of the Pushchino Radio Astronomy Observatory with a spectral resolution of 0.101 km/s (0.0822 km/s after the end of 2005). Based on the data throughout the monitoring since 1980, two very different cycles of maser activity were found. The first (1980–1992) is characterized by high activity within a broad range of radial velocities. Emission at velocities near 7 km/s predominated in 1980–1986, and emission near 14–16 km/s, in 1987–1992. In 1997–2008, the maser intensity was appreciably lower than in the first activity cycle. Numerous flares of individual emission features were observed. Identifications based on VLA data show that strong flares took place in both maser sources, IRS1 and IRS3. Both sources demonstrated a low level of maser activity during essentially the same epochs (1977, 1995–1997, and the close of 2009 through the beginning of 2010), although the sources are separated by at least 2000 AU.

Results of a multi-faceted study of the H2O maser emission in the region ON2 N carried out on the Very Large Array (VLA, NRAO) and 22-m radio telescope of the Pushchino Radio Astronomy Observatory are reported. The envelope around the ultracompact HII region is fairly extended and has a composite, strongly fragmented structure. The maser emission zone consists of single spots and spot clusters arranged along an arc, which is associated with a ram shock front. This shock front is nonsta-tionary, and its position changes with time. The front position probably depends on the state of activity of the central star. There can be turbulent motions of material in clusters as well as individual maser spots (such as turbulent vortices). In the turbulent-vortex model, the size of an H2O maser spot is estimated to be 0.07–0.1 AU. Flux-correlated radial-velocity drifts of emission features have been detected, which can be accompanied by spatial displacement (proper motion) of maser spots.

We report the results of a study of fast variations of the H2O maser emission toward NGC 7538 IRS 1, which is associated with a star-forming region. The study is based on monitoring data in the 1.35 cm line obtained in 1996–2003 on the 22-meter radio telescope of the Pushchino Radio Astronomy Observatory (Russia). Periodic flux variations of four long-lived emission features with an average period of about 0.9 year have been detected. The flux variations of these features are correlated, suggesting that the detected variability is a consequence of pulsation of, e.g., stellar wind from the protostar in NGC 7538 IRS 1, with a period of about 0.9 year (0.87 ± 0.03 year). These pulsations are superimposed on long-term variability of the integrated maser emission with a period of 13 years.

Strong flares of the H2O maser emission in sources associated with active star-forming regions are analyzed. The main characteristics of 13 flares in nine sources selected using special criteria are presented. The observed phenomena are explained as flares in double emission features. The approach of two emission features in the spectrum with increasing flux and their recession with decreasing flux is explained using a model with two physically related clumps of material that are partially superposed in the line of sight. Calculations have shown that, in this type of model, exponential amplification (unsaturated maser emission) in the overlapping parts of the clumps can produce the observed line narrowing with increasing flux. In most cases, the maser spots are inhomogeneous. During the evolution of some flares, the maser condensations may split into separate fragments. A less catastrophic evolutionary path may be an initial stage of formation of chainlike structures, which are fairly widespread in envelopes around ultracompact HII regions.

A model of the source associated with VLA 1 in W75N is constructed based on monitoring of the H2O maser carried out on the 22-m radio telescope of the Pushchino Radio Astronomy Observatory and NRAO VLA maps of the source at epochs 1992, 1996, 1998, and 1999. The source has a complex hierarchical structure. Individual maser spots form compact clusters in the form of filaments or chains, as well as more extended arc-like clusters. There are radial-velocity gradients in the chains and filaments. All these structures are arranged in a very elongated disk. This complex configuration could arise due to various-scale turbulent motions of the emitting material, from microturbulence to large-scale turbulence. The strong elongation could be due to an outflow of material from the star. No radial motions of individual clusters of spots with respect to the central source in VLA 1 have been found. The presence of these spots at various epochs could be due to the passage of MHD waves, which excite the maser emission in corresponding zones of the maser source. This process could have a cyclic character, and be associated with the flare activity of the central star.

