Maser Region Research Articles

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.

Water vapour masers in long-period variable stars

Context. Water vapour maser emission from late-type stars characterises them as asymptotic-giant-branch stars with oxygen-rich chemistry that are losing mass at a substantial rate. Further conclusions on the properties of the stars, however, are hampered by the strong variability of the emission. Aims. We wish to understand the reasons for the strong variability of H2O masers in circumstellar shells of late-type stars. In this paper we study RX Bootis and SV Pegasi as representatives of semiregular variable stars (SRVs). Methods. We monitored RX Boo and SV Peg in the 22-GHz maser line of water vapour with single-dish telescopes. The monitoring period covered two decades for RX Boo (1987–2007) and 12 years for SV Peg (1990–1995, 2000–2007). In addition, maps were obtained of RX Boo with the Very Large Array over several years. Results. We find that most of the emission in the circumstellar shell of RX Boo is located in an incomplete ring with an inner radius of 91 mas (15 AU). A velocity gradient is found in a NW–SE direction. The maser region can be modelled as a shell with a thickness of 22 AU, which is only partially filled. The gas crossing time is 16.5 years. The ring-like structure and the velocity gradient remained stable for at least 11 years, while the maser line profiles varied strongly. This suggests that the spatial asymmetry is not accidental, so that either the mass loss process or the maser excitation conditions in RX Boo are not spherically symmetric. The strong variability of the maser spectral features is mainly due to incoherent intensity fluctuations of maser emission spots, which have lifetimes of the order of 1 year. We found no correlation between the optical and the maser variability in either star. The variability properties of the SV Peg masers do not differ substantially from those of RX Boo. There were fewer spectral features present, and the range of variations was narrower. The maser was active on the >10-Jy level only 1990–1992 and 2006/2007. At other times the maser was either absent (<1 Jy) or barely detectable. Conclusions. The variability of H2O masers in the SRVs RX Boo and SV Peg is due to the emergence and disappearance of maser clouds with lifetimes of ∼1 year. The emission regions do not evenly fill the shell of RX Boo leading to asymmetry in the spatial distribution, which persists at least an order of magnitude longer.

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.

VLBA SiO observations of bright O-rich AGB stars

We present the results of observations of the SiO masers at 7 mm wavelength in several oxygen rich Asymptotic Giant Branch (AGB) stars. These observations were coordinated with observations using the ESO VLTI/MIDI interferometer at 10 μm wavelength. These stars are o Ceti (Mira), U Herculis (U Her), OH26.5+0.6, R Leonis (R Leo), and W Hydrae (W Hya). Both total intensity and linear polarizations are imaged. Linear radial features seen in o Ceti may be associated with an X-ray flare seen a year earlier by [CITE]. Similar linear features are also seen in R Leo, U Her and OH26.5+0.6. These features typically show systematic radial velocity variations and a tendency for the linear polarization to be perpendicular to the direction of elongation, suggesting that magnetic fields are dynamically important. A wide radial extent of the maser rings observed in U Herculis and OH26.5+0.6 is interpreted as meaning that the outer boundaries of the masing regions in these stars are not truncated by the condensation of SiO onto dust grains.

Water Maser Kinematics in the Jet of OH 12.8−0.9

We present Very Long Baseline Array observations of the kinematics of the water masers associated with OH 12.8-0.9, the fourth member of the so-called "water fountain" class of sources. We find that the masers occupy two distinct regions at the ends of a bipolar jet-like structure oriented north-south, with the blue-shifted masers located to the north and the red-shifted masers to the south. The masers are distributed along arc-like structures 12-20 mas across oriented perpendicular to the separation axis with an angular separation of ~110 mas on the sky. Our multi-epoch observations, show the two maser arcs to be expanding away from each other along the axis of separation. The relative proper motions of the two maser regions is 2.7 mas/yr (~105 km/s at the assumed distance of 8 kpc). The measured radial velocity difference between the northern, blue-shifted masers and the southern, red-shifted masers is 48.4 km/s. The radial velocity, when combined with the proper motion, yields a three-dimensional expansion velocity of 58 km/s and an inclination angle of 24 deg. for the jet. By combining our radial velocities with historical values, we estimate the three dimensional acceleration of the masers to be ~0.63 km/s/yr and a dynamical age for the collimated outflow of ~90 yr.

