We investigate the development of bipolar outflows during the early post-AGB evolution. A sample of ten OH/IR stars is observed at high angular resolution, including bipolar nebulae (OH231.8+4.2), bright post-AGB stars (HD 101584) and reflection nebulae (e.g. Roberts 22). The IRAS colour--colour diagram separates the sample into different types of objects. One group may contain the progenitors to the (few) extreme bipolar planetary nebulae. Two objects show colours and chemistry very similar to the planetary nebulae with late IR-[WC] stars. One object is a confirmed close binary. A model is presented consisting of an outer AGB wind which is swept up by a faster post-AGB wind, with either wind being non-spherically symetric. The interface of the two winds is shown to exhibit a linear relation between velocity and distance from the star. The OH data confirms the predicted linear velocity gradients, and reveals torus-like, uniformly expanding components. All sources are discussed in detail using optical/HST images where available. ISO data for Roberts 22 reveal a chemical dichotomy, with both crystalline silicates and PAHs features being present. IRAS 16342-3814 shows a dense torus; HST data shows four point-like sources located symmetrically around the nebula, near the outer edge of the dense torus. Lifetimes for the bipolar OH/IR stars are shown to be in excess of 10^4 yr, longer than normal post-AGB timescales. This suggests that the disks are near-stationary. We suggest that accretion from such a disk slows down the post-AGB evolution. Such a process could explain the link between the long-lived bipolar nebular geometry and the retarded star.Comment: 31 pages, LaTeX using mn.sty. MNRAS, accepted for publicatio

MERLIN polarization measurements of 1665‐,1667‐ and 1720‐MHz OH masers in the bipolar outflow source W75N are presented. The OH masers are distributed over a region 5 arcsec across (10 000 au at 2 kpc), which is elongated in a north‐west–south‐east (NW–SE) direction. The distribution is interpreted in terms of a rotating disc orthogonal to the bipolar outflow that is seen in CO and H2. A compact cluster of OH masers with a large velocity spread appears to mark the source of the outflow. Most of the OH masers detected have strong circular polarization up to 100 per cent. Seven Zeeman groups are identified, including the first example of a complete Zeeman triplet (two σ‐components and one π‐component). The field strength measured from Zeeman pairs of opposite circular polarization ranges from −8 to +8 mG, with a field reversal on opposite sides of the disc. This is interpreted in terms of a toroidal component of the magnetic field in the disc. Linear polarization is also detected in many of the OH masers with some 1665‐MHz masers showing up to 100 per cent linear polarization. The polarization position angles are preferentially along the direction of the bipolar outflow or perpendicular to it. The polarization data lend support to the twisted magnetic field model originally proposed by Uchida & Shibata.

Abstract. We present MERLIN observations of OH, water and methanol masers towards the young high mass stellar object IRAS 20126+4104. Emission from the 1665-MHz OH, 22-GHz H 2 O and 6.7-GHz CH 3 OH masers is detected and all originates very close to the central source. The OH and methanol masers appear to trace part of a circumstellar disk around the central source. The positions and velocities of the OH and CH 3 OH masers are consistent with Keplerian rotation around a central mass of ∼5 M . The water masers are offset from the OH and CH 3 OH masers and have significantly changed since they were last observed, but still appear to be associated to the outflow from the source. All the OH masers components are circularly polarised, in some cases reaching 100 percent while some OH components also have linear polarisation. We identify one Zeeman pair of OH masers and the splitting of this pair indicates a magnetic field of strength ∼11 mG within ∼0.5 (850 AU) of the central source. The OH and CH 3 OH maser emission suggest that the disk material is dense, n > 10 6 cm −3 , and warm, T > 125 K and the high abundance of CH 3 OH required by the maser emission is consistent with the evaporation of the mantles on dust grains in the disk as a result of heating or shocking of the disk material.

This paper presents observations of ultralow‐frequency (ULF) waves from Van Allen Probes. The event that generated the ULF waves occurred 2 days after a minor geomagnetic storm during a geomagnetically quiet time. Narrowband pulsations with a frequency of about 7 mHz with moderate amplitudes were registered in the premidnight sector when Probe A was passing through an enhanced density region near geosynchronous orbit. Probe B, which passed through the region earlier, did not detect the narrowband pulsations but only broadband noise. Despite the single‐spacecraft measurements, we were able to determine various wave properties. We find that (1) the observed waves are a second harmonic poloidal mode propagating westward with an azimuthal wave number estimated to be ∼100; (2) the magnetic field fluctuations have a finite compressional component due to small but finite plasma beta (∼0.1); (3) the energetic proton fluxes in the energy ranging from above 10 keV to about 100 keV exhibit pulsations with the same frequency as the poloidal mode and energy‐dependent phase delays relative to the azimuthal component of the electric field, providing evidence for drift‐bounce resonance; and (4) the second harmonic poloidal mode may have been excited via the drift‐bounce resonance mechanism with free energy fed by the inward radial gradient of ∼80 keV protons. We show that the wave active region is where the plume overlaps the outer edge of ring current and suggest that this region can have a wide longitudinal extent near geosynchronous orbit.