We investigate the influence of stochastic Doppler width fluctuations on the shape of spectral lines. The photospheres and atmospheres of stars, and the interstellar medium, possess stochastic behavior especially near nonstationary objects such as active galactic nuclei, quasars, flare stars, and regions of star formation. In reality, we observe the mean values of intensities from these objects. In most situations, the spectral line extinction coefficient has a Gaussian shape with the stochastic Doppler width determined by thermal and small-scale turbulent motions of atoms or molecules. For small-scale turbulent motions (short-correlated turbulence) the propagation of radiation is described by the average extinction factor. This coefficient depends on the level of the Doppler width fluctuations {eta}. We show that these fluctuations change both the value of intensity and the shape of spectral lines. We consider distortions of the spectral line shapes for the absorption and emission lines for various values of the parameter {eta}. For a number of H{sub 2}O maser sources we estimate the values of this parameter, the optical depths of the inverted media, and the mean effective Doppler velocities. Maser emission lines with non-Gaussian shape can serve as an additional method for the investigation of the physical parameters inmore » maser 'spots'.« less

The results of analyzing asymmetry in the H2O maser lines at λ = 1.35 cm are presented. We investigated in detail the known mechanism of the asymmetry origin due to the hyperfine structure of the 616 → 523 transition and different (not equidistant) positions of the corresponding triplet components. The triplet of the lines is characterized by the sum of extinction factors that are continuously transformed into a single-peaked asymmetric extinction coefficient, with the increase in the Doppler widths (characterizing by effective Doppler velocity uD). A noticeable asymmetry corresponds to uD ≤ 0.5− 1k m s −1 . For higher values of uD the extinction factor approaches a Gaussian shape with the decreasing magnitude of the profile asymmetry. An appreciable asymmetry in unsaturated maser lines can only arise at uD ≤ 0. 5k m s −1 , if there are very narrow linewidths (Δu ≈ 0.2−0. 4k m s −1 ). But observed saturated masers possess a fairly high degree of the line asymmetry, 10−40%. Most probably, this asymmetry is a result of rebroadening of the linewidth up to the value that is comparable to the width of the asymmetric extinction coefficient. Long-term monitoring of the H2O maser emission sources at λ = 1.35 cm in active star-forming regions has shown the existence of separate lines (emission features) with various linewidths and shapes. The linewidths of the emission from such single features can be from 0.35 to 0.9 km s −1 . The linewidth is determined by thermal and small-scale turbulent motions of H2O molecules in a maser spot. The line shape is determined by possible superposition of some components with similar radial velocities. In the case of a single spectral line, which is emitted by a separate feature, the line shape is basically approximated by the usual Gaussian curve. The monitoring has shown that there are single lines with non-Gaussian shapes, and these lines mostly have an asymmetric shape.

Results of monitoring of the H2O maser observed toward the infrared source IRAS 21078+5211 in the giant molecular cloud Cygnus OB7 are presented. The observations were carried out on the 22-m radio telescope of the Pushchino Radio Astronomy Observatory (Russia) from April 1992 to March 2006. Five cycles of maser activity at various levels were observed. In the periods of highest activity, the spectrum of the H2O maser emission extended from −43 to 12 km/s. During strong flares, the flux densities in some emission features reached nearly 600 Jy. The protostar has a small peculiar velocity with respect to the CO molecular cloud (∼2 km/s). Based on the character of the radial-velocity variations and the tendency for the linewidth to increase with the flux, it is concluded that the medium is strongly fragmented and that there is a small-scale turbulent outflow of ga in the H2O maser region, which may impede the formation of an HII region. The asymmetric distribution of the maser components in V LSR, the difference in the average linewidths of the central and lateral clusters of components, and the fairly high radial velocities relative to the molecular cloud (especially during periods of the highest maser activity) suggest that the maser spots belong to different clusters and different structures of the source: a disk and bipolar outflow.

