H2O Maser Sources Research Articles

New Compton-thick AGN in the circumnuclear H2O maser hosts UGC 3789 and NGC 6264

Large column densities, derived from X-ray studies, are typically measured towards AGN hosting water masers, especially when the H2O emission is associated with the nuclear accretion disk. In addition, possible correlations between the intrinsic X-ray luminosity and the characteristics of the H2O maser emission have been put forward that, however, require confirmation. We have performed high-sensitivity XMM-Newton observations of a sample of five H2O maser sources confidently detected in our ongoing survey with the Swift satellite of all known water masers in AGN, in order to obtain detailed X-ray information of these promising targets and to set up a systematic detailed study of the X-ray/H2O-maser relation in AGN. For three galaxies, NGC 613, VIIZw73, and IRAS 16288+3929, the amount of intrinsic absorption has been estimated, indicating column densities of 4-6x10^23 cm^-2. For UGC 3789 and NGC 6264 (the two confirmed disk-maser galaxies in our sample), column densities in excess of 1x10^24 cm^-2 are inferred from the large EW of the Fe Kalpha line. By adding our results to those obtained in past similar studies, we find that the percentage of water masers sources that host highly-obscured (NH>10^23 cm^-2) and Compton-thick AGN is 96% (45/47) and 57% (27/47), respectively. In addition, 86%, 18/21, of disk maser galaxies host Compton-thick AGN. The correlation between the galaxies' bolometric luminosity and accretion disk radius, suggested in previous works, is also confirmed.

WATER MASER SURVEY ONAKARIANDIRASSOURCES: A SEARCH FOR “LOW-VELOCITY” WATER FOUNTAINS

We present the results of a 22GHz H2O maser survey toward a new sample of AGB and post-AGB star candidates. Most of the objects are selected for the first time based on the AKARI data, which have high flux sensitivity in the mid-infrared ranges. We aim at finding H2O maser sources in the transient phase between the AGB and post-AGB stage of evolution, where the envelopes start to develop large deviations from spherical symmetry. The observations were carried out with the Effelsberg 100m radio telescope. Among 204 observed objects, 63 detections (36 new) were obtained. We found 4 objects which may be "water fountain" sources (IRAS 15193+3132, IRAS 18056-1514, OH 16.3-3.0, and IRAS 18455+0448). They possess an H2O maser velocity coverage much smaller than those in other known water fountains. However, the coverage is still larger than that of the 1612MHz OH maser. It implies that there is an outflow with a higher velocity than the envelope expansion velocity (typically <25 km/s), meeting the criterion of the water fountain class. We suggest that these candidates are possibly oxygen-rich late AGB or early post-AGB stars in a stage of evolution immediately after the spherically symmetric AGB mass-loss has ceased.

ATCA survey of H2O masers in the Large Magellanic Cloud

Abstract We have analysed archival data taken with the Australia Telescope Compact Array (ATCA) during 2001–2003 and detected nine new interstellar and circumstellar H2O masers in the Large Magellanic Cloud (LMC). This takes the total number of star formation H2O masers in the LMC to 23, spread over 14 different star-forming regions and 3 evolved stars. Three H2O maser sources (N105a/MC23, N113/MC24, N157a/MC74) have been detected in all the previous observations that targeted these sites, although all show significant variability on time-scales of decades. The total number of independent H2O maser sources now known in the LMC means that through very long baseline interferometry astrometric measurements it will be possible to construct a more precise model of the galactic rotation of the LMC and its orbital motion around the Milky Way Galaxy.

SIMULTANEOUS OBSERVATIONS OF SiO AND H2O MASERS TOWARD KNOWN STELLAR H2O MASER SOURCES

We present the results of simultaneous observations of SiO v = 1, 2, 29SiO v = 0, J = 1-0, and H2O 616-523 maser lines toward 152 known stellar H2O maser sources using the Yonsei 21 m radio telescope of the Korean VLBI Network from 2009 June to 2011 January. Both SiO and H2O masers were detected from 62 sources with a detection rate of 40.8%. The SiO-only maser emission without H2O maser detection was detected from 27 sources, while the H2O-only maser without SiO maser detection was detected from 22 sources. Therefore, the overall SiO maser emission was detected from 89 sources, resulting in a detection rate of 58.6%. We have identified 70 new detections of the SiO maser emission. For both H2O and SiO maser detected sources, the peak and integrated antenna temperatures of SiO masers are stronger than those of H2O masers in both Mira variables and OH/IR stars and the relative intensity ratios of H2O to SiO masers in OH/IR stars are larger than those in Mira variables. In addition, distributions of 152 observed sources were investigated in the IRAS two-color diagram.

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 Nançay (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 35–41 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.

