H2O Masers In Regions Research Articles

Investigating Asymmetry Development from SiO to H2O Maser Regions in VX Sagittarii

Simultaneous very-long-baseline interferometry monitoring observations of H2O and SiO masers toward VX Sagittarii were conducted from 2014 February to 2019 January. Thirty epochs of observations revealed that the H2O and SiO masers had asymmetric and ring-like structures, respectively. However, from 2017 September to 2018 March, the SiO maser transformed from a ring-like structure to a northeast–southwest (NE–SW) extension, and the 43.1 and 86.2 GHz SiO maser components had velocities of 39.48 and 10.65 km s−1 in the NE–SW direction, suggesting a possible localized strong shock wave. The H2O maser had a double-sided structure oriented in the NE–SW direction with near-stellar velocity components, which aligned with the extended direction of the SiO maser. The nonregular optical brightness and maser intensity variations were speculated to be related to the morphological evolution of the SiO maser. During the stable states attained by regular pulsations, the SiO maser region was presumed to experience radial acceleration, which reverted the SiO maser to a ring-like structure. However, the H2O maser region, where the acceleration almost terminates, retained its asymmetric morphology due to the prior influence of external forces. The results suggest that substantial energy transfer can alter the dynamics of the SiO maser and surrounding atmosphere, leading to an asymmetric distribution in the H2O maser region.

Simultaneous Time-monitoring Observations of H2O and SiO Masers toward the Supergiant VX Sagittarii

We performed simultaneous monitoring observations of the 22.2 GHz H2O and 43.1/42.8/86.2/129.3 GHz SiO masers toward the red supergiant VX Sagittarii using the Korean VLBI Network single-dish telescopes. The observations were conducted about every 2 months from 2013 May to 2019 January (30 epochs in total). They included four optical maxima in the active phase of the optical pulsation cycles. The line profile of a H2O maser always comprised various velocity components with a wider velocity range and varied from highly redshifted to blueshifted velocities with respect to the stellar velocity, in contrast to those of the SiO masers. We examined the relation between peak intensities and velocities of 11 detailed components in the line profile of the H2O maser and the pulsation phases. The peak intensity of each component generally exhibited a better correlation with the pulsation phases than that of total intensity. The peak velocities of several components gradually decreased or increased with respect to the stellar velocity, implying an accelerating motion and the development of asymmetries in the H2O maser region. The characteristics of four transition SiO maser properties were compared according to the stellar pulsation phases. The intensity and velocity variation trend of the 43.1 GHz SiO maser was similar to that of the 42.8 GHz SiO maser. However, the variation trend of the 43.1 and 42.8 GHz SiO masers was different from that of the 86.2 and 129.3 GHz SiO masers. This difference stems from the different location of each maser reflecting a different excitation condition.

Rapidly evolving episodic outflow in IRAS 18113−2503: clues to the ejection mechanism of the fastest water fountain

ABSTRACT Water fountains are evolved stars showing early stages of collimated mass-loss during transition from the asymptotic giant branch, providing valuable insight into the formation of asymmetric planetary nebulae. We report the results of multi-epoch VLBI observations, which determine the spatial and three-dimensional kinematic structure of H2O masers associated with the water fountain IRAS 18113−2503. The masers trace three pairs of high-velocity (∼150–300 km s−1) bipolar bow shocks on a scale of 0${^{\prime\prime}_{.}}$18 (∼2000 au). The expansion velocities of the bow shocks exhibit an exponential decrease as a function of distance from the central star, which can be explained by an episodic, jet-driven outflow decelerating due to drag forces in a circumstellar envelope. Using our model, we estimate an initial ejection velocity ∼840 km s−1, a period for the ejections ∼10 yr, with the youngest being ∼12 yr old, and an average envelope density within the H2O maser region $n_{\text{H}_2}{\approx }10^{6}$ cm−3. We hypothesize that IRAS 18113−2503 hosts a binary central star with a separation of ∼10 au, revealing novel clues about the launching mechanisms of high-velocity collimated outflows in water fountains.

Flares of the H2O Maser Emission in the Young Stellar Object GH2O 092.67+03.07 (IRAS 21078+5211)

The results of monitoring the H2O maser observed toward the region GH2O 092.67+03.07 (IRAS 21078+5211) located in the Giant Molecular Cloud Cygnus OB7 are presented. The observations were carried out with the 22-m radio telescope of the Pushchino Radio Astronomy Observatory in 2006–2017. Strong flares of the H2O maser emission with flux densities up to 19 800 Jy were detected. The flares exhibited both global (over the source) and local characters. All the flares were accompanied by strong variations in the H2O spectra within the corresponding radial-velocity ranges. Individual H2O components form both compact clusters and chains 1–2-AU long. Analysis of the variations of the fluxes, radial velocities, and line shapes of features during the flares showed that the medium may be strongly fragmented, with small-scale turbulent motions taking place in the H2Omaser region.