A probable model for the circumstellar envelope associated with the source VLA 2 in W75N has been constructed, based on H2O-maser monitoring toward W75N carried out on the 22-m radio telescope in Pushchino, as well as VLA maps for 1992, 1996, 1998, and 1999. The envelope has a complex hierarchical structure, including individual maser spots, clusters and chains of spots, inhomogeneous filaments, individual arc-shaped layers, and other complicated features. Most widespread are multi-link chains or filaments with sizes of 1–2 AU. This pattern arises due to the complex hierarchical structure of turbulent motions of material on various scales, from microturbulence to large-scale chaotic motions. No expansion of individual layers in the envelope of VLA 2 has been found. The appearance of the layers is due to the passage of MHD waves that excite the maser emission in consecutive shells in the envelope. This process is fairly cyclic, and is related to the flare activity of the star.

Results of monitoring of H2O maser in the infrared source IRAS 20126+4104, which is associated with a cool molecular cloud, are presented. The observations were carried out on the 22-meter radio telescope of the Pushchino Radio Astronomy Observatory (Russia) between June 1991 and January 2006. The spectrum of the H2O maser emission extends from − 16.7 to 4.8 km/s and splits into separate groups of emission features. Cyclic variations of the integrated maser flux with a period from 3.4 to 5.5 years were detected, together with strong flares of up to 220 Jy in individual emission features. It is shown that large linewidths in periods of high maser activity are due to small-scale turbulent motions of the material. An expanding envelope around a young star is accepted as a model for the source. The protostar has a small peculiar velocity with respect to the molecular cloud (∼2 km/s). Individual emission features form organized structures, including multi-link chains.

Rapidly-evolving red supergiants (RSG) lose half or more of their mass before ending their lives as supernovae. Masers allow us to study the mass loss from 4 nearby RSG in AU-scale detail using MERLIN and EVN/global VLBI. The water maser clouds are over-dense and over-magnetised with respect to the surrounding wind. In most cases, the brighter an individual maser component is the smaller its apparent (beamed) FWHM appears, as predicted for approximately spherical clouds. Individual water maser features have a typical half-life of 5-10 yr, but comparison with single dish monitoring suggests that the water vapour clouds themselves survive many decades (the water maser shell crossing time), within which the local masers wink on and off. OH mainline masers are found in the tenuous surrounding gas, overlapping the water maser shell, surrounded by OH 1612-MHz masers at a greater distance from the star.

An analysis of the H2O maser emission toward the source NGC 7538N, which is associated with an active star-forming region, is reported. The analysis is based on 24 years of monitoring in the 1.35-cm line using the the 22-m radio telescope of the Pushchino Radio Astronomy Observatory in 1981–2005 with a spectral resolution of 0.101 km/s. Individual spectral components have been isolated, and temporal drifts in their radial velocities found. From time to time, the drifts were accompanied by velocity jumps. This can be explained if there are chains consisting of clumps of material that are elongated in the radial direction toward the star and have a radial-velocity gradient. In 1982–2005, two maser activity cycles were observed, during which the chains were activated. We propose that shocks consecutively cross the chain elements and excite maser emission in them. The longest chain, at a radial velocity of −58 km/s, has not fewer than 15 links. For a shock velocity of 15 km/s, the chain step is estimated to be ≤1.5 AU. The chains could be located in a circumstellar disk with a width of ≤1015 cm. A structure in the form of a rotating nonuniform vortex with the rotation period of about 1.6 years has also been detected. The translational motion of the vortex may be a consequence of its orbital motion within the protoplanetary disk.