The G305 star-forming complex: embedded massive star formation discovered byHerschelHi-GAL

We present a Herschel far-infrared study towards the rich massive star-forming complex G305, utilizing PACS 70, 160 μm and SPIRE 250, 350, and 500 μm observations from the Hi-GAL survey of the Galactic plane. The focus of this study is to identify the embedded massive star-forming population within G305, by combining far-infrared data with radio continuum, H2O maser, methanol maser, MIPS and Red MSX Source survey data available from previous studies. By applying a frequentist technique we are able to identify a sample of the most likely associations within our multiwavelength data set, which can then be identified from the derived properties obtained from fitted spectral energy distributions (SEDs). By SED modelling using both a simple modified blackbody and fitting to a comprehensive grid of model SEDs, some 16 candidate associations are identified as embedded massive star-forming regions. We derive a two-selection colour criterion from this sample of log (F70/F500) ≥ 1 and log (F160/F350) ≥ 1.6 to identify an additional 31 embedded massive star candidates with no associated star formation tracers. Using this result we can build a picture of the present-day star formation of the complex, and by extrapolating an initial mass function, suggest a current population of ≈2 × 104 young stellar objects (YSOs) present, corresponding to a star formation rate (SFR) of 0.01–0.02 M⊙ yr−1. Comparing this resolved SFR, to extragalactic SFR tracers (based on the Kennicutt–Schmidt relation), we find that the star formation activity is underestimated by a factor of ≥2 in comparison to the SFR derived from the YSO population.

Water fountains: bipolar fast stellar jets traced by water vapor maser emission

AbstractHighly collimated, bipolar fast jets are found in asymptotic giant branch (AGB) and post-AGB stars as well as in active galactic nuclei and young stellar objects. It is still unclear how to launch such jets from dying stars that were originally spherically symmetric. Exploration of the stellar jet evolution is also expected to probe its role in shaping a planetary nebula. Interestingly, some of stellar H2O maser sources — water fountains — exhibit stellar jets with spatially and kinematically high collimation in the earliest phase (&lt;1000 years) of the jet evolution. Such water fountains have been identified in 14 sources to date. We have recently conducted interferometric (VLBA, EVN, VERA, VLA) maser and the single-dish (ASTE) CO J = 3 → 2 line observations of the water fountains. They have revealed a typical dynamical age (&lt; 100 yr) and the detailed kinematical structures of the water fountains, possibility of the coexistence of “equatorial flows”, and their locations and kinematics in the Milky Way. Based on these results, the masses and evolutionary statuses of the host stars are also estimated.

Analysis of H2O masers in Sharpless 269 using VERA archival data—Effect of maser structures on astrometric accuracy

Astrometry using H2O maser sources in star forming regions is expected to be a powerful tool to study the structures and dynamics of our Galaxy. Honma et al. (2007) (hereafter H2007) claimed that the annual parallax of Sharpless 269 is determined within an error of 0.008marcs (mas), concluding that S269 is located at 5.3kpc ±0.2kpc from the sun, and its galactrocetnric distance is R=13.1kpc. From the proper motion, they claimed that the galacto-centric rotational velocity of S269 is equal to that of the sun within a 3% error. This small error, however, is hardly understood when taking into account the results of other observations and theoretical studies of galactic dynamics. We here reanalyzed the VERA archival data using the self calibration method (hybrid mapping), and found that clusters of maser features of S269 are distributed in much wider area than that investigated in H2007. We confirmed that, if we make a narrow region image without considering the presence of multiple maser spots, and only the phase calibration is applied, we can reproduce the same maser structures in a maser feature investigated in H2007. The distribution extent of maser spots in the feature differs 0.2mas from east to west between our results and H2007. Moreover, we found that change of relative positions of maser spots in the cluster reaches 0.1mas or larger between observational epochs. This suggests that if one simply assumes the time-dependent, widely distributed maser sources as a stable single point source, it could cause errors of up to 0.1 mas in the annual parallax of S269. Taking into account the internal motions of maser spot clusters, the proper motion of S269 cannot be determined precisely. We estimated that the peculiar motion of S269 with respect to a Galactic circular rotation is ∼20kms−1. These results imply that the observed kinematics of maser emissions in S269 cannot give a strong constraint on dynamics of the outer part of the Galaxy, in contrast to the claim by H2007.

