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.     

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.     

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.     

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.     

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.     

Observations of OH and H<Subscript>2</Subscript>O Maser Emission in the Star-Forming Region S128

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.     

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.     

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).     

Cyclic activity of the H2O maser emission towards NGC 2071

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.     

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.     

Evolution of the H2O maser emission in G10.6-0.4

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.     

Observations of late-type variable stars in the water vapor radio line. The long-period variable R cassiopeia

Observations of circumstellar maser emission from the long-period variable R Cas in the 1.35-cm water-vapor line are reported. The observations were carried out on the 22-m radio telescope of the Pushchino Radio Astronomy Observatory in 1980–2003 (JD=2444409–2452724). Over the 23 years of observations, strong flares in the H₂O line profile were recorded in 1982 (with a peak flux density up to 400 Jy) and 1986–1989 (up to 750 Jy). Subsequently, from 1990 to March 2003, the H₂O line flux was usually below the detection threshold of the radio telescope (<5–10 Jy). Episodic small increases of the emission with peak flux densities of 20–60 Jy were observed. The variations of the H₂O line flux F are correlated with variations in the visual brightness of the star. The phase delay Δ_γ of the F variations relative to the optical light curve of R Cas ranged from 0.2–0.3P during the observations (P=430.46^d is the star's period). A model for the variability of the H₂O maser in R Cas is discussed. If the variations are due to periodic impacts by shock waves driven by the stellar pulsations, the time for the shock to travel from the photosphere to the inner boundary of the H₂O-masing shell may reach 2–4P. The flares could be due to transient episodes of enhanced mass loss by the star or to the propagation of an exceptionally strong shock from the stellar surface.     

Variability of the OH and H<Subscript>2</Subscript>O maser emission toward AS 501

The results of observations of OH (λ = 18 cm) and H₂O (λ = 1.35 cm) masers toward AS 501 obtained with the Nançay 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.     

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.     

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 of the water-vapor maser in NGC 7538: 1981–1992

We report the results of monitoring the H₂O maser in NGC 7538, which is associated with a star-forming region. The observations were carried out on the 22-meter telescope of the Pushchino Radio Astronomy Observatory. Two intervals of long-term variability of the integrated flux that reflect the cyclic activity of the maser have been distinguished (1981–1992 and 1993–2003); the data for the earlier activity cycle, 1981–1992, have been analyzed. The period of the long-time-scale variations is about 13–14 years. Flares of individual spectral features and of two groups of features with mean radial velocities of ?60 and ?46.6 km/s have been observed. The flares lasted from 0.3 to 1 year. The emission features observed during the 1984–1985 flare at radial velocities between ?62 and ?58 km/s probably form a spatially compact group of spots (<10¹⁵ cm) in NGC 7538 IRS 1. The triplet structure of the spectra can be traced. The observed anticorrelations and correlations of the fluxes of the triplet components suggest that the maser spots may be located either in a protoplanetary disk or in a high-velocity gaseous outflow.     

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.     

H<Subscript>2</Subscript>O masers and protoplanetary disk dynamics in IC 1396 N

We report H₂O maser line observations of the bright-rimmed globule IC 1396 N using a ground-space interferometer with the 10-m RadioAstron radio telescope as the space-based element. The source was not detected on projected baselines >2.3. Earth diameters, which indicates a lower limit on the maser size of L > 0.03 AU and an upper limit on the brightness temperature of 6.25 × 10¹² K. Fringe-rate maps are prepared based on data from ground-ground baselines. Positions, velocities and flux densities of maser spots were determined. Multiple low-velocity features from ?4.5 km/s to +0.7 km/s are seen, and two high-velocity features of V _LSR = ?9.4 km/s and V _LSR = +4.4 km/s are found at projected distances of 157 AU and 70 AU, respectively, from the strongest low-velocity feature at V _LSR = ~+0.3 km/s. Maser components from the central part of the spectrum fall into four velocity groups but into three spatial groups. Three spatial groups of low-velocity features detected in the 2014 observations are arranged in a linear structure about ~200 AU in length. Two of these groups were not detected in 1996 and possibly are jets which formed between 1996 and 2014. The putative jet seems to have changed direction in 18 years, which we explain by the precession of the jet under the influence of the gravity of material surrounding the globule. The jet collimation can be provided by a circumstellar protoplanetary disk. There is a straight line orientation in the “V _LSR-Right Ascension” diagram between the jet and the maser group at V _LSR = ~+0.3 km/s. However, the central group with the same position but at the velocity V _LSR ~ ?3.4 km/s falls on a straight line between two high-velocity components detected in 2014. Comparison of the low-velocity positions from 2014 and 1996, based on the same V _LSR-Right Ascension diagram for low-velocity features, shows that the majority of the masers maintain their positions near the central velocity V _LSR = ~0.3 km/s during the 18 year period.     

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.     

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.