We present the results of a variability study of some H2O maser-emission components of Sgr B2, which is located in an active star-forming region. Our monitoring was conducted in 1982–2004 with the 22-m radio telescope of the Pushchino Radio Astronomy Observatory. We analyze brightness variations for the strongest groups of emission features in the H2O spectra, mainly during periods of maser flaring activity. Each of these groups contains many components, whose radial velocities and fluxes we determined. Most of the components displayed radial-velocity drifts. We detected a correlation between the flux and radial-velocity variations for some of the components. Variability of the emission can be explained in a model in which the maser spots form elongated chains and filaments with radial-velocity gradients. During H2O flares, the flux increases of some maser spots were accompanied by acceleration, while flux decreases were accompanied by deceleration of their motion in the dense circumstellar matter. Spectral groups of emission features are probably spatially compact structures.

The results of a search for ultrarapid variability of galactic water maser sources at the wavelength 1.35 cm on a timescale of several minutes to 1 h are reported. A special observational technique was developed. Using this technique, several observational sessions for more than 30 maser sources were carried out from 2002 to 2006. Among those, several sources that have shown such variability were found. The causes of this are discussed. The most interesting of these causes are internal processes within the sources ongoing in regions with a characteristic size of about 0.1 AU. Clear evidence for such a variability type was found in at least two sources: W49N and W33B. Variability of this kind displays itself only in certain states of the water masers.

Context. The results of monitoring of the water-vapour maser in the 1.35-cm line toward ON2 in 1995-2004 are reported. Aims. The main goal was to study variations of the H 2 O maser emission on a long (10 years) time interval, in particular, velocity drifts and correlation between fluxes in various spectral features. Methods. The observations were carried out on the RT-22 radio telescope of Pushchino Radio Astronomy Observatory. Results. The emission of the northern component (ON2 N) took place in a broad interval of radial velocities, from -12 to 9 km s -1 . The total H 2 O emission has two variability components: slow and flaring. The period of the former component is most likely between 25 and 30 years. The flaring component has a cyclic character with a period from 1.1 to 2.6 years. The alternation of the activity cycles was accompanied by changes in the velocity structure of the H 2 O spectra. There is good correlation between the variations in the integrated flux and velocity centroid, as well as between the emissions in various segments of the H 2 O spectrum. We suggest that long-term variations and oscillations of the water-maser emission can be related to the non-stationarity and anisotropy of bipolar molecular outflow from the B-type star in the ultracompact H II region.

The water-vapor maser emission in the source G10.6-0.4 associated with an active starforming region (OB star cluster) is analyzed. The maser was monitored from 1981–2004 using the 22-meter radio telescope of the Pushchino Radio Astronomy Observatory. Statistical processing of the results revealed the presence of structural formations on various scales. The individual H2O maser features may form ordered structures with velocity (V LSR) gradients, localized in separate clusters of maser features. The statistical variations of the V LSR values for the maser components may be due to the accretion of material onto the OB star cluster in G10.6-0.4 together with the rotation of the molecular cloud core. A model with a rotating, nonuniform condensation of accreted material in the vicinity of the stellar cluster is proposed to explain the variations of the velocity centroid of the H2O spectra. The integrated flux variations are explained well by a model in which the central source is an OB star cluster, possibly containing five to six stars. An important role in the evolution of the maser emission, as well as of the source as a whole, may be played by turbulent motions of the gas.

We consider the propagation of spectral-line radiation in a correlated turbulent atmosphere. The ensembles of turbulent velocities u(r,t) and optical depths, τν, are assumed to be Gaussian. We investigate the explicit analytical solution of the stochastic radiative transfer equation for the intensity Iν of radiation. The scattering term is not taken into account. It is shown that, in addition to the usual Doppler broadening of the spectral line, correlated turbulent motions of atoms and molecules give rise to considerable changes in the shape of a spectral line. We find that the mean intensity I(0)ν (Iν=I(0)ν+I′ν, I′ν = 0) obeys the usual radiative transfer equation with renormalized extinction factor αeffν if the correlation length R0 of the turbulence is small as compared to a photon free path. A simple analytical expression for αeffν is given. This expression integrally depends on the two-point correlation function of the turbulent velocity field.