A Search for Formaldehyde 6 cm Emission toward Young Stellar Objects. II. H 2 CO and H110α Observations

We report the results of our second survey for Galactic H2CO maser emission toward young stellar objects. Using the GBT and the VLA in the A configuration we observed 58 star-forming regions and discovered the fifth H2CO 6 cm maser region in the Galaxy (G23.71-0.20). We have discussed the detection toward G23.71-0.20 in a previous paper. Here we present all the other results from our survey, including detection of H2CO absorption features toward 48 sources, detection of the H110α recombination line toward 29 sources, detection (including tentative detections) of the carbon recombination line C110α toward 14 sources, subarcsecond angular resolution images of 6 cm continuum emission toward five sources, and observations of the H2CO masers in IRAS 18566+0408 and NGC 7538. In the case of NGC 7538, we detected the two main H2CO maser components, and our observations confirm variability of the blueshifted component recently reported by Hoffman et al. The variability of both maser components in NGC 7538 could be caused by a shock wave that reached the redshifted component approximately 14 yr before reaching the blueshifted component. If that were the case, we would expect to detect an increase in the flux density rate of change of the blueshifted component sometime after the year 2009. Our data also support the use of H2CO/H110α observations as a tool to resolve the kinematic distance ambiguity of massive star-forming regions in the Galaxy.

The structure accompanying young star formation

AbstractWe studied the structure of the H2O super maser region in Orion with VLBI angular resolution of 0.1 mas or 0.05 AU. The maser emission (F ∼ 8 MJy) was determined by highly organized structure: accretion disk, bipolar outflow, torus and surrounding shell. The accretion disk, divided into protoplanetary rings, is viewed edge-on. The disk rotates as a rigid body with velocities V ∼ ΩR and the rotation period is ∼ 170 yrs. The highly collimated bipolar outflow has a size of 9×0.7 AU, a velocity of ∼ 10 km/s. In the center a bright compact (≤0.05 AU) source – ejector is located, surrounded by a torus 0.6 AU in diameter. The outflow has a helix structure, which is determined by precession with a period of T ∼ 10 yrs. Comet-like bullets were observed on distances up to 80 AU.

Masers as probes of supersonic turbulence

AbstractA possible intimate connection between astrophysical masers in regions of star formation and turbulence has been a subject of increasing interest during the last two decades. Evidence for the presence of a residual turbulent component in the observed expansion and rotation of clusters of water masers was shown by multi-epoch VLBI maps. The water maser hot spots demonstrate self-similar (fractal) spatial clustering and a power-law two-point velocity correlation function similar to that of incompressible turbulence – with the power index close to “Kolmogorov's” 1/3. The possibility of using maser sources for studying supersonic turbulence critically depends on whether the observed hot spots are an integral effect of radiative transfer over a large distance, comparable to the size of the whole maser source, or whether they are compact local physical objects, such as small random shocks, in which the mechanical energy of turbulence dissipates. If the latter hypothesis is correct, the compact and bright maser hot spots may be excellent local probes of the spatial and kinematic structure of supersonic turbulence. Observational and theoretical arguments for and against these hypotheses are discussed and the first quantitative results about supersonic turbulence obtained by statistical analysis of maser sources are presented in this review.

A review of H2CO 6cm masers in the Galaxy

AbstractWe present a review of the field of formaldehyde (H2CO) 6cm masers in the Galaxy. Previous to our ongoing work, H2CO 6cm masers had been detected in the Galaxy only toward three regions: NGC7538 IRS1, Sgr B2, and G29.96–0.02. Current efforts by our group using the Very Large Array, Arecibo, and the Green Bank Telescope have resulted in the detection of four new H2CO 6cm maser regions. We discuss the characteristics of the known H2CO masers and the association of H2CO 6cm masers with very young regions of massive star formation. We also review the current ideas on the pumping mechanism for H2CO 6cm masers.

Investigating high-mass star formation through maser surveys

AbstractInterstellar masers are unique probes of the environments in which they arise. In studies of high-mass star formation their primary function has been as signposts of these regions and they have been used as probes of the kinematics and physical conditions in only a few sources. With a few notable exceptions, we know relatively little about the evolutionary phase the different maser species trace, nor their location with respect to other star formation tracers. While detailed studies of a small number of maser regions can reveal much about them, other information can only be obtained through large, systematic searches. In particular, such surveys are vital in efforts to determine an evolutionary sequence for the common maser species, and there is growing evidence that methanol masers may trace an earlier phase than the other common maser species of OH and water.

First Detection of an H 2 CO 6 cm Maser Flare: A Burst in IRAS 18566+0408

We report the discovery of a short-duration (less than 3 months) outburst of the H2CO 6 cm maser in IRAS 18566+0408 (G37.55+0.20). During the flare, the peak flux density of the maser increased by a factor of 4; after less than a month, it decayed to the preflare value. This is the first detection of a short, burstlike variability of an H2CO 6 cm maser. The maser shows an asymmetric line profile that is consistent with the superposition of two Gaussian components. We did not detect a change in the velocity or the line width of the Gaussian components during the flare. If the two Gaussian components trace two separate maser regions, then very likely an event outside the maser gas triggered simultaneous flares at two different locations.

