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.     

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.     

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.     

Flares of H2O maser emission from SGR B2 in 2005–2012

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.     

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.     

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.     

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.     

Triplet spectra of H<Subscript>2</Subscript>O masers and protoplanetary disks

An analysis of the H₂O maser emission associated with protoplanetary disks is presented. Triplet H₂O spectra can be formed at certain stages in the evolution of Keplerian disks. The dependence of the mass of the central star in the Keplerian disk on the disk radius is derived. The calculations are based on the distribution of the water-vapor molecules (maser spots). In S140, the observed elongated maser spots (chains) with a smoothly varying line-of-sight velocity are interpreted as protoplanetary structures with a small intrinsic rotation.     

The H<Subscript>2</Subscript>O supermaser region in Orion KL: Epoch 1982.9

Radio interferometric observations of an H₂O maser flare in the Orion Nebula at epoch 1982.9 have been used to determine the flare’s spatial structure. Antennas in the Crimea, Effelsberg, and Onsala were used. The emission region consists of three groups of components. The angular sizes of the components are 0.2–0.9 mas, and the widths of the emitted lines are 0.2–0.7 km/s. The velocities of the components are correlated with their relative positions, which correspond to expanding concentric rings. Assuming a 1 M_⊙ protostar in a Keplerian approximation, the radius of the inner ring R is 15 AU, the velocity of its rotation V_rot is 8.98 km/s, and the radial component of the velocity V_rad is 1.79 km/s. For the outer ring, R=15.7 AU, V_rot=8.79 km/s, and V_rad=2.61 km/s.     

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.     

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.     

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.     

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.     

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.     

H<Subscript>2</Subscript>O maser emission in the direction of Sagittarius B2: Results of monitoring for 1982–1992

The results of monitoring the water-vapor maser at λ=1.35 cm in Sgr B2 are presented. The observations were carried out on the 22-m radio telescope of the Pushchino Radio Astronomy Observatory (Russia) in 1982–1992. A strong flare of the maser radiation associated with Sgr B2(N) was detected in this period. The absolute strength of this flare is comparable to the megamaser emission observed in Orion in 1979–1987. The flare is probably due to a strengthening of the flow of material from the rotating accretion disk, in which are embedded the three ultracompact HII regions K1, K2, and K3. A subsequent excitation of emission features at increasingly higher radial velocities was observed, associated with a gradient of V_LSR along the direction of the outflow. The large width of the lines (>0.86 km/s) could reflect a complex structure for the maser spots, such as a chain or filamentary structure, as has been observed in Orion and S140.     

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.     

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.     

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.     

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.     

Cyclic activity of the water-vapor maser in S128

The results of observations of the S128 H₂O maser carried out from February 1995 to March 2001 on the 22-m radio telescope of the Pushchino Radio Astronomy Observatory are presented. Two activity cycles of the H₂O maser with a period of about 10 years were observed during the total monitoring interval (1981–2001). This may be connected either with cyclic activity of the central star in S128 during its formation or with the influence on the H₂O masering region of shocks arising near an ionization front at the interface of two colliding CO clouds. The emission at radial velocities from ?73 to ?70 km/s consists of four emission features. The emission feature at ?71.8 km/s exhibits a flux dependence on linewidth that is typical of an unsaturated maser.