Results of a study of a strong flare of H2O maser emission in the star-forming region Sgr B2(M) in 2004 are reported. The observations were carried out on the 22-m radio telescope of the Pushchino Radio Astronomy Observatory. The main emission, with its flux density reaching 3800 Jy, was concentrated in a narrow radial-velocity interval (about 3 km/s) and was most likely associated with the compact group r, while the emission at VLSR > 64 km/s came from group q. After 1994, the variations of the H2O maser emission in Sgr B2(M) became cyclic with a mean period of 3 years.

We present the results from observing the circumstellar maser emission of the M-type supergiant S Per in the 6 16 –5 23 water-vapour line at 1.35 cm. The observations were carried out in 1981–2002 ($\rm JD=2\,444\,900$–2 452 480) on the RT-22 radio telescope of the Pushchino Radio Astronomy Observatory, Astrospace Center of the Lebedev Institute of Physics, Russian Academy of Sciences. The H 2 O spectra obtained represent an unprecedented long, uniform dataset on this star. We discuss the properties of the optical and maser variations of S Per, together with particulars of the available VLBI maps. The close relation between maser and optical variations favors a model in which mass-loss is episodic. Changes observed in the total H 2 O line flux follow the visual light curve with a delay of 0.01 to 0.5 P , where $P\approx 800$ d is the mean light cycle for S Per. The feature at $V_{\mathrm{LSR}}=-44$ km s -1 flared in July 1988, which seemed to be the response of the maser to an unusually bright optical maximum. The position of the -44-km s -1 feature on the VLBI maps coincides with the direction toward the optical stellar disc, which can be explained by amplification of enhanced stellar continuum by the H 2 O line.

We analyze our monitoring data for the water-vapor maser in the source W31(2), associated with a region of vigorous star formation, a cluster of OB stars. The monitoring was performed with the 22-m radio telescope at Pushchino Radio Astronomy Observatory during 1981–2004. The variability of the H2O maser in W31(2) was found to be cyclic, with a mean period of 1.9 yr. Two flares were most intense (superflares): in 1985–1986 and 1998–1999. In each activity cycle, we observed up to several short flares, subpeaks. The fluxes of many emission features during the flares were correlated. We also observed successive activation of individual emission features in order of increasing or decreasing radial velocity, suggesting an ordered structure and, hence, a radial-velocity gradient of the medium. There is a clear correlation of the emission peaks of the main components in the spectra at radial velocities of −1.7, −1.3, 0.5, and 1.3 km s−1 with activity cycles and of the emission at VLSR < −8 km s−1 with short flares. During the superflares, the emission in the low-velocity part of the H2O spectrum and a number of other phenomena related to coherent maser-emission properties were suppressed. The maser spots are assumed to form a compact structure, to have a common pumping source, and to be associated with an accretion flow onto the cluster of OB stars.

A catalog of water-vapor maser spectra at 1.35 cm for the sourceW31(2), which is associated with an active star-forming region, is presented. The observations were carried out in 1981–2003 on the 22-m antenna of the Pushchino Radio Astronomy Observatory with a spectral resolution of 0.101 km/s. The mean interval between observations was about 1.5 months. The total velocity range in which emission was observed during the monitoring is from −14 to +14 km/s. The spectrum is strongly variable and contains a large number of emission features. Two strong flares with an interval between their emission maxima (integrated flux) of about 12 years (1985–1986 and 1998–1999) were observed, as well as fast variations on a timescale of 0.5–2 years with amplitudes of up to 600 Jy km/s. No long-period component of the variations was found. A drift of the velocity centroid has been detected; it is well approximated by a third-power polynomial corresponding to a period of about 31–33 years. The two strong flares fall on different phases of this curve: the first (1985–1986) is located near the minimum, while the second (1998–1999) is at the maximum. The observed character of the variability of the emission is well explained by the existing model for the region of G10.6-0.4. The drift of the velocity centroid is probably associated with the nonstationary accretion of material onto an HII region formed by a cluster of OB stars.