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.     

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 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.     

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.     

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.     

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.     

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.     

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.     

The evolution of H<Subscript>2</Subscript>O maser spots in star-forming regions

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.     

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.     

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.     

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.     

Evolution of the H2O Maser Emission in the Star-Forming Region S252A

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.

     

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.     

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.     

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.     

Polarization observations of circumstellar OH masers

Results of observations of circumstellar OH masers in lines with wavelengths near 18 cm are reported. The observations were carried out on the radio telescope of the Nan cay Radio Astronomy Observatory (France). In 2007–2009, 70 late-type stars were observed (including Mira and semi-regular variables). For 53 of these, emission was detected in at least one of three OH lines (1612, 1665, or 1667 MHz). Circular and linear polarization of the maser emission was measured, yielding all four Stokes parameters. Polarized emission features were detected in the OH line spectra of 41 stars. A summary of all the observations is given. The results obtained for T Lep, R LMi, and R Crt are discussed. Emission in the 1665 and 1667 MHz OH lines was detected in T Lep for the first time. Features probably due to Zeeman splitting were detected in the OH line profiles of all three stars. Estimates of the magnetic-field strengths in the maser sources were obtained (0.46–2.32 mG). Variability of the polarization characteristics of the maser emission of the stars on time intervals of several months was found.     

Pulsations of the water-vapor maser in NGC 7538 IRS 1 with a period of about 0.9 year

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.     

Evolution of Maser Emission in the Region of Active Star Formation G43.8–0.1. I. OH Maser Emission at 18 cm

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.

     

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.