Water Vapor Masers Research Articles

H2O 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.

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.

A search for extragalactic H2O maser emission towards IRAS galaxies - II. Discovery of an H2O maser in the type 1 Seyfert galaxy NGC 4051

Water vapor maser emission in the 6_{16}-5_{23} transition towards the narrow-line Seyfert 1 (NLS1) galaxy NGC 4051 has been discovered during an ongoing single-dish extragalactic water maser survey. The Doppler-shifted maser components appear to bracket maser components lying near the systemic velocity of the galaxy symmetrically. The tentative result of a Very Large Array (VLA) snapshot observation is that the masers are confined within 0.1 arcsec (5 pc at a distance D = 9.7 Mpc) of the radio continuum peak seen at 8.4 GHz. The low luminosity of the maser (~2 Lsun) is not typical for masers that coincide with the radio continuum nucleus and appear associated with Active Galactic Nucleus (AGN) activity. A low-luminosity maser in a Type 1 Seyfert nucleus could be explained by a low maser gain resulting from the lower inclination of an obscuring disk around an active nucleus.

Water and OH Maser Emission from the Planetary Nebula K3-35

Water-vapour masers, typical of the envelopes in giant stars, are not expected to persist in planetary nebulae due to the ultraviolet radiation of the remnant star that progressively destroys the molecules. Recently, we have reported the first unambiguous detection of water maser emission in a planetary nebula, K 3–35 (Miranda et al. 2001). The water masers in K3–35 were detected at the center of the nebula, along the minor axis, at a radius of ~85 AU and also at the surprisingly large distance of 5000 AU from the star, at the tips of the bipolar lobes. The existence of these water molecules is puzzling, and probably we are observing the very moment of transformation of a giant star into a planetary nebula. Miranda et al. (2001) also report the presence of polarization in the OH 1665 MHz masers, which are distributed towards the central star in a torus-like structure. Here we review the main results on this source.

Detection of water vapor maser in a narrow-line Seyfert 1 nucleus

We have discovered 22 GHz water vapor maser toward a narrow-line type 1 (NLS1) Seyfert galaxy, NGC4051. The detection of water masers in such galaxies is very rare. The Doppler-shifted maser components symmetrically straddle a component lying at about 712 km s -1 . The location of each maser component is of great interest for studying galactic dynamical structures around an active nucleus. We report on preliminary results of VLA observations.

A collimated jet of molecular gas from a star on the asymptotic giant branch.

Evolved stars of about one solar mass are in general spherically symmetric, yet the planetary nebulae that they produce in the next phase of their evolution tend not to exhibit such symmetry. Collimated 'jets' and outflows of material have been observed up to approximately 0.3 parsec from the central stars of planetary nebulae, and precession of those jets has been proposed to explain the observed asymmetries. Moreover, it has recently been shown theoretically that magnetic fields could launch and collimate such jets. Here we report the detection of a collimated and precessing jet of molecular gas that is traced by water-vapour maser spots approximately 500 astronomical units (au) from the star W43A in Aquila. We conclude that the jet is formed in the immediate vicinity of the star, and infer that elongated planetary nebulae are formed by jets during the short period, of less than 1,000 years, when the star makes its transition through the proto-planetary nebula phase to become a planetary nebula.

Water vapor masers in stars departing from the AGB

The 22 GHz water vapor masers of four \non-variable OH/IR stars considered to be young Proto- Planetary Nebulae (PPNs) were monitored for more than 10 years. In two of them, OH 15.7+0.8 and OH 17.7{2.0 having Mira-like line proles, the maser flux decreased and the maser nally disappeared. The decrease of the maser brightness is spectacular for OH 17.7{2.0 in particular. The maser had a flux density of several hundred Jansky in the eighties and is now absent since 1990. The masers of OH 15.7+0.8 and OH 17.7{2.0 probably emerged in the \dying AGB wind during the mass loss reduction process accompanying the departure from the AGB. The other two, OH 12.8{0.9 and OH 37.1{0.8, have double-peaked proles with a considerably larger velocity spread than the OH masers in these sources. The velocities are incompatible with the spherical symmetric mass loss process on the AGB and give evidence for the presence of probably bipolar outflows with a projected velocity of ve 28 km s 1 during the very early PPN phase. The gross structure of their proles was rather stable during the monitor period while individual maser lines appeared and disappeared. An increase of the velocity range was found in both sources implying that the outflow is accelerating. Analysis of the prole variations of all four objects suggests a lifetime of individual maser components of 1{3 years. There is no evidence for ordered motion traced by the masers. The H2O maser properties of all these objects are strongly dierent from those of ordinary OH/IR stars, allowing to nd new young PPNs among optically still hidden objects by looking for irregular maser proles. In contrast to current denitions in the literature \non-variable OH/IR stars are proposed to be classied as (young) PPNs. It is argued that beyond the end of the AGB, maser properties cannot be used to infer the evolutionary stage of the objects in transition to Planetary Nebulae.

