Flares of the H<Subscript>2</Subscript>O Maser Emission in the Young Stellar Object GH<Subscript>2</Subscript>O 092.67+03.07 (IRAS 21078+5211)

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.     

A strong flare of the H<Subscript>2</Subscript>O maser in Sagittarius B2(M)

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 V_LSR > 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.     

Evolution of selected components of the H<Subscript>2</Subscript>O maser in Sgr B2

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.     

Chain-type structure in the H<Subscript>2</Subscript>O maser NGC 7538N

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.     

Results of Long-Term Monitoring of Maser Emission in the Star-forming Region G 10.623–0.383

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.     

Evolution of the H<Subscript>2</Subscript>O Maser Emission Zone in ON2 N

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.     

Long-term monitoring and interpretation of flares in the H<Subscript>2</Subscript>O maser emission of IRAS 16293–2422

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.     

Evolution of the H<Subscript>2</Subscript>O and OH Maser Emission in W75 N

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.     

The source of maser emission in W33C (G12.8-0.2)

The results of observations of the H₂O and OH maser sources toward the region of W33C (G12.8-0.2) are reported. The observations were carried out on the 22-m radio telescope of the Pushchino Radio Astronomy Observatory in the 1.35-cm water-vapor line and on the Large Radio Telescope at Nan?ay (France), in the main (1665 and 1667 MHz) and satellite (1612 and 1720 MHz) OH lines. Multiple, strongly variable, short-lived H₂O emission features were detected in a broad interval of radial velocities, from ?7 to 55 km/s. OH maser emission in the 1667-MHz line was detected at velocities of 35?C41 km/s. The Stokes parameters of the maser emission in the main OH lines 1665 and 1667 MHz were measured. Zeeman splitting was detected in the 1665-MHz line at 33.4 and 39.4 km/s, and in the 1667 MHz line only at 39.4 km/s. The magnetic-field intensity was estimated. Appreciable variability of the Zeeman splitting components was observed at 39 and 39.8 km/s in both main lines. The extended spectrum and fast variability of the H₂O maser emission, together with the variability of the Zeeman-splitting components in the main OH lines, may indicate a composite clumpy structure of the molecular cloud and the presence of large-scale rotation, bipolar outflows, and turbulent motions of material in this cloud.     

Masers in the cool molecular cloud L 379

We present the results of monitoring the H₂O masers in the IR sources IRAS 18265-1517 and IRAS 18277-1516 associated with the cool molecular cloud L 379, which contains high-velocity bipolar molecular jets. The sources were observed in the 1.35 cm H₂O line using the 22-m radio telescope of the Pushchino Radio Astronomy Observatory (Russia) during 1991–2004. We detected H₂O maser emission from IRAS 18265-1517 at radial velocities of 17.8 and 18.4 km/s, virtually coincident with the velocity of the molecular cloud derived from CO-line observations (18.4 km/s). The maser emission towards the other source, IRAS 18277-1516, was at higher velocities than the central velocity of the CO molecular cloud. The H₂O maser spots are most likely associated with a redshifted region of CO emission. Cyclic variability of the integrated H₂O maser emission that may be related to cyclic activity of the central star was detected for IRAS 18277-1516. The strongest and most long-lived component (V_LSR ≈ 20.6 km/s) displays a radial-velocity drift, which could be due to deceleration of a dense clump of matter (maser condensation) in the circumstellar medium during the descending branch of a strong flare. We found numerous emission features for both IRAS 18265-1517 and IRAS 18277-1516, providing evidence for fragmentation of the medium surrounding their central objects.     

Twenty-year-long monitoring of the H2O maser S269

Results of observations of the H₂O maser in S269 carried out from October 1980 to February 2001 on the 22-m telescope (RT-22) of the Pushchino Radio Astronomy Observatory are presented. During the monitoring of S269, variability of the integrated flux of the maser emission with a cyclic character and an average period of 5.7 years was observed. This may be connected with cyclic activity of the central star during its formation. Emission at radial velocities of 4–7 km/s was detected. Thus, the H₂O maser emission in S269 extends from 4 to 22 km/s, and is concentrated in three radial-velocity intervals: 4–7, 11–14, and 14–22 km/s. In some time intervals, the main group of emission features (14–22 km/s) had a triplet structure. The central velocity of the total spectrum is close to the velocity of the CO molecular cloud and HII region, differing from it by an amount that is within the probable dispersion of the turbulent gas velocities in the core of the CO molecular cloud. A radial-velocity drift of the component at V _LSR≈20 km/s with a period of ≈26 years has been detected. This drift is likely due to turbulent (vortical) motions of material.     