Simultaneous observations of SiO and H2O masers toward known stellar SiO and/or H2O maser sources

AbstractWe present the results of simultaneous observations of SiO v = 1, 2, 29SiO v = 0, J = 1−0 and H2O 616-523 maser lines toward 318 known stellar SiO and/or H2O maser sources using the Yonsei 21-m radio telescope of the Korean VLBI Network. Toward 166 known SiO and H2O maser sources, both SiO and H2O maser emissions were detected from 112 sources giving a detection rate of 67.5 %. On the other hand, toward 152 known H2O-only maser sources, both SiO and H2O maser emissions were detected from 62 sources, giving a detection rate of 40.8 %. Characteristics of all observed sources in the IRAS two-color diagram is investigated including their evolutionary sequence and mutual relations between SiO and H2O maser properties.

Astrometry of water masers in post-AGB stars

AbstractMeasurements of trigonometric parallaxes and secular motions of evolved stars, especially post-AGB stars including central objects of planetary nebulae and water fountain sources as well as peculiar or unclassified stars, provide unambiguous source distance scales and information on their orbits in the Milky Way Galaxy. True source luminosities and kinematical properties should lead us to elucidate the true characteristics and evolutional tracks of these stars. Here we present the recent results of astrometry towards H2O maser sources with the VLBI Exploration of Radio Astormetry (VERA). The target sources include a planetary nebula (K3–35), a pre-PN (IRAS 19312+1950), a water fountain (IRAS 18286−0959) and a K-type star (IRAS 22480+6002). We have demonstrated that parental stars of the former three sources should be intermediate-mass stars from their luminosities and orbits in the Milky Way. It is suggested that IRAS 22480+6002 should be a K-type supergiant previously suggested rather than an RV Tau variable star.

Trigonometric parallax measurements of post-AGB stars with VERA

AbstractA trigonometric parallax distance is measurable for a planetary nebula if it harbors H2O (or OH) maser spots. We have demonstrated it for H2O maser sources, K3-35, IRAS 19312+1950 and IRAS 18286-0959 using the VLBI Exploration of Radio Astrometry (VERA). They are post-AGB stars and exhibit peculiar morpho-kinematical structures like a bipolar planetary nebula or a bipolar molecular jet (water fountain). The luminosities and secular motions of these sources derived from the trigonometric distances and proper motions of the H2O masers suggest that their parental stars may be intermediate-mass (2M⊙ &lt; M* &lt; 8 M⊙) evolved stars.

Extragalactic H2O maser sources and their properties

The properties of a sample of extragalactic H 2 O maser sources over a wide spectral range are discussed. Based on a sample of 81 maser galaxies it is shown that mega- and kilomasers have completely different properties. In particular, for megamasers the strongest observed relationships are between the parameters of the H 2 O line and the mass of the galactic nucleus, while the parameters of the line are uncorrelated with the x-ray, infrared, and radio emission. A weak correlation between megamaser emission and the surface (column) density of hydrogen is observed. As for kilomasers, their H 2 O luminosity depends weakly on the x-ray emission, although in the case of soft x rays this dependence is significant. The H 2 O luminosity of kilomasers is moderately correlated with the infrared and radio continuum luminosities, but the line parameters are independent of the mass of the nucleus and the surface density of hydrogen.

SIMULTANEOUS OBSERVATIONS OF SiO AND H 2 O MASERS TOWARD KNOWN STELLAR SiO AND H 2 O MASER SOURCES. I.

We present the results of simultaneous observations of both SiO and H2O masers toward 166 known SiO and H2O maser sources using the Korean VLBI Network Yonsei 21 m radio telescope during 2009 June. Both SiO and H2O maser emission were detected from 112 sources giving a detection rate of 67% at one epoch observation. SiO-only maser emission was detected from 42 sources, while H2O-only maser emission was detected from four sources. Most of the SiO masers appear around the stellar velocity, while H2O masers show different characteristics compared with SiO masers. There are more than 20 sources that show a one-way peak or double peaks with respect to the stellar velocity and SiO maser peak velocity. The H2O maser peak and integrated intensity ratios with respect to those of SiO (v = 1) show increasingly larger values from Mira variables, to OH/IR stars, to semi-regular variables. In addition, the IRAS two-color diagram of SiO and H2O maser observational results is discussed.

THE DISTANCE TO A STAR-FORMING REGION IN THE OUTER ARM OF THE GALAXY

We performed astrometric observations with the VLBA of WB89-437, an H2O maser source in the Outer spiral arm of the Galaxy. We measure an annual parallax of 0.167 +/- 0.006 mas, corresponding to a heliocentric distance of 6.0 +/- 0.2 kpc or a Galactocentric distance of 13.4 +/- 0.2 kpc. This value for the heliocentric distance is considerably smaller than the kinematic distance of 8.6 kpc. This confirms the presence of a faint Outer arm toward l = 135 degrees. We also measured the full space motion of the object and find a large peculiar motion of ~20 km/s toward the Galactic center. This peculiar motion explains the large error in the kinematic distance estimate. We also find that WB89-437 has the same rotation speed as the LSR, providing more evidence for a flat rotation curve and thus the presence of dark matter in the outer Galaxy.