Detection of Water Maser Flare in the Seyfert/LINER, NGC 6240

Abstract We report on a flare-up of water-vapor maser emission in a prototypical ultraluminous FIR galaxy with an AGN, NGC 6240. The flux density of the detected maser line was $ \approx 0.1 \,\mathrm{Jy}$ with $ \mathit{FWHM} \approx 5 \,\mathrm{km} \,\mathrm{s}^{-1}$, indicating an apparent isotropic luminosity of $\approx 100 \,{L}_{\odot}$ at a distance of 100 Mpc, which is typical for water megamasers. The maser line was redshifted by $ \approx 200 \,\mathrm{km} \,\mathrm{s}^{-1}$ relative to the systemic velocity of the galaxy, while no systemic-velocity feature was detected. The previous possible detection or observed upper limits were at a level lower than the flux density in the present paper, indicating that the water maser in NGC 6240 flared up by more than a factor of 5 in 2001.

Long-term study of water maser emission associated with young stellar objects

We present the results of more than 10 years of monitoring of the water vapor maser emission in 14 star forming regions obtained with the Medicina 32-m radiotelescope. The sample of objects covers a large range of luminosities of the associated FIR sources. In order to present in a compact and representative way the large amount of data available, we give for each source: a time-velocity-flux density plot, the time dependent integrated flux, the lower and upper envelopes of the maser emission, the frequency of the maser occurrence as a function of velocity, and the mean velocity averaged over the observing period. A brief morphological description of the environment of the maser source is also given. The present data form the basis for a discussion of the main properties of the water vapor maser emission to be presented in a forthcoming paper.

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.

VLBI Monitoring Observations of Water Masers around the Semi-Regular Variable Star R Crateris

Abstract We monitored water-vapor masers around the semi-regular variable star R Crateris with the Japanese VLBI Network (J-Net) at the 22 GHz band during four epochs with intervals of one month. The relative proper motions and Doppler-velocity drifts of twelve maser features were measured. Most of them existed for longer than 80 days. The 3-D kinematics of the features indicates a bipolar expanding flow. The major axis of the asymmetric flow was estimated to be at P.A.$= 136^{\circ}$. The existence of a bipolar outflow suggests that a Mira variable star had already formed a bipolar outflow. The water masers are in a region of apparent minimum radii of $1.3 \times 10^{12} \,\mathrm{m}$ and maximum radii of $2.6 \times 10^{12} \,\mathrm{m}$, between which the expansion velocity ranges from 4.3 to $7.4 \,\mathrm{km} \,\mathrm{s}^{-1}$. These values suggest that the water masers are radially accelerated, but still gravitationally bound, in the water-maser region. The most positive and negative velocity-drifting features were found relatively close to the systemic velocity of the star. We found that the blue-shifted features are apparently accelerated and the red-shifted apparently decelerated. The acceleration of only the blue-shifted features seems to be consistent with that of the expanding flow from the star.

Water-maser emission from a planetary nebula with a magnetized torus.

A star like the Sun becomes a planetary nebula towards the end of its life, when the envelope ejected during the earlier giant phase becomes photoionized as the surface of the remnant star reaches a temperature of approximately 30,000 K. The spherical symmetry of the giant phase is lost in the transition to a planetary nebula, when non-spherical shells and powerful jets develop. Molecules that were present in the giant envelope are progressively destroyed by the radiation. The water-vapour masers that are typical of the giant envelopes therefore are not expected to persist in planetary nebulae. Here we report the detection of water-maser emission from the planetary nebula K3-35. The masers are in a magnetized torus with a radius of about 85 astronomical units and are also found at the surprisingly large distance of about 5,000 astronomical units from the star, in the tips of bipolar lobes of gas. The precessing jets from K3-35 are probably involved in the excitation of the distant masers, although their existence is nevertheless puzzling. We infer that K3-35 is being observed at the very moment of its transformation from a giant star to a planetary nebula.