A Collimated Jet and an Infalling-Rotating Disk in G192.16−3.84 Traced by H2O Maser Emission

We report on $\mathrm{H}_2 \mathrm{O}$ masers associated with the massive-star forming region G192.16$-$3.84 observed with the new Japanese VLBI network at three epochs spanned for two months, which have revealed the three-dimensional kinematical structure of the whole $\mathrm{H}_2 \mathrm{O}$ maser region in G192.16$-$3.84, containing two young stellar objects separated by ${\sim 1200 \,\mathrm{AU}}$. The maser spatio-kinematical structure has well persisted since previous observations, in which the masers are expected to be associated with a highly collimated bipolar jet and an infalling-rotating disk in the northern and southern clusters of $\mathrm{H}_2 \mathrm{O}$ maser features, respectively. We estimated a jet expansion speed of ${\sim 100 \,\mathrm{km} \,\mathrm{s}^{-1}}$ and re-estimated the dynamical age of the whole jet to be $5.6 \times 10^{4} \,\mathrm{yr}$. We investigated the spatial distribution of the Doppler velocities during the previous and present observations and the relative proper motions of $\mathrm{H}_2 \mathrm{O}$ maser features in the southern cluster, as well as the relative bulk motion between the two maser clusters. They are well explained by a model of an infalling-rotating disk with a radius of $\sim 1000 \,\mathrm{AU}$ and a central stellar mass of 5–10${M}_{\odot}$, rather than by a model of a bipolar jet perpendicular to the observed CO outflow. Based on the derived $\mathrm{H}_2 \mathrm{O}$ maser spatio-kinematical parameters, we discuss the formation mechanism of the massive young stellar objects and the outflow development in G192.16$-$3.84.

Correlation of Mira’s SiO Maser Properties

The v = 0, 1, and 2, J = 1-0 SiO maser spectra of Mira have been observed with high phase resolution over approximately three periods of the star. A time lag cross-correlation analysis of the integrated flux density and the velocity centroid of the emission indicates that the velocity structure varies in a consistent manner with emission during some oscillation periods of the star. Over two periods of the star the v = 1 integrated flux density and velocity centroid were correlated at the 0.67 level. The velocity centroid reached a maximum 0.066 phase earlier than the integrated flux density. These quantities were anticorrelated at the 0.71 level, with the velocity centroid reaching a minimum 0.59 phase earlier than the integrated flux density maximum. The other maser transitions show lower levels of correlation and other phase relationships. The velocity centroids of the spectra have been analyzed and compared with theoretical predictions of shock passage effects. These observations are relevant for kinematic models of the maser region and for maser-pumping models.

Methanol maser polarization in W3(OH)

Abstract We present the first 6.7-GHz methanol maser linear polarization map of the extended filamentary maser structure around the compact H ii region W3(OH). The methanol masers show linear polarization of up to ∼8 per cent and the polarization angles indicate a magnetic field direction along the north–south maser structure. The polarization angles are consistent with those measured for the OH masers, taking into account external Faraday rotation toward W3(OH), and confirm that the OH and methanol masers are found in similar physical conditions. Additionally we discuss the Zeeman splitting of the 6.7-GHz methanol transition and present an upper limit of ∼22 mG for the magnetic field strength in the maser region. The upper limit is fully consistent with the field strengths derived from OH maser Zeeman splitting.

Sensitive Very Long Baseline Interferometry Studies of the OH Megamaser Emission from IRAS 17208−0014

We present phase-referenced VLBI results on the radio continuum and the OH 18 cm megamaser emission from the ultraluminous infrared galaxy, IRAS 17208-0014. The observations were carried out at 1599 MHz using the Very Long Baseline Array, the phased VLA, and the Green Bank Telescope. The highest resolution radio continuum results show several compact sources with brightness temperatures on the order of 106 K. These sources are more likely to be clustered supernova remnants and/or luminous radio supernovae. However, the agreement between the number of observed and expected compact sources above the 5 σ level supports the possibility that each one of the compact sources could be dominated by a recently detonated luminous radio supernova. The continuum results suggest that there are no radio-loud AGNs in the nuclear region of this galaxy. The OH 18 cm megamaser emission in IRAS 17208-0014 is detected at various angular resolutions. It has an extent of 170 pc × 110 pc and is mostly localized in two regions separated by 61 pc. The structure and dynamics of the maser emission seem to be consistent with a clumpy, rotating, ringlike geometry, with the two dominant maser regions marking the tangential points of the proposed rotating-ring distribution. Assuming Keplerian motion for the rotating maser ring, the enclosed dynamical mass and the mass density within a radius of 30.5 pc are about 3 × 107( i) M☉ and 281( i) M☉ pc-3, respectively.