An Infrared Photometric Study of Galaxies with Extragalactic H 2 O Maser Sources

All galaxies with extragalactic H2O maser sources observed so far are collected. With the 2MASS and the IRAS photometric data an infrared study is performed on those galaxies. By a comparison between the H2O maser detected sources and non-detected sources in the infrared it is indicated that infrared properties in the IRAS 12–25 μm and 60–100 μm are important for producing H2O masers in galaxies. It is also found that the H2O maser galaxies with different nuclear activity types have rather different infrared properties mainly in the IRAS 12–60 μm region.

The distance to an outer Galaxy star forming region

AbstractWe performed phase-referencing VLBI astrometric observations of the H2O maser source IRAS 02395+6244, located well beyond the solar circle. We measured its heliocentric distance to be 5.49 ± 0.80 kpc, implying a Galactocentric distance of 12.5 ± 0.5 kpc and a distance of 270 ± 40 pc above the Galactic plane.

Absolute Proper Motions of H2O Masers Away from the Galactic Plane Measured with VERA in the “Superbubble” Region NGC 281

Abstract We report on absolute proper-motion measurements of an H2O maser source in the NGC 281 West molecular cloud, which is located $\sim$ 320 pc above the Galactic plane and is associated with an H I loop extending from the Galactic plane. We conducted multi-epoch phase-referencing observations of the maser source with VERA (VLBI Exploration of Radio Astrometry) over a monitoring period of 6 months, since 2006 May. We find that the H2O maser features in NGC 281 West are systematically moving toward the southwest and further away from the Galactic plane with a vertical velocity of 20–30 km s$^{-1}$ at its estimated distance of 2.2–3.5 kpc. Our new results provide the most direct evidence that the gas in the NGC 281 region on the H I loop was blown out from the Galactic plane, most likely in a superbubble driven by multiple or sequential supernova explosions in the Galactic plane.

Statistical properties of extragalactic H2O maser sources

AbstractRecently, with the development of sensitive maser receiver and wide-band spectrometer, maser emission were detected toward more and more galaxies. 69 galaxies beyond the Magellanic Clouds has been reported with H2O maser emission (to 2006.12). The statistical properties of the sample of extragalactic H2O masers are not well explored. Here we collect observation data of all these detected extragalactic H2O maser sources and give a statistical analysis to investigate the statistical properties of the sample.

Twenty-six-year monitoring of water masers

AbstractSince 1980 variability of a sample of H2O maser sources has been monitored on the 22-metre radio telescope in Pushchino, Russia. The interval between successive observational sessions is 1–2 months. The sample includes 125 maser sources in star-forming regions (SFR) and late-type variable stars. Twenty-six-year time series of H2O line profiles have detected flares and velocity drift of spectral features. Very fast variations in the H2O maser flux (Δt ≲ 1 hour) have been detected in several SFR sources, in particular, W33B. Variations of circumstellar H2O masers in late-type stars correlate with visual light curves with a time lag of 0.3-0.4P (P is the star's period). Exceptionally strong H2O maser flares were recorded in SFR sources (Sgr B2 and others) and in the stars W Hya, R Cas and U Ori. Models for H2O maser variability are reviewed. For stellar masers shock-wave excitation of H2O line variability is discussed.

Astrometry of H2O maser sources in nearby molecular clouds with VERA

AbstractWe have carried out multi-epoch VLBI observations of the H2O maser sources associated with young stellar objects (YSOs) in nearby molecular clouds with VERA (VLBI Exploration of Radio Astrometry), which is a newly constructed VLBI network in Japan (Kobayashiet al. 2003). The main goal of our study is to measure the absolute proper motions and distances to nearby molecular clouds within 1 kpc from the Sun, to reveal their 3-dimensional structures and dynamical properties. Using the VERA dual-beam receiving system (Honmaet al. 2003), we have carried out phase-referencing VLBI observations and measured annual parallaxes and absolute proper motions of the H2O maser features with respect to the extragalactic radio sources. We have successfully detected the annual parallax of one of the H2O maser features in Orion KL to be 2.29±0.10 mas, corresponding to the distance of 437±19 pc from the Sun (Hirotaet al. 2007). In addition, the annual parallax of SVS13 in NGC 1333 is also determined to be4.10±0.17 mas, corresponding to the distance of 244±10 pc from the Sun, although the life time of the maser features are only 6 months. The absolute proper motions of the H2O maser features associated with Orion KL and NGC 1333 are derived, possibly indicating the outflow motions from the YSOs as well as the systemic motions of the powering sources.