We briefly report the results of polarimetric observations of SiO emission from late-type stars taken with a millimeter polarimeter installed at the Nobeyama 45m telescope. We found that the SiO J = 2 − 1 emission in the v = 0 state from R Cas contains a highly linearly polarized component, followed by the detection of highly linearly polarized emission of the same transition from VY CMa (Shinnaga et al. 1999). The high polarization degree is a strong evidence that the v = 0 emission originates from maser action. We found out that, not only a peculiar supergiant star, but also a Mira variable star is associated with the SiO v = 0 maser.

We present 86 GHz (v = 1, J = 2 → 1) SiO maser line observations with the 30-m IRAM telescope of a small sample of late-type stars in the inner Galaxy. The stars were selected on basis of their infrared magnitudes and colors, and our first results show an instantaneous detection rate of ∼65%. The derived SiO maser fluxes and line-of-sight velocities indicate that the stars are located in the Galactic Bulge. The goals of the project are to study the relation between 86 GHz SiO maser emission and infrared properties of the stars, and to enlarge the number of known stellar line-of-sight velocity measurements. Combining the detections with measurements from other surveys (e.g. OH maser surveys) will allow a kinematic analysis of stellar orbits, and probe the dynamics of the inner Galaxy and its probable triaxial mass distribution.

We have performed VLBA observations of the SiO v = 1 and v = 2 J = 1-0 masers in two AGB stars, TX Cam and IRC +10011. We confirm the ring-like spatial distribution, previously found in several AGB objects, as well as the tangential polarization pattern, already reported for TX Cam. Both properties, that seem to be systematic in this kind of objects, are characteristic of radiatively pumped SiO masers. On the contrary, we do not confirm the previous report on the spatial coincidence between the J = 1-0 v = 1 and 2 masers, a result that would have argued in favor of collisional pumping. We find that both lines sometimes arise from nearby spots, typically separated by 1-2 mas, but are rarely coincident. The discrepancy with previous results is explained by the very high spatial resolution of our observations, ∼ 0.5 mas, an order of magnitude better than in the relevant previously published experiment. Moreover, we have been able to measure a probable rotation of the inner shell of a few km/s. Rotation of circumstellar shells is assumed by the most convincing models explaining the drastic change of symmetry between the AGB envelopes (spherical symmetry) and Proto Planetary Nebulae (axial symmetry).

The Orion-KL nebula is the closest (450pc) site of high-mass star formation and exhibits powerful outflows associated with protostars. It is also one of only three known star forming regions to exhibit SiO maser emission. Emission in three SiO maser transitions (v=1 J=1 → 0, v=1 J=2 → 1, and v=2 J=1 → 0) imaged by VLBI exhibits an “X” morphology suggesting that the Orion masers form along the outlines of two opposing conical outflows to the NW and SE. At the center of this “X”, VLA observations find emission from an HII region presumably associated with a young star whose wind drives the outflow. The SiO masers probably form along the interface between the stellar wind and surrounding parent cloud. We find that SiO maser emission from different transitions preferentially occurs at different radii from the central star implying that the masers are tracers for physical conditions in the wind-cloud interaction region. On the smallest scales, some individual maser features in each transition overlap both spatially and in velocity providing strong evidence that more than one transition can mase within the same volume of gas.

We have made a blind survey of SiO masers, of the J=2-1 v = 1 transition, toward 106 bright OH/IR stars which have been previously detected in the OH main line at 1662 MHz with the ATCA in the galactic bulge by Sevenster et al (2000) and in the galactic disk at J=1-0 (v = 1 − 2) line by Miyazaki (2001). A comparison with published data shows that new SiO masers are detected for OH/IR stars in the galactic center, suggesting that those stars are undergoing strong mass loss with dense circumstellar envelopes with apparent angular sizes far smaller than the OH maser shell as obtained by interferometric observations. At the moment, our detection rate is about 23% and it may be increased when the project is completed.

We have measured significant proper motions using the water maser clouds in the outflows from RT Vir (6 epochs) and VX Sgr (2 epochs). In both cases proper motions are measured in the bright emission found perpendicular to the OH emission. This strongly suggests latitude dependent mass-loss with the water masers tracing a dense equatorial outflow and the OH emission tracing a less dense polar outflow. In both cases the rotational velocities are < 1 km s−1 thus ruling out a strongly rotating circumstellar envelope. This suggests the outflow is not shaped by an orbiting companion. In the case of VX Sgr the proper motions are contained in a wedge perpendicular to the measured magnetic field axis, thus strongly suggesting that the magnetic field is helping to shape the mass outflow.