A study on evolved stars by simultaneous observations of H2O and SiO masers using KVN

AbstractThe Korean VLBI Network (KVN) is a unique millimeter VLBI system which is consisted of three 21 m telescopes with relatively short baselines. We present the preliminary results of simultaneous monitoring observations of the 22.2 GHz H2O and 43.1/42.8/86.2/129.3 GHz SiO masers based on the KVN Key Science Project (KSP). We obtained the astrometrically registered maps of the H2O and SiO masers toward nine evolved stars using the source frequency phase referencing method (SFPR). The SFPR maps of the H2O and SiO masers enabled us to investigate the spatial structure and kinematics from the SiO to H2O maser regions including the development of an outward motion from the ring-like or elliptical structures of SiO masers to the asymmetric structures of the 22.2 GHz H2O maser features. In particular, the 86.2/129.3 GHz SiO (v=1, J=2–1 and J=3–2) masers were clearly imaged toward several objects for the first time. The SiO v=1, J=3–2 maser shows different distributions compared to those of the SiO v=1, 2, J=1–0 and v=1, J=2–1 masers implying a different physical condition.

Water Maser Emission Around Low/Intermediate Mass Evolved Stars

AbstractWe present results of Very Long Baseline Array (VLBA) polarimetric 22 GHz H2O maser observations of a number of low/intermediate mass evolved stars. We observed 3 Miras (Ap Lyn, IK Tau and IRC+60370), 1 semi-regular variable (RT Vir) and 1 pPN (OH231.8+4.2). Circular polarization is detected in the H2O maser region of OH231.8+4.2 and we infer a magnetic field of |B||| = ~45 mG. This implies an extrapolated magnetic field of ~2.5 G on the surface of the central star. The preliminary results on RT Vir and IRC+60370 also indicate the first detection of weak H2O maser linear polarization.

Water maser polarization of the water fountains IRAS 15445−5449 and IRAS 18043−2116

We present the morphology and linear polarization of the 22-GHz H2O masers in the high-velocity outflow of two post-AGB sources, d46 (IRAS 15445-5449) and b292 (IRAS 18043-2116). The observations were performed using The Australia Telescope Compact Array. Different levels of saturated maser emission have been detected for both sources. We also present the mid-infrared image of d46 overlaid with the distribution of the maser features that we have observed in the red-shifted lobe of the bipolar structure. The relative position of the observed masers and a previous radio continuum observation suggests that the continnum is produced along the blue-shifted lobe of the jet. It is likely due to synchrontron radiation, implying the presence of a strong magnetic field in the jet. The fractional polarization levels measured for the maser features of d46 indicate that the polarization vectors are tracing the poloidal component of the magnetic field in the emitting region. For the H2O masers of b292 we have measured low levels of fractional linear polarization. The linear polarization in the H2O maser region of this source likely indicates a dominant toroidal or poloidal magnetic field component. Since circular polarization was not detected it is not possible to determine the magnetic field strength. However, we present a 3-sigma evaluation of the upper limit intensity of the magnetic field in the maser emitting regions of both observed sources.

On the magnetic field of OH 231.8+4.2

AbstractDuring the transition from an AGB star to a planetary nebula, a large number of low/intermediate initial mass stars loses its spherical symmetry. The process responsible for that change of morphology is, so far, not well understood. The candidates responsible for shaping these objects are (i) a companion to the star (binary/heavy planet) and its tidal forces, (ii) disk interaction and (iii) magnetic fields - or a combination of these. In particular a binary induced magnetic field is a promising option. To study this we observed the polarization of H2O masers in the known binary pre-Planetary Nebula (pPN) OH231.8+4.2. Our results show a magnetic field B|| of ~45 mG is present in the H2O maser region of this pPN.