ChandraObservations and the Nature of the Anomalous Arms of NGC 4258 (M106)

This paper presents high-resolution X-ray observations with Chandra of NGC 4258 and infers the nature of the so-called in this galaxy. The anomalous arms dominate the X-ray image; diffuse X-ray emission from the plateau regions, seen in radio and Hα imaging, is also found. X-ray spectra have been obtained at various locations along the anomalous arms and are well described by thermal (MEKAL) models with kT in the range 0.37-0.6 keV. The previously known kiloparsec-scale radio jets are surrounded by cocoons of X-ray emitting gas for the first 350 pc of their length. The radio jets, seen in previous VLBA and VLA observations, propagate perpendicular to the compact nuclear gas disk (imaged in water vapor maser emission). The angle between the jets and the rotation axis of the galactic disk is 60°. The jets shock the normal interstellar gas along the first 350 pc of their length, causing the hot, X-ray emitting cocoons noted above. At a height of z = 175 pc from the disk plane, the jets exit the normal gas disk and then propagate though the low-density halo until they reach hot spots (at 870 pc and 1.7 kpc from the nucleus), which are seen in radio, optical line, and X-ray emission. These jets must drive mass motions into the low-density halo gas. This high-velocity halo gas impacts on the dense galactic gas disk and shock heats it along and around a of damage, which is the projection of the jets onto the galactic gas disk as viewed down the galaxy disk rotation axis. However, because NGC 4258 is highly inclined (i = 64°), the line of damage projects on the sky in a different direction from the jets themselves. We calculate the expected P.A. of the line of damage on the sky and find that it coincides with the anomalous arms to within 2°. It is therefore proposed that the anomalous arms, which are known to lie in the galactic disk, represent disk gas that has been shocked by mass motions driven by the out-of-plane radio jets. In the inner ( 1 kpc), some of the disk gas is blown out of the disk plane toward the halo on the opposite side of the disk from the relevant radio jet; this effect causes the arms to curve and is also responsible for the so-called plateau emission. An alternative is that the jet directions were different in the past, so they projected onto the disk in different directions. Our picture accounts for: (1) the diffuse character of the anomalous arms, (2) the inferred ionization of the optical line-emitting gas by shock waves, (3) the angle between the anomalous arms and the radio jets, (4) the sharp brightness gradients along the outer edges of the anomalous arms (these edges represent the standing shock where the high velocity, jet-driven halo gas meets the disk), (5) the existence of the plateaus, and (6) the wide range of radial velocities observed in the plateaus.

Chandra Observations and the Nature of the Anomalous Arms of NGC 4258 (M106)

This paper presents high resolution X-ray observations with Chandra of NGC 4258 and infers the nature of the so called ``anomalous arms'' in this galaxy. The anomalous arms dominate the X-ray image; diffuse X-ray emission from the ``plateaux'' regions, seen in radio and H$\alpha$ imaging, is also found. X-ray spectra have been obtained at various locations along the anomalous arms and are well described by thermal (mekal) models with kT in the range 0.37 - 0.6 keV. The previously known kpc-scale radio jets are surrounded by cocoons of hot X-ray emitting gas for the first 350 pc of their length. The radio jets, seen in previous VLBA and VLA observations, propagate perpendicular to the compact nuclear gas disk (imaged in water vapor maser emission). The angle between the jets and the rotation axis of the galactic disk is 60$^{\circ}$. The jets shock the normal interstellar gas along the first 350 pc of their length, causing the hot, X-ray emitting cocoons noted above. At a height of z = 175 pc from the disk plane, the jets exit the normal gas disk and then propagate though the low density halo until they reach ``hot spots'' (at 870 pc and 1.7 kpc from the nucleus), which are seen in radio, optical line and X-ray emission. These jets must drive mass motions into the low density halo gas. This high velocity halo gas impacts on the dense galactic gas disk and shock heats it along and around a ``line of damage'', which is the projection of the jets onto the galactic gas disk as viewed down the galaxy disk rotation axis. However, because NGC 4258 is highly inclined ($i$ = 64$^{\circ}$), the ``line of damage'' projects on the sky in a different direction to the jets themselves. We calculate the expected p.a. of the ``line of damage'' on the sky and find that it coincides with the anomalous arms to within 2$^{\circ}$. (Abstract truncated).