The proper motion results have thrown up a puzzle. For VX Sgr the material is being radially accelerated and material enters the water maser zone at 4 km s−1. This is consistent with the mass loss model of Bowen (1988). However the RT Vir proper motion results show no such radial acceleration. The masers have already received their acceleration before they reach the water maser zone at 3-AU (our numerical models seem to support the small radius). This was probably done in the pulsation zone. The acceleration mechanism is unclear for this source.

During the red supergiant (RSG) stage of massive star evolution, emission from dust and molecules allows the copious stellar winds to be studied in great detail. This help us understand not only the evolutionary stages of the star (which are highly dependent on mass loss rates), but also the morphology of the eventual supernova remnant. Maser emission from OH and H2O has been mapped with milli-arcsec resolution (using MERLIN and the EVN/global VLBI) around RSG including VY CMa, S Per and VX Sgr. The H2O masers originate in clouds accelerating away from the star and OH mainlines masers interleave the outer parts of the H2O maser shell. Zeeman splitting of OH maser lines reveals the orientation and strength of stellar-centred magnetic fields.

Over the past decade we observed a sample of late-type stars (supergiants, semi-regular variables, OH/IR stars, and Mira's) 3-4 times per year in the 1.3-cm line of H2O. The observations were carried out with the Medicina 32-m and the Effelsberg 100-m telescopes. In addition, a sub-sample of these stars was observed at several epochs with the VLA. In our analysis we also use data from the literature, for instance on mass loss and optical variability. The aim is to investigate the properties of the circumstellar outflows and to elucidate the maser-pumping mechanism. In the conference poster we presented some preliminary results of a 4-star sub-set of the data base; in the small space available here, we briefly describe the monitoring program, and discuss the Mira-variable R Cas in some detail.

The SEST telescope has been used for a submillimeter water maser survey toward late-type stars. Six new 321 GHz water masers were detected. These, as well as the earlier reported sources, VY CMa and o Cet, were also observed in the vibrationally excited water maser line at 96 GHz. However, only VY CMa and o Cet showed the 96 GHz emission. The line velocity of o Cet is blueshifted, suggesting that the hot water vapor at 96 GHz takes part in the pulsation-shock motion of the Mira variable.

We present here some noteworthy results of two related studies on oxygen-rich late type stars. The aim of this work was to study the OH circumstellar shell properties in terms of evolution. These studies are based on an OH monitoring programme carried out with the Nançay Radio Telescope. The first study concerns seven Miras distributed along the colour-colour diagram. They were observed at two or three different epochs covering one to seven cycles over a period from 1980 to 1995 at 1612, 1667 and 1665 MHz in both circular polarizations. The second study concerns thirty objects covering a wide range of mass loss rate from Miras to OH/IR stars. They were observed in 1994 at 1665 & 1667 MHz in both circular polarizations.

Drifts in radial velocities of OH maser features of the three semiregular variable stars R Crt, RT Vir and W Hya have been analyzed. The radial velocities of the extreme blue- and red-shifted features drifted by about 0.2 kms−1 to 1 km s−1 on time-scales of few months to several years. In the case of W Hya a blending effect of spectral features can account for those drifts. In the two other sources the velocity drifts are likely to be intrinsic to the clumps for which the velocities of peak emission vary due to appearance or disappearance of maser emission in the regions where circumstellar matter is still accelerated.

Using VLBI phase referencing, we have been registering the position of the most blue-shifted OH 1667 MHz maser spot with respect to nearby extragalactic reference sources for almost seven years. The data for these 10 epochs allow us to determine the proper motion and parallax of U Her. It also yields the opportunity to compare the maser position to the position of the star, as measured by the Hipparcos satellite. Results support the theory that the compact bright spot is the amplified stellar image.

The ATCA/VLA Galactic Plane survey detected OH 1612 MHz maser emission from a total of 766 sources. In most cases the detected sources have double-peaked spectral profiles which are characteristic of OH/IR stars. A small number of sources however have irregular spectra and uncertain classifications. We discuss the maser results for two sources with irregular OH spectra, D046 (OH 326.5-0.4) and B292 (OH009.1-0.4). For B292 we detected OH 1720 MHz emission - a transition previously unknown for evolved stars. D046 has exceptionally broad maser profiles and strong non-thermal radio continuum emission. Both are likely to be bipolar post-AGB sources.

Post-AGB sources can be identified from their MSX far-infrared properties. For the ATCA/VLA OH-selected sample, most of the likely post-AGB stars do not have unusual OH 1612 MHz spectra.

