Water Fountain Source Research Articles

Excited-state OH Masers in the Water Fountain Source IRAS 18460-0151

Water fountain objects are generally defined as evolved stars with low to intermediate initial mass accompanied by high-velocity molecular jets detectable in the 22.235 GHz H2O maser line. They are the key objects of understanding the morphological transitions of circumstellar envelopes during the post asymptotic giant branch phase. Masers are useful tools to trace the kinematic environments of the circumstellar envelopes. In this Letter we report the discovery of exceptionally uncommon excited-state hydroxyl (ex-OH) masers at 4660 and 6031 MHz toward the water fountain source IRAS 18460−0151. These are the brightest ex-OH masers discovered in late-type objects to date. To the best of our knowledge, prior to the current work, no evolved stellar object has been observed in the 4660 MHz ex-OH maser line. The ground-state hydroxyl (g-OH) masers at 1612 and 1665 MHz are also observed. The velocity components of the 4660 MHz ex-OH maser line and the much weaker 1665 MHz g-OH maser line all can be seen in the 1612 MHz g-OH maser line profile. The blueshifted components of the three masers are more intense than the redshifted ones in contrast to the ex-OH maser line at 6031 MHz. The relevance of the behaviors of the ex-OH masers to the circumstellar environments is unclear.

Evolution of the Outflow in the Water Fountain Source IRAS 18043–2116* *Released on 2021 March 1.

We present the spectral and spatial evolution of H2O masers associated with the water fountain source IRAS 18043−2116, found in observations with the Nobeyama 45 m Telescope and the Australia Telescope Compact Array. We have found new highest-velocity components of the H2O masers (at the redshifted side V LSR ≃ 376 km s−1 and at the blueshifted side V LSR ≃ −165 km s−1), and the resulting velocity spread of ≃540 km s−1 breaks the speed record of fast jets/outflows in this type of sources. The locations of those components have offsets from the axis joining the two major maser clusters, indicating a large opening angle of the outflow (∼60°). The evolution of the maser cluster separation of ∼2.9 mas yr−1 and the compact (∼0.″2) CO emission source mapped with the Atacama Large Millimeter-submillimeter Array suggest a very short (∼30 yr) timescale of the outflow. We also confirmed an increase in the flux density of the 22 GHz continuum source. The properties of the jet and the continuum sources and their possible evolution in the transition to the planetary nebula phase are further discussed.

Water Fountain Sources Monitored in FLASHING

AbstractWe have investigated the spectral evolutions of H2O and SiO masers associated with 12 “water fountain” sources in our FLASHING (Finest Legacy Acquisitions of SiO-/H2O-maser Ignitions by Nobeyama Generation) project. Our monitoring observations have been conducted using the Nobeyama 45 m telescope every 2 weeks–2 months since 2018 December except during summer seasons. We have found new extremely high velocity H2O maser components, breaking the records of jet speeds in this type of sources. Systematic line-of-sight velocity drifts of the H2O maser spectral peaks have also been found, indicating acceleration of the entrained material hosting the masers around the jet. Moreover, by comparing with previous spectral data, we can find decadal growths/decays of H2O maser emission. Possible periodic variations of the maser spectra are further being inspected in order to explore the periodicity of the central stellar system (a pulsating star or a binary). Thus we expect to see the real-time evolution/devolutions of the water fountains over decades.

Observational identification of a sample of likely recent common-envelope events

One of the most poorly understood stellar evolutionary paths is that of binary systems undergoing common-envelope evolution, when the envelope of a giant star engulfs the orbit of a companion. Although this interaction leads to a great variety of astrophysical systems, direct empirical studies are difficult because few objects experiencing common-envelope evolution are known. We present ALMA observations towards sources known as water fountains that reveal they had low initial masses ($<4~{\rm M}_\odot$) and ejected a significant fraction of it over less than a few hundred years. The only mechanism able to explain such rapid mass ejection is common-envelope evolution. Our calculations show that the water-fountain sample accounts for a large fraction of the systems in our Galaxy which have just experienced the common-envelope phase. Since water-fountain sources show characteristic fast bipolar outflows, outflows and jets likely play an important role right before, during or immediately after the common-envelope phase.