H2O maser polarization of the water fountains IRAS 15445–5449 and IRAS 18043–2116

AbstractWe present the morphology and linear polarization of the 22-GHz H2O masers in the high-velocity outflow of two post-AGB sources, d46 (IRAS 15445–5449) and b292 (IRAS 18043–2116). The observations were performed using The Australia Telescope Compact Array. Different levels of saturated maser emission have been detected for both sources. We also present the mid-infrared image of d46 overlaid with the distribution of the maser features that we have observed in the red-shifted lobe of the bipolar structure. The relative position of the observed masers and a previous radio continuum observation suggests that the continnum is produced along the blue-shifted lobe of the jet. It is likely due to synchrontron radiation, implying the presence of a strong magnetic field in the jet. The fractional polarization levels measured for the maser features of d46 indicate that the polarization vectors are tracing the poloidal component of the magnetic field in the emitting region. For the H2O masers of b292 we have measured low levels of fractional linear polarization. The linear polarization in the H2O maser region of this source likely indicates a dominant toroidal or poloidal magnetic field component.

MAGNETIC FIELDS IN EVOLVED STARS: IMAGING THE POLARIZED EMISSION OF HIGH-FREQUENCY SiO MASERS

We present Submillimeter Array observations of high frequency SiO masers around the supergiant VX Sgr and the semi-regular variable star W Hya. The J=5-4, v=1 28SiO and v=0 29SiO masers of VX Sgr are shown to be highly linearly polarized with a polarization from ~5-60%. Assuming the continuum emission peaks at the stellar position, the masers are found within ~60 mas of the star, corresponding to ~100 AU at a distance of 1.57 kpc. The linear polarization vectors are consistent with a large scale magnetic field, with position and inclination angles similar to that of the dipole magnetic field inferred in the H2O and OH maser regions at much larger distances from the star. We thus show for the first time that the magnetic field structure in a circumstellar envelope can remain stable from a few stellar radii out to ~1400 AU. This provides further evidence supporting the existence of large scale and dynamically important magnetic fields around evolved stars. Due to a lack of parallactic angle coverage, the linear polarization of masers around W Hya could not be determined. For both stars we observed the 28SiO and 29SiO isotopologues and find that they have a markedly different distribution and that they appear to avoid each other. Additionally, emission from the SO 5_5-4_4 line was imaged for both sources. Around W Hya we find a clear offset between the red- and blue-shifted SO emission. This indicates that W Hya is likely host to a slow bipolar outflow or a rotating disk-like structure.

10.1007/s11444-008-3004-8

Results of monitoring of the H2O maser observed toward the infrared source IRAS 21078+5211 in the giant molecular cloud Cygnus OB7 are presented. The observations were carried out on the 22-m radio telescope of the Pushchino Radio Astronomy Observatory (Russia) from April 1992 to March 2006. Five cycles of maser activity at various levels were observed. In the periods of highest activity, the spectrum of the H2O maser emission extended from −43 to 12 km/s. During strong flares, the flux densities in some emission features reached nearly 600 Jy. The protostar has a small peculiar velocity with respect to the CO molecular cloud (∼2 km/s). Based on the character of the radial-velocity variations and the tendency for the linewidth to increase with the flux, it is concluded that the medium is strongly fragmented and that there is a small-scale turbulent outflow of ga in the H2O maser region, which may impede the formation of an HII region. The asymmetric distribution of the maser components in V LSR, the difference in the average linewidths of the central and lateral clusters of components, and the fairly high radial velocities relative to the molecular cloud (especially during periods of the highest maser activity) suggest that the maser spots belong to different clusters and different structures of the source: a disk and bipolar outflow.

The magnetic field of the evolved star W43A

The majority of the observed planetary nebulae exhibit elliptical or bipolar structures. Theoretical modeling has indicated that magnetically collimated jets may be responsible for the formation of the non-spherical planetary nebulae. The aim of this project is to measure the Zeeman splitting caused by the magnetic field in the OH and H2O maser regions occurring in the circumstellar envelope and bipolar outflow of the evolved star W43A. We report a measured magnetic field of approximately 100 micro-gauss in the OH maser region of the circumstellar envelope around W43A. The GBT observations reveal a magnetic field strength B|| of ~30 mG changing sign across the H2O masers at the tip of the red-shifted lobe of the bipolar outflow. We also find that the OH maser shell shows no sign of non-spherical expansion and that it probably has an expansion velocity that is typical for the shells of regular OH/IR stars. The GBT observations confirm that the magnetic field collimates the H2O maser jet, while the OH maser observations show that a strong large scale magnetic field is present in the envelope surrounding the W43A central star. The magnetic field in the OH maser envelope is consistent with the one extrapolated from the H2O measurements, confirming that magnetic fields play an important role in the entire circumstellar environment of W43A.