In the last years various OH surveys at 1612 MHz have been carried out in the galactic bulge region with high spatial resolution (Lindqvist et al. 1992; Sevenster et al. 1997; Sjouwerman et al. 1998) which have resulted in the detection of over five hundred double-peaked objects commonly associated with oxygen-rich AGB stars. More recently, the ISOGAL survey, which is part of the ISO mission, scanned several regions along the galactic plane, including the inner bulge, at 6.8 and 15 micron wavelengths. In this work we report an analysis of 104 OH/IR stars in the bulge, based on near and mid-infrared observations. Mass-loss rates, luminosities and masses are determined for a sub-sample of stars, for which near-infrared data are available in the literature.

We present high resolution observations of HCN(J=1-0) line in the bright carbon star Y CVn. With an angular resolution of 0.5 arcsec we are able to resolve the HCN envelope around Y CVn. The HCN(J=1-0) line profile toward the central position exhibits several strong spikes having brightness temperature in excess of 1000 K. This result indicates that the J=1-0 line is masing. We also discuss the pump mechanism of this maser and the implication for the determination of the HCN abundance.

We performed the first VLBI observations of the SiO v = 1 and v = 2 J = 1-0 masers in a Proto Planetary Nebula OH 231.8+4.2 (also known as the Rotten Egg nebula), at milliarcsecond resolution. Only the v=2 maser transition was detected. We detect several maser spots lying along a line which is almost perpendicular to the axis of symmetry of the Nebula. We find that all the emission is concentrated in an area of ∼ 10 mas, indicating that the SiO masers are originated very close to the surface of the star. One of the two emission areas presents an elongated structure with a clear velocity gradient. The detected emission is consistent with a torus, or disk, in rotation with a velocity of ∼ 6 km/s and with an infall velocity of ∼ 10 km/s.

We present Very Large Array (VLA) observations of H2O and OH (1612 MHz) maser emission toward the protoplanetary nebula OH 231.8+4.2. The H2O observations show two features at VLSR of 23.1 and 44.5 km s−1, coinciding in position within error with the SiO (v=1; J=2−1) maser position from Sánchez Contreras et al. (2000). The H2O and SiO masers are most likely tracing the position of QX Pup, the Mira near the center of OH 231.8+4.2. This position is, however, clearly offset by ∼1 from the axis of the bipolar outflow traced by the OH maser emission. These results suggest the presence of a binary system: one invisible star powering the bipolar outflow and the other, the Mira star QX Pup, associated with the H2O and SiO masers. This scenario requires that the two stars have evolved at very similar rates.

One of the most exciting challenge facing theories of post-main sequence evolution today is to understand how Asymptotic Giant Branch (AGB) stars transform themselves into aspherical planetary nebulae (PNe). Recently, high-resolution imaging surveys of young planetary nebulae and protoplanetary nebulae (PPNe: objects in transition between the AGB and PN phases) have revealed that the majority of these objects are characterised by multipolar bubbles distributed roughly point-symmetrically around the central star. These data strongly suggest that the current model for the shaping of PNe is no longer adequate. High angular-resolution kinematic information is sorely needed to complement the imaging data in order to test new hypotheses, such as our proposal that episodic high-speed jet-like outflows, operating during the protoplanetary or very late-AGB phase, are the primary agent.

We have therefore begun a program of using interferometric mapping of OH (and H2O, when feasible) maser emission in order to trace the kinematics of the structures discovered in protoplanetary nebulae with HST. These masers provide a unique and crucial probe of the kinematics of the circumstellar material in PPNe, because of the lack of other emission-line diagnostics. Although our work is still in its infancy (only two objects have been studied in detail), we find that the OH masers indicate the presence of multiple low-latitude outflows and an increase of outflow velocity with latitude. This paper summarises our progress so far, the state of current studies, and future prospects.

This paper reviews observational studies of maser polarization, with particular reference to observational results reported in the intervening eight years since the last maser conference (Clegg & Nedoluha 1993). The scientific role of maser polarization observations is discussed and the theoretical interpretation of such data is considered. Recent observational results are presented for several common maser transitions, including OH, H2O, and SiO.

Full-polarization spectral-line VLBA observations were made of the ground state, main-line, 2π3/2J = 3/2 OH masers in two Galactic star-forming regions: the “e1”/ “e2” region of W51 and G351.78-0.54. Two especially interesting results are presented. (1) Two of the 27 Zeeman pairs in W51 were found to be associated with unusually strong magnetic fields (≈ 20 mG), more than twice as strong as for any previously reported OH maser. (2) G351.78-0.54 was found to exhibit a high degree of linear polarization (up to 61%), which constrains the three dimensional structure of its magnetic field.