Towards continuous viewing of circumstellar maser sources over decades

AbstractThe brightness of maser features are fascinating and give valuable insight for circumstellar physics of oxygen-rich, intermediate-mass stars, in particular the final evolution of circumstellar envelopes (CSEs). The variety of accompanying masers such as SiO, H2O, and OH in the CSEs may provide unique probes into different stages of rapid CSE evolution. However, with only sparse monitoring of these masers one can sometimes find it difficult to accurately interpret their spatio-kinematics, origins and excitation mechanisms. Examples can be seen in the variety of proposed models for water masers associated with “water fountains” and for silicon-monoxide masers. In order to better understand these issues, one needs to consider continuous monitoring of the individual maser gas clumps over a few stellar cycles or episodic ejection events. Here I present our previous long-term monitoring observations, especially for the water fountain source W43A. Our current efforts involve programs of intensive monitoring observations of circumstellar maser sources over decadal time periods. These programs with the East Asia VLBI Network observe H2O and SiO maser lines simultaneously mapped at high cadence (2–8 weeks) with VLBI observations.

Tianma 65-m telescope detection of new OH maser features towards the water fountain source IRAS 18286−0959

Tianma 65-m telescope detection of new OH maser features towards the water fountain source IRAS 18286−0959

WATER MASERS IN W43A: EARLY MORPHOLOGICAL CHANGES OF A FUTURE PLANETARY NEBULA

We present the distribution patterns of H2O maser features in the water fountain source (WF) W43A and show that they are closely related to the early morphological changes in a planetary nebula (PN). Using the Very Long Baseline Array, we have detected H2O maser features in W43A in 13 epochs across yr. W43A is the only WF that has been observed for over a decade. We introduce a new cavity model scenario—a halo with a bipolar evacuated volume that has a partially enhanced wall—and compare it with the traditional precessing jet model particularly in terms of explaining the bow-shaped distribution patterns of H2O maser features in the most recent epochs and their temporal evolution. Long-term observations show that the distribution patterns require more than a single jet to form. Moreover, we have identified six groups of H2O maser features at both the redshifted and blueshifted sides with point symmetry. The six groups are believed to correspond to periodic mass profiles in the envelope and have recorded the mass-loss history. Together with a geometric similarity to the mid-infrared morphology of W43A, the results suggest that H2O masers can be used to trace the inner morphology and rapid temporal changes in evolved stars, especially where the PN shaping has just started.

MASER AND INFRARED STUDIES OF OXYGEN-RICH LATE/POST-ASYMPTOTIC GIANT BRANCH STARS AND WATER FOUNTAINS: DEVELOPMENT OF A NEW IDENTIFICATION METHOD

We explored an efficient method to identify evolved stars with oxygen-rich envelopes in the late AGB or post-AGB phase of stellar evolution, which include a rare class of objects - the "water fountains". Our method considers the OH and H2O maser spectra, the near infrared Q-parameters (these are colour indices accounting for the effect of extinction), and far-infrared AKARI colours. Here we first present the results of a new survey on OH and H2O masers. There were 108 colour-selected objects: 53 of them were observed in the three OH maser lines (1612, 1665, and 1667 MHz), with 24 detections (16 new for 1612 MHz); and 106 of them were observed in the H2O maser line (22 GHz) with 24 detections (12 new). We identify a new potential water fountain source, IRAS19356+0754, with large velocity coverages of both OH and H2O maser emission. In addition, several objects with high velocity OH maser emission are reported for the first time. The Q-parameters as well as the infrared [09]-[18] and [18]-[65] AKARI colours of the surveyed objects are then calculated. We suggest that these infrared properties are effective in isolating aspherical from spherical objects, but the morphology may not necessarily be related to the evolutionary status. Nonetheless, by considering altogether the maser and infrared properties, the efficiency of identifying oxygen-rich late/post-AGB stars could be improved.