The magnetic field of the proto-planetary nebula candidate IRAS 19296+2227

Context. Magnetic fields are thought to be one of the possible mechanis ms responsible for shaping the generally spherical outflow of evolved stars into often aspherical planetary nebulae. However, direct measurements of magnetic fields during the tr ansition to the planetary nebula phase are rare. Aims. The aim of this project is to expand the number of magnetic fiel d measurements of stars in the (proto-)planetary nebula phase and find if the magnetic field strength is su ffi cient to affect the stellar outflow. Methods. We used Very Long Baseline Array observations to measure the circular polarization due to the Zeeman splitting of 22 GHz H2O masers in the envelope of the proto-planetary nebula candidate star IRAS 19296+2227 and the planetary nebula K3-35. Results. A strong magnetic field of B|| = −135± 28 is detected in the H2O maser region of the proto-planetary nebula candidate IRAS 19296+2227. The H2O masers of K3-35 are too weak to detect circular polarization although we do present the measurements of weak linear polarization in those masers. Conclusions. The field measured in the masers of IRAS 19296+2227 is dynamically important and, if it is representative o f the large scale field, is an important factor in driving the stellar mas s loss and shaping the stellar outflow.

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.

H2O Masers and Supersonic Turbulence

We use unpublished and published VLBI results to investigate the geometry and the statistical properties of the velocity field traced by H2O masers in five galactic regions of star formation: Sgr B2(M), W49N, W51(MAIN), W51N, and W3(OH). In all sources the angular distribution of the H2O hot spots demonstrates approximate self-similarity (fractality) over almost 4 orders of magnitude in scale, with the calculated fractal dimension d between � 0.2 and 1.0. In all sources, the lower order structure functions for the line-ofsight component of the velocity field are satisfactorily approximated by power laws, with the exponents near their classic Kolmogorov values for high Reynolds number incompressible turbulence. These two facts, as well as the observed significant excess of large deviations of the two-point velocity increments from their mean values, strongly suggest that the H2O masers in regions of star formation trace turbulence. We propose a new conceptual model of these masers in which maser hot spots originate at the sites of ultimate dissipation of highly supersonic turbulence produced in the ambient gas by the intensive gas outflow from a newly born star. Because of the high brightness and small angular sizes of masing hot spots and the possibility of measuring their positions and velocities with high precision, they become a unique probe of supersonic turbulence. Subject headings: ISM: jets and outflows — masers — turbulence

Sub-au imaging of water vapour clouds around four asymptotic giant branch stars

We present MERLIN maps of the 22-GHz H2O masers around four low-mass late-type stars (IK Tau, U Ori, RT Vir and U Her), made with an angular resolution of ∼15 milliarcsec and a velocity resolution of 0.1 km s −1 . The H2O masers are found in thick expanding shells with inner radii ∼6 to 16 au and outer radii four times larger. The expansion velocity increases radially through the H2O maser regions, with logarithmic velocity gradients of 0.5‐0.9. IK Tau and RT Vir have well-filled H2O maser shells with a spatial offset between the near and far sides of the shell, which suggests that the masers are distributed in oblate spheroids inclined to the line of sight. U Ori and U Her have elongated poorly filled shells with indications that the masers at the inner edge have been compressed by shocks; these stars also show OH maser flares. MERLIN resolves individual maser clouds, which have diameters of 2‐4 au and filling factors of only ∼0.01 with respect to the whole H2O maser shells. The circumstellar envelope velocity structure gives additional evidence the maser clouds are densitybounded. Masing clouds can be identified over a similar time-scale to their sound crossing time (∼2 yr) but not longer. The sizes and observed lifetimes of these clouds are an order of magnitude smaller than for those around red supergiants, similar to the ratio of low-mass:high-mass stellar masses and sizes. This suggests that cloud size is determined by stellar properties, not local physical phenomena in the wind.

Circular polarization of water masers in the circumstellar envelopes of late type stars

We present circular polarization measurements of circumstellar H2O masers. The magnetic fields in circumstellar envelopes are generally examd. by polarization observations of SiO and OH masers. SiO masers C probe the high temp. and d. regime close to the central star. OH masers are found at much lower densities and temps., generally much further out in the circumstellar envelope. The circular polarization detected in the (616-523) rotational transition of the H2O maser can be attributed to Zeeman splitting in the intermediate temp. and d. regime. The magnetic fields are derived using a general, LTE Zeeman anal. as well as a full radiative transfer method (non-LTE), which includes a treatment of all hyperfine components simultaneously as well as the effects of satn. and unequal populations of the magnetic substates. The differences and relevances of these interpretations are discussed extensively. We also address a non-Zeeman interpretation as the cause for the circular polarization, but this is found to be unlikely. We favor the non-LTE anal. The H2O masers are shown to be unsatd., on the basis of their line widths and the lack of linear polarization. The field strengths are compared with previous detections of the magnetic field on the SiO and OH masers. Assuming a r-2 dependence of the magnetic field on the distance to the star, similar to a solar-type magnetic field, our results seem to indicate that we are probing the highest d. maser clumps at the inner edge of the H2O maser region. This allows us to est. the d. of the clumps, and the extent of the H2O maser region. We show that the magnetic pressure dominates the thermal pressure by a factor of 20 or more. We also give an order of magnitude est. of the magnetic field on the surface of the stars. In particular we discuss the differences between supergiants and Mira variable stars. [on SciFinder (R)]