Spherical episodic ejection of material from a young star

The exact processes by which interstellar matter condenses to form young stars are of great interest, in part because they bear on the formation of planets like our own from the material that fails to become part of the star. Theoretical models suggest that ejection of gas during early phases of stellar evolution is a key mechanism for removing excess angular momentum, thereby allowing material to drift inwards towards the star through an accretion disk. Such ejections also limit the mass that can be accumulated by the stellar core. To date, these ejections have been observed to be bipolar and highly collimated, in agreement with theory. Here we report observations at very high angular resolution of the proper motions of an arc of water-vapour masers near a very young, massive star in Cepheus. We find that the arc of masers can be fitted to a circle with an accuracy of one part in a thousand, and that the structure is expanding. Only a sphere will always produce a circle in projection, so our observations strongly suggest that the perfectly spherical ejection of material from this star took place about 33 years earlier. The spherical symmetry of the ejecta and its episodic nature are very surprising in the light of present theories.

High–resolution maser studies of galactic nuclei

Water vapour masers are proving to be remarkably useful probes of the neutral accretion discs surrounding black holes at radii of a few tenths of a parsec. This paper describes the observational results on NGC 4258, including measurements of the magnetic field (less than 300 mG), the accelerations of the high–velocity features (−0.8 to 0.4 km s−1yr−1), and the distance (7.2±0.5 Mpc). The status of measurements of other masers with resolved structure is also described.

Optical and Radio Monitoring of a Sample of Late-Type Variables

AbstractResults of regular observations of about 20 Miras and semiregular variables (∘ Cet, R Leo, R Aql, U Her, χ Cyg, R Tri, and others) in 1994–1999 are presented. The monitoring includes optical spectroscopy, optical (UBV) and infrared (JHKLM) photometry, high-resolution radio spectroscopy in the 1.35-cm water-vapour maser line (on the 22-m radio telescope in Pushchino). Optical spectroscopy shows that the appearance of strong Balmer emission lines in Miras may be a rare event, and the presumed shock waves are thus weak. The H2O maser flux generally correlates with visual light curves of the stars with some delay Δφ = 0.1 – 0.4P (P is the variability period). For R Leo, R Cas, and U Aur strong flares in the Hφ line, observed in 1997–1998, were followed by corresponding outbursts of the H2O maser emission about 2P later. This sequence of events is probably caused by a shock wave propagating outward in the stellar atmosphere and consecutively initiating the Hα emission and, later, the maser flare at a larger distance from the stellar surface.

Determining the period of the long-period activity of the water-vapor maser in W75N

We present the results of our study of the H2O maser emission from the source W75N, which is associated with a star-forming region, between November 1994 and March 1999. The observations were carried out with the RT-22 radio telescope of the Pushchino Radio Astronomy Observatory (Lebedev Physical Institute). The maser emission in 1994–1999 can be represented as a superposition of flares of separate components with a duration from two to six months, which occurred mainly in the radial-velocity range 8–17.5 km s−1. We detected a regular drift of the velocity centroid from 13 to 9 km s−1 and an abrupt change in its velocity from 9 to 5 km s−1, which took place at the initial stage of maser activity. Based on the variability of the total H2O flux in all years of our observations of W75N (from December 1979 through March 1999), we conclude that the long-period variability of the water-vapor maser emission has a period of ∼11.5 years. We give arguments that this variability is mainly associated with the most compact group of maser spots, whose positions coincide with the position of the continuum source VLA 2.

Observational evidence for massive black holes in the centers of active galaxies

Naturally occurring water vapor maser emission at 1.35 cm wavelength provides an accurate probe for the study of accretion disks around highly compact objects, thought to be black holes, in the centers of active galaxies. Because of the exceptionally fine angular resolution, 200 microarcseconds, obtainable with very long baseline interferometry, accompanied by high spectral resolution, <0.1 km s-1, the dynamics and structures of these disks can be probed with exceptional clarity. The data on the galaxy NGC 4258 are discussed here in detail. The mass of the black hole binding the accretion disk is 3·9 × 107 M⊙. Although the accretion disk has a rotational period of about 800 years, the physical motions of the masers have been directly measured with VLBI over a period of a few years. These measurements also allow the distance from the earth to the black hole to be estimated to an accuracy of 4 per cent. The status of the search for other maser/black hole candidates is also discussed.

Observation of water masers and a study of their time variation

Using the 25-m telescope of Shanghai Observatory, Joint Radio Astronomy Laboratory, 19 known maser sources and 20 candidates in H II regions were surveyed for water vapour maser emission at 22.2 GHz. 21 sources were detected, of which two are for the first time. Combining with some previous data, the time variations in the flux and velocity are obtained and discussed. Time variation in the peak flux density is common and is of two types, a change in the intensity of the same peak, and a swapping-over of two peaks. The corresponding IRAS sources were identified and the relation between the maser emission and the infrared parameters discussed. It is shown that maser production rate is more sensitive to the infrared flux intensity than the infrared colour distribution.