EXPLORATION OF A RELIC CIRCUMSTELLAR ENVELOPE IN THE “WATER FOUNTAIN” SOURCE IRAS 18286–0959

The water fountain source IRAS 18286−0959 (I18286) was reported as an object exhibiting highly collimated, double-helix stellar jets traced by the H2O maser emission. Using the European VLBI Network, we measured the absolute coordinates of the 1612 MHz OH maser emission in I18286, which is very likely associated with a relic of a circumstellar envelope (CSE) developed in the asymptotic giant branch stage of the central star. The location of the OH maser is near the center of the H2O maser feature cluster, where one of the originating points of the two jets is located. We also mapped 22.2 GHz H2O maser emission in this object using the Very Long Baseline Array in the seasons of 2006–2007 as well as 2008–2009. In the first three epochs in 2006–2007, we detected at least 90 maser features per epoch. In six epochs in 2008–2009, when the observation data had already been published and analyzed to identify the double-helix jets, we newly identified 14 proper motions of H2O maser features which could not be measured in the previous analysis due to their short lifetimes. In this paper, together with the OH maser emission, we focus on "outlier" H2O maser features, which exhibit slow expansion velocities (Vexp ≲ 30 km s−1) and are likely associated with either the relic CSE or an equatorial flow of I18286. They were marginally distinguishable from the maser features associated with the jets in I18286.

Annual Parallax Distance and Secular Motion of the Water Fountain Source IRAS 18286–0959

Abstract We report on results of astrometric observations of H$ _{2}$ O masers in the ``water fountain'' source IRAS 18286$-$ 0959 (I18286) with the VLBI Exploration of Radio Astrometry (VERA). These observations yielded an annual parallax of IRAS 18286$-$ 0959, $ \pi$$ =$ 0.277 $ \pm$ 0.041 mas, corresponding to a heliocentric distance of $ D$$ =$ 3.61$ ^{+0.63}_{-0.47}$ kpc. The maser feature, whose annual parallax was measured, showed an absolute proper motion of ($ \mu_{\alpha}$ , $ \mu_{\delta}$ ) $ =$ ($-$ 3.2 $ \pm$ 0.3, $-$ 7.2 $ \pm$ 0.2) [mas yr$ ^{-1}$ ]. The intrinsic motion of the maser feature in the internal motions of the cluster of features in I18286 does not seem to trace the motion of the bipolar jet of I18286. Taking into account this intrinsic motion, the derived motion of the maser feature is roughly equal to that of the maser source I18286 itself. The proximity of I18286 to the Galactic midplane ($ z$$ \approx$ 10 pc) suggests that the parental star of the water fountain source in I18286 should be an intermediate-mass AGB/post-AGB star, but the origin of a large deviation of the systemic source motion from that expected from the Galactic rotation curve is still unclear.

Extremely Strong 13CO J = 3→2 Line in the ``Water Fountain'' IRAS 16342−3814: Evidence for the Hot-Bottom Burning

Abstract We observed four ``water fountain'' sources in the CO $ J$$ =$ 3$ \rightarrow$ 2 line emission with the Atacama Submillimeter Telescope Experiment (ASTE) 10 m telescope in 2010–2011. The water fountain sources are evolved stars that form high-velocity collimated jets traced by H$ _{2}$ O maser emission. The CO line was detected only from IRAS 16342$-$ 3814. The present work confirmed that the $ ^{12}$ CO to $ ^{13}$ CO line intensity ratio is $ \sim$ 1.5 at the systemic velocity. We discuss the origins of the very low $ ^{12}$ CO-to-$ ^{13}$ CO intensity ratio, as possible evidence for ``hot-bottom burning'' in an oxygen-rich star, and the CO intensity variation in IRAS 16342$-$ 3814.