Dynamics of the superfine structure in the Orion KL jet

We have studied the superfine structure of the active H2O maser region in Orion KL with an angular resolution of ≤ 0.3 × 0.7 mas. The high level of H2O maser emission from 1979-1988 was due to an accretion disk, which is divided into five groups of protoplanetary rings. The peak brightness temperatures of the structures was Tpeak = 1013-14 K. The region is located in the OMC-1 molecular cloud, VLSR ⋍ 7.74 km/s. The cloud amplifies by more than two orders of magnitude the emission from the structures, whose radial velocities are within the maser window ±0.3 km/s. Due to this, the velocity of the H2O super maser emission is constant. In the quiescent period of 1995 a 6 AU jet was discovered, PA = −32°, Tb ⋍ 1011K. In 1998 the jet's brightness temperature increased by more than 3 orders of magnitude. Initially the jet's position angle was PA = −45°, and then changed to PA = −38°. During the period of decreasing emission in 1999 the jet had changed its form and became a helix, that suggests the precession of the rotation axes. In the central part of the jet there is a compact bright source - “the ejector” - with Tejc = 1017 K.

MERLIN observations of water maser proper motions in VY Canis Majoris

ABSTRA C T MERLIN observations of the 22-GHz water masers in the circumstellar envelope of the supergiant VY CMa show an ellipsoidal distribution with a maximum extent of 700 mas east‐ west and 400 mas north‐south. Comparison with observations made nine years earlier shows that the majority of maser features have survived and show proper motions throughout the region. The mean change in position is 28 mas and the proper motions are generally directed away from the assumed stellar position, and tend to be larger for features at greater projected distances. If the H2O maser region is modelled as a partially filled thick spherical shell, and VY CMa is at a distance of 1.5 kpc, then the proper motion velocities in the direction of expansion are between 8 km s π1 at a distance of 75 mas from the assumed stellar position and 32 km s π1 at 360 mas. These velocities are consistent with the H2O maser spectral line velocities which correspond to a maximum expansion velocity of 36 km s π1 at 400 mas from the assumed stellar position. These observations are consistent with radiation pressure on dust providing the force to accelerate the stellar wind as it passes through the H2O maser shell. The H2O maser region is elongated in the same direction as the dusty nebula around VY CMa. The water masers illuminate the small-scale dynamics and clumpiness which show the role of dust in driving the outflow. The overall ellipsoidal shape may be due to properties of the dust, such as its behaviour in the stellar magnetic field, or to interaction between the wind and circumstellar material. Maser monitoring also shows the difference between changes on the time-scale of stellar variability (a few years) and possible stages in the evolution of VY CMa to its likely fate as a supernova.

The Meaning of the OH-H_{2};O Maser Maps

view Abstract Citations (48) References (24) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Meaning of the OH-H_{2};O Maser Maps Litvak, M. M. Abstract The approximate conditions of density, diameter, and temperature of the OH and H2O maser regions are deduced from the very long baseline interferometry, from the presence of microwave saturation from the energy requirements, from the absence of excessive pressure or thermal broadening, from the microwave optical depths, and from the selection of hyperfine components. The amplifier surrounding each emission point has a diameter comparable to the distance between neighboring points and a mass close to 1 M0. Calculations indicate that the nonlinear effect of self-focusing is probably not occurring, that stimulated Raman scattering is yielding only the lowest hyperfine component in H2O with a lineshape asymmetry toward lower frequency, and that for both OH and 1120 the interferometers are observing "hot spots" of unsaturated amplification. Ordinary scattering from spatial fluctuations in electron density probably accounts for the observed angular sizes, especially for 18-cm OH signals. The region appears to consist of condensations each of about 1 Al0 on the edges of two parallel, slowly corotating disks each about 100 Alo. A stable, large-scale rotational discontinuity (of velocity and magnetic field) may be propagating between the two disks. Publication: The Astrophysical Journal Pub Date: November 1971 DOI: 10.1086/151189 Bibcode: 1971ApJ...170...71L full text sources ADS |