The past decade has seen tremendous growth in the study of cosmic maser sources,. Radio interferometers have fully resolved the structure of the sources on all angular scales, and have enabled large numbers of sources to be studied in detail for the first time. We now have a far clearer picture of the disposition of hydroxyl, water and silicon monoxide masers in star-forming regions and in circumstellar envelopes than was possible ten years ago. There have also been major new developments, including the discovery of extremely powerful megamasers in the nuclei of distant galaxies, the discovery of strong 12 GHz methanol masers in star-forming regions and the discovery of widespread 89 GHz hydrogen cyanide masers in circumstellar envelopes. Large numbers of new maser sources have been found using the all-sky infrared survey by the IRAS satellite to provide candidates; Maser sources of different types can be identified by their characteristic infrared emission. IRAS follow-up surveys have also revealed new types of circumstellar maser associated with stars in unusual stages of their evolution.On the theoretical front there have been major advances in our understanding of saturation and competitive gain. New mechanisms for producing polarisation in saturated masers have been found, and new pump mechanisms have been proposed for several maser lines, including the first satisfactory pump for strong water masers. This review summarises these and other important developments, concentrating mainly on the new observations and the new types of astrophysical investigation which these have made possible. The use of masers for astronomical distance measurements is also discussed.

Context. Masers are important tracers of the early evolution of young high mass stars, but the relationship between different types of maser and the evolutionary state of the exciting source remains unclear. Aims. To determine whether OH masers are common towards candidate high mass protostellar objects. Methods. We present a survey of OH maser emission towards a sample of high mass protostellar objects made using the Nançay and GBT telescopes. Results. OH maser emission was detected towards 63 objects with 36 new detections. There are 56 star-forming regions and 7 OH/IR candidates. Nearly half of the detected sources have OH flux densities < ∼ 1 Jy. There is no evidence that sources with OH masers have a different range of luminosities from the non-maser sources. The results of this survey are compared with previous H 2 O and class II CH 3 OH maser observations of the same objects. Some of the detected sources are only associated with OH masers and some sources are only associated with the 1720 MHz OH maser line. The velocity range of the maser emission suggests that the water maser sources may be divided into two groups. The detection rates and velocity range of the OH and Class II CH 3 OH masers support the idea that there is a spatial association of the OH and Class II CH 3 OH masers. The sources span a wide range in R, the ratio of the methanol maser peak flux to OH 1665 MHz maser peak flux, however there are only a few sources with intermediate values of R, 8 < R < 32, which has characterised previous samples. The majority of the sources are either methanol-favoured or OH-favoured. Sources which have masers of any species, OH, water or methanol, have redder [100 µm-12 µm] IRAS colours than those without masers. However, there is no evidence for different maser species tracing different stages in the evolution of these young high mass sources. Conclusions. The detection of OH masers towards 26% of a sample of 217 sources should remove any doubt about the existence of OH maser emission towards these objects or this early evolutionary stage. Previous observations which have shown that the OH maser emission from similar sources traces the circumstellar disks around the objects. This combined with the sensitivity of the OH emission to the magnetic field, make the newly detected sources interesting candidates for future follow-up at high angular resolution.

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MERLIN polarization measurements of the OH 1665‐MHz maser emission from the bipolar outflow source G 35.2‐0.74N are presented. The masers have an elongated distribution, and a velocity gradient which is interpreted in terms of a disc surrounding the source of the outflow. The radius of the maser disc is 1.3 arcsec (2600 au at 2 kpc). Analysis of the OH maser polarization reveals four Zeeman pairs, with field strengths ranging from ‐2.5 to +5.3 mG. The direction of the field reverses on opposite sides of the disc. Linear polarization is found in five of the masers, and is used to infer the three‐dimensional magnetic field direction at these points. The data lend support to the hydromagnetic model proposed by Uchida & Shibata.