Dust shell model of the water fountain source IRAS 16342–3814

We investigate the circumstellar dust shell of the water fountain source IRAS 16342-3814. We performed two-dimensional radiative transfer modeling of the dust shell, taking into account previously observed spectral energy distributions (SEDs) and our new $J$-band imaging and $H$- and $K_S$-band imaging polarimetry obtained using the VLT/NACO instrument. Previous observations expect an optically thick torus in the equatorial plane because of a striking bipolar appearance and a large viewing angle of 30 - 40$\degr$. However, models with such a torus as well as a bipolar lobe and an AGB shell cannot fit the SED and the images simultaneously. We find that an additional optically and geometrically thick disk located inside a massive torus solves this problem. The masses of the disk and the torus are estimated to be 0.01 $M_{\sun}$ at the $a_\mathrm{max}=100 \mu$m dust and 1 $M_{\sun}$ at $a_\mathrm{max}=10 \mu$m dust, respectively. We discuss a possible formation scenario for the disk and torus based on a similar mechanism to the equatorial back flow. IRAS 16342-3814 is expected to undergo mass loss at a high rate. The radiation from the central star is shielded by the dust that was ejected in the subsequent mass loss event. As a result, the radiation pressure on dust particles cannot govern the motion of the particles anymore. The mass loss flow can be concentrated in the equatorial plane by help of an interaction, which might be the gravitational attraction by the companion, if it exists in IRAS 16342-3814. A fraction of the ejecta is captured in a circum-companion or circum-binary disk and the remains are escaping from the central star(s) and form the massive torus.

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.

Water Fountains in Pre-Planetary Nebulae: The Case of IRAS16342–3814

AbstractWe present a brief review of Very Long Baseline Array (VLBA) observations of water masers in the so-called water fountain pre-planetary nebulae, and report on new VLBA and Very Large Array (VLA) data for the water masers in the prototypical water fountain source IRAS16342–3814, taken approximately monthly in 2008–2009. A new and very strong water maser is found at an LSR velocity of −3 km s−1, which is offset from the central (likely stellar) velocity. The new VLBA observations still show a similar general structure as was observed in 2002, but there are details which are difficult to explain.

Maser emission during post-AGB evolution

AbstractThis contribution reviews recent observational results concerning astronomical masers toward post-AGB objects with a special attention to water fountain sources and the prototypical source OH 231.8+4.2. These sources represent a short transition phase in the evolution between circumstellar envelopes around asymptotic giant branch stars and planetary nebulae. The main masing species are considered and key results are summarized.

The kinematics and magnetic fields in water-fountain sources based on OH maser observations

Context. water-fountain sources are a class of post-AGB objects that exhibit bipolar jet-like structures traced by H2 O maser emission. The circumstellar envelopes of these objects show deviations from spherical symmetry. While the H2 O masers originate in a collimated outflow in the polar region, the OH masers may be distributed in the equatorial region or a biconical outflow in the circumstellar shell. Magnetic fields could play an important role in collimating the jet and shaping the circumstellar envelope of these objects.Aims. The aim of this project is to understand the morphology of the OH masers in three water-fountain sources, for which we obtained high-resolution interferometric data. We compare the observational parameters of the spectral profile and spatial distribution with our models in order to constrain the morphology and velocity fields of the OH maser shell. We would also like to understand the role of the magnetic field in shaping the circumstellar envelope of these stars. Methods. We observed the OH masers of three water-fountain sources (OH 12.8-0.9, OH 37.1-0.8, and W43A) in full polarization spectral line mode using the UK Multi-Element Radio Linked Interferometer Network. Additionally, we performed reconstruction models of OH maser shells distributed uniformly in equatorial or biconical outflows and compared them with the OH maser observations.Results. The OH masers of W43A seem to be located in the equatorial region of the circumstellar shell, while for OH 12.8-0.9 the masers are likely distributed in a biconical outflow. We measured magnetic fields of ~360 μ G and ~29 μ G on average for OH 37.1-0.8 and OH 12.8-0.9, respectively. The measured field strengths show that a large-scale magnetic field is present in the circumstellar environment of these stars. Conclusions. Our observations show that the OH maser region of the three water-fountain sources studied in this work show signs of aspherical expansion. Reconstruction of the OH maser distribution of these objects show both biconical and equatorial distributions. Magnetic fields likely play an important role in shaping the OH maser region of water-fountain sources.