The H2O super maser emission of Orion KL accretion disk, bipolar outflow, shell

The H2O super maser outbursts were observed in Orion KL in active periods 1979-1987 (F ≤ 8 MJy) and 1998-1999 (F ≤ 4 MJy). The line velocity was VLSR = 7.65 km/s and line width ΔV ~ 0.5 km/s. The emission was linear polarized m ≤ 75%. We studied structure of H2O super maser region with VLBI angular resolution 0.1 mas or 0.05 AU. The emission was determined by high organized structure: a chain of bright (Tb ~ 1016K) compact components (~ 0.05 AU), which are distributed along thin S-form structure 27× 0.3 AU, Tb ~ 1011K, Fig. 1. The brightest components have velocities V ~ 7.65 km/s. The components correspond to tangential direction of the rings, velocities of which are V ~ ΩR and rotation period is ~180 yrs. The highly collimated bipolar outflow 9x0.7 AU and comet-like bullets were observed in the quiescent period 1995 (F= 1 kJy) and second activity period 1998-1999. The central compact (0.05 AU) bright (Tb ~ 1016K) source is ejector of bipolar outflow, which surrounded by a torus. Compact bright features are located in the outflow, which velocities are V ~ 10 km/s in the beginning of activity, and V ~ 3 km/s in the end. The helix structure of outflow is determined by precession with period T ~ 10 yrs. The bullets were ejected in the first period activity. Extraordinary changing of the polarization position angle dX/dV ~ 23o/km/s is determined by nozzle emission.

Polarization of H$_\mathsf{2}$O masers in the presence of velocity and magnetic field gradients

Through polarization observations H 2 O masers are excellent probes of magnetic fields in the maser region. Magnetic field strengths, such as those in the H 2 O masers regions of the envelopes of late-type stars and star-forming regions, are typically determined using a direct relation between the field strength and the observed circular polarization. Here it is shown that velocity and magnetic field gradients along the maser have a significant effect on the field strengths obtained from circular polarization observations. Due to velocity gradients the actual magnetic field strength could be up to $100\%$ higher than the field strength derived from the observations. Additionally, when a magnetic field gradient is present, the resulting circular polarization derived is caused predominantly by the average magnetic field in the unsaturated maser core. Measurements of the fractional linear polarization are not affected by velocity or magnetic field strength gradients, though changes in the magnetic field angle along the maser do quench the linear polarization intensity when the maser saturates.

OH Megamasers, Starburst and AGN Activity in Markarian 231

We present MERLIN observations of OH maser and radio continuum emission from the Ultra Luminous IR Galaxy Markarian 231. The 1665- and 1667-MHz transitions have a combined velocity extent of 720 km/s and show a similar position-velocity structure including a gradient of 1.7 km/s/pc from NW to SE along the 420-pc major axis, steeper in the inner few tens of pc. The maser distribution is modelled as a torus rotating about an axis inclined at ~45deg. We estimate the enclosed mass density to be 320(90) Msun in a flattened distribution, including a central unresolved mass of /=1E+05 K, giving a maser gain of </=2.2. The 1667:1665-MHz line ratio is close to the LTE ratio of 1.8 consistent with radiatively pumped, unsaturated masers. The size of individual masing regions is in the range 0.25-4 pc with a covering factor close to unity. There are no very bright compact masers, in contrast to galaxies such as the Seyfert 2 Markarian 273 where the masing torus is viewed nearer edge-on. The comparatively modest maser amplification seen from Markarian 231 is consistent with its classification as a Seyfert 1. Most of the radio continuum emission on 50-500 pc scales is probably of starburst origin but the compact peak is 0.4 per cent polarized by a magnetic field running north-south, similar to the jet direction on these scales. There is no close correlation between maser and continuum intensity. Comparisons with other data show that the jet changes direction close the nucleus and suggest that the sub-kpc disc hosting the masers and starburst activity is severely warped.

Ring shaped 6.7 GHz methanol maser emission around a young high-mass star

We report on EVN imaging of the 6.7 GHz methanol maser emission from the candidate high-mass protostar G23.657-0.127. The masers originate in a nearly circular ring of 127 mas radius and 12 mas width. The ring structure points at a central exciting object which characteristics are typical for a young massive star; its bolometric luminosity is estimated to be <3.2*10^4 L_sun and <1.2*10^5 L_sun for near (5.1 kpc) and far (10.5 kpc) kinematic distances, respectively. However, the spatial geometry of the underlying maser region remains ambiguous. We consider scenarios in which the methanol masers originate in a spherical bubble or in a rotating disc seen nearly face-on.

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.