An H<Subscript>2</Subscript>O maser in the infrared source IRAS 20126+4104

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.     

Observations of Extended Green Objects in the 1.35-cm H<Subscript>2</Subscript>O Line on the 22-m Pushchino Radio Telescope

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 maser source S140-H2O as a protoplanetary disk

We present the observations of the H₂O maser in S140 with the 22-m radio telescope of the Pushchino Radio Astronomy Observatory in 1992–1999. The H₂O maser emission is mainly concentrated in three symmetrically placed narrow radial-velocity intervals. In contrast to our earlier observations (1981–1991), we did not detect emission simultaneously in all three intervals; instead, the emission appeared successively in each. We discuss possible origins for this behavior. To explain the flux variability and radial-velocity drift of the main components, we propose a model with Keplerian orbital motion of clumps (protoplanets) and calculate their orbital parameters.     

Maser emission toward the very young massive star GH2O 092.67+03.07 (IRAS 21078+5211)

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.     

The fast variable maser source IRAS 05338-0624

A new OH maser was detected in January 2008 toward the infrared source IRAS 05338-0624 in the dark cloud L1641N. The observations were carried out on the Nan cay Radio Telescope (France) in the 1667 and 1665 MHz OH lines. In the spectra of both lines, thermal OH emission from the surrounding molecular cloud is present at radial velocities V _LSR = 6–9 km/s. In addition, a narrow maser feature is present in both lines at V _LSR = 2 km/s in the profiles obtained on January 7, 2008; the peak flux densities at 1667 and 1665 MHz are 1.5 and 0.4 Jy, respectively. No OH maser emission was detected in February–July 2008. Then, a maser feature was again observed in the 1665 MHz line on August 20, 2008, at the same velocity as in January, V _LSR = 2 km/s, with a peak flux density of 0.4 Jy. No 1667 MHz counterpart was observed with an upper limit of ～0.1 Jy. Emission in both OH lines was again absent on September 18. The source was also observed in the H₂O line at λ = 1.35 cm on the 22-m radio telescope of the Pushchino Radio Astronomy Observatory (Russia) on February 7 and 13, 2008. In both cases, a maser feature was detected at V _LSR = 9 km/s, with peak flux densities of 35 and 15 Jy, respectively. After the its apparent absence in April, H₂O maser emission reappeared on May 14, 2008, at V _LSR = 7 km/s with a flux density of about 15 Jy. The history of previous observations of the object in the OH and H₂O lines is traced. The maser displays strong and rapid flux variability in the lines of both molecules, as is typical of young low-luminosity stellar objects at early stages of their evolution.     

Long-term monitoring of the W44C (G 34.3+0.15) maser in the 1.35 cm water vapor and 18 cm hydroxyl lines

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 Nançay 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).     

Variability of the H2O maser emission in S255

The paper reports the results of observations of the H₂O maser in S255 carried out in 1982–1985 and 1990–2000 on the 22-meter telescope of the Pushchino Radio Astronomy Observatory. The H₂O maser emission extends from ?2 to 14 km/s and is mainly concentrated in three radial-velocity intervals. The velocity of the central group of emission features coincides with that of the molecular cloud, while the two lateral groups (blueshifted and redshifted) are positioned in the spectrum more or less symmetrically relative to the central feature. During the monitoring of S255, two phenomena were observed. First, the integrated flux of the H₂O maser emission varied in a cyclic manner with a period of two to four years; this may be connected with activity of the protostar. Second, the fluxes of emission features (or groups of features) were anticorrelated. The emission of the three groups of features noted above dominated in succession. In some time intervals, a triplet spectral structure with anticorrelation between the fluxes of the lateral components and of the central and lateral components was observed. The flux anticorrelation between groups of features and individual features could be due to competition between spatial emission pumping modes in a nonuniform Keplerian disk.     

Variability of the spectrum and spatial structure of the H<Subscript>2</Subscript>O maser in W75N-VLA 1

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.     

Superflares of H<Subscript>2</Subscript>O Maser Emission Toward the Protostellar Object G25.65+1.05 (IRAS 18316−0602)

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.