High-velocity H2O maser emission from the post-asymptotic-giant-branch star OH 009.1-0.4

Abstract Observations of H2O masers towards the post-asymptotic-giant-branch star and water fountain source OH 009.1-0.4 were made as part of The H2O southern galactic Plane Survey, with the Mopra radiotelescope. Together with followup observations using the Australia Telescope Compact Array (ATCA), we have identified H2O maser emission over a velocity spread of nearly 400 km s-1 (-109 to +289 km s-1). This velocity spread appears to be the largest of any known maser source in our Galaxy. High-resolution observations with the ATCA indicate that the maser emission is confined to a region 0.3 × 0.3 arcsec and shows weak evidence for a separation of the red- and blueshifted maser spots. We are unable to determine if the water fountain is projected along the line of sight, or is inclined, but either way OH 009.1-0.4 is an interesting source, worthy of followup observations.

H2O Maser Observations of Candidate Post‐AGB Stars and Discovery of Three High‐Velocity Water Sources

We present the results of 22 GHz H2O maser observations of a sample of 85 post-asymptotic giant branch (post-AGB) candidate stars, selected on the basis of their OH 1612 MHz maser and far-infrared properties. All sources were observed with the Tidbinbilla 70 m radio telescope, and 21 detections were made; 86 GHz SiO Mopra observations of a subset of the sample are also presented. Of the 21 H2O detections, 15 are from sources that are likely to be massive AGB stars and most of these show typical, regular H2O maser profiles. In contrast, nearly all the detections of more evolved stars exhibited high-velocity H2O maser emission. Of the five sources seen, v223 (W43A, IRAS 18450-0148) is a well-known "water-fountain" source that belongs to a small group of post-AGB stars with highly collimated, high-velocity H2O maser emission. A second source in our sample, v270 (IRAS 18596+0315), is also known to have high-velocity emission. We report the discovery of similar emission from a further three sources, d46 (IRAS 15445-5449), d62 (IRAS 15544-5332), and b292 (IRAS 18043-2116). The source d46 is an evolved post-AGB star with highly unusual maser properties. The H2O maser emission from d62 is probably associated with a massive star. The source b292 is a young post-AGB star that is highly likely to be a water-fountain source, with masers detected over a velocity range of 210 km s-1.

Stellar molecular jets traced by maser emission

AbstractHighly collimated jets found in AGB and post-AGB stars are expected to play an important role for shaping planetary nebulae. Recent VLBI observations of H2o maser sources have revealed that some of the spatio-kinematical structures of H2o maser sources exhibit stellar jets with extremely spatially and kinematically high collimation. Such stellar H2o maser jets, so-called “water fountains”, have been identified in about 10 sources to date. Here we review recent VLBI observations and relevant observational results of the water fountain sources. They have revealed a typical dynamical age and the detailed kinematical structures of the water fountains, the possibility of the existence of “equatorial flows”, and the evolutionary status of the host stars. The location and kinematics of one of the water fountain sources in the Galaxy is also revealed and shown here.

OH 12.8-0.9: A New Water-Fountain Source

We present observational evidence that the OH/IR star OH 12.8-0.9 is the fourth in a class of objects previously dubbed sources. Using the Very Long Baseline Array, we produced the first images of the H2O maser emission associated with OH 12.8-0.9. We find that the masers are located in two compact regions with an angular separation of ~109 mas on the sky. The axis of separation between the two maser regions is at a position angle of 15 east of north, with the blueshifted (-80.5 to -85.5 km s-1) masers located to the north and the redshifted (-32.0 to -35.5 km s-1) masers to the south. In addition, we find that the blue- and redshifted masers are distributed along arclike structures ~10-12 mas across, oriented roughly perpendicular to the separation axis. The morphology exhibited by the H2O masers is suggestive of an axisymmetric wind with the masers tracing bow shocks formed as the wind impacts the ambient medium. This bipolar jet-like structure is typical of the three other confirmed water-fountain sources. When combined with the previously observed spectral characteristics of OH 12.8-0.9, the observed spatiokinematic structure of the H2O masers provides strong evidence that OH 12.8-0.9 is indeed a member of the water-fountain class.