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When low- and intermediate-mass stars pass through the Asymptotic Giant Branch (AGB) they experience dramatic changes in their circumstellar shell (CSE) influenced by their mass loss, the possible presence of a (closeby) companion and the magnetic field. Masers, well spread in this environment, provide a powerful tool to reveal the CSE changes occurring when the stars undergo a transitional phase on the AGB. These can be indirect, via for instance the modification of the pumping conditions or a direct consequence of e.g. a companion and/or of the magnetic field. Evidences of such changes have been observed towards Miras, materialized by strong - both in intensity and degree of polarisation - (OH) flaring events and towards stars believed to be transitioning from the Mira to the OH/IR phase, showing an unusual high degree of polarisation. How OH maser emission can be used as a signpost of transitional phases along the AGB is explored. [ABSTRACT FROM AUTHOR]

We present high-angular resolution observations of OH maser emission towards the high-mass star-forming region IRAS 06056+2131. The observations were carried out using the UK radio interferometer array, Multi-Element Radio Linked Interferometer Network (MERLIN) in the OH main lines at 1665 and 1667 MHz, in addition to the OH satellite line at 1720 MHz. The results of this study reveal the small upper limit to the size of emission in the 1665-MHz line with an estimated total intensity of ∼4 Jy. We did not detect any emission from the 1667 and 1720-MHz lines. The full polarization mode of MERLIN enables us to investigate the magnetic field in the OH maser region. In this transition, a Zeeman pair is identified from which a magnetic strength of ∼−1.5 mG is inferred. Our results show that IRAS 06056+2131 is highly polarized, with ∼ 96 |${{\ \rm per\ cent}}$| circular polarization and ∼6 |${{\ \rm per\ cent}}$| linear polarization. The linear polarization angle is ∼29°, implying a magnetic field which could be aligned with the outflow direction detected towards this region, but the actual magnetic field direction has an uncertainty of up to 110° due to the possible effects of Faraday rotation. The star-forming evolutionary status of the embedded protostellar object is discussed. [ABSTRACT FROM AUTHOR]

Aims. Polarisation observations of circumstellar dust and molecular (thermal and maser) lines provide unique information about dust properties and magnetic fields in circumstellar envelopes of evolved stars. Methods. We use Atacama Large Millimeter/submillimeter Array (ALMA) Band 5 science verification observations of the red supergiant VY CMa to study the polarisation of SiO thermal/maser lines and dust continuum at ∼1.7 mm wavelength. We analyse both linear and circular polarisation and derive the magnetic field strength and structure, assuming the polarisation of the lines originates from the Zeeman effect, and that of the dust originates from aligned dust grains. We also discuss other effects that could give rise to the observed polarisation. Results. We detect, for the first time, significant polarisation (∼3%) of the circumstellar dust emission at millimeter wavelengths. The polarisation is uniform with an electric vector position angle of ∼8 ◦ . Varying levels of linear polarisation are detected for the J = 4−3 28SiO v = 0, 1, 2, and 29SiO v = 0, 1 lines, with the strongest polarisation fraction of ∼30% found for the 29SiO v = 1 maser. The linear polarisation vectors rotate with velocity, consistent with earlier observations. We also find significant (up to ∼1%) circular polarisation in several lines, consistent with previous measurements. We conclude that the detection is robust against calibration and regular instrumental errors, although we cannot yet fully rule out non-standard instrumental effects. Conclusions. Emission from magnetically aligned grains is the most likely origin of the observed continuum polarisation. This implies that the dust is embedded in a magnetic field >13 mG. The maser line polarisation traces the magnetic field structure. The magnetic field in the gas and dust is consistent with an approximately toroidal field configuration, but only higher angular resolution observations will be able to reveal more detailed field structure. If the circular polarisation is due to Zeeman splitting, it indicates a magnetic field strength of ∼1–3 Gauss, consistent with previous maser observations. [ABSTRACT FROM AUTHOR]

We present the analysis of the onset of the new 2010s OH flaring event detected in the OH ground-state main line at 1665 MHz towards o Ceti and compare its characteristics with those of the 1990s' flaring event. This is based on a series of complementary single-dish and interferometric observations both in OH and H2O obtained with the Nançay Radio telescope, the Medicina and Effelsberg Telescopes, the European VLBI Network and (e)Multi-Element Radio Linked Interferometer Network. We compare the overall characteristics of o Ceti's flaring events with those that have been observed towards other thin-shell Miras, and explore the implication of these events with respect to the standard OH circumstellar-envelope model. The role of binarity in the specific characteristics of o Ceti's flaring events is also investigated. The flaring regions are found to be less than ~400 ± 40 mas (i.e. ≤40 ± 4 au) either side of o Ceti, with seemingly no preferential location with respect to the direction to the companion Mira B. Contrary to the usual expectation that the OH maser zone is located outside the H2O maser zone, the coincidence of the H2O and OH maser velocities suggests that both emissions arise at similar distances from the star. The OH flaring characteristics of Mira are similar to those observed in various Mira variables before, supporting the earlier results that the regions where the transient OH maser emission occurs are different from the standard OH maser zone. [ABSTRACT FROM AUTHOR]

ABSTRACT MERLIN phase-referenced polarimetric observations towards the W51 complex were carried out in 2006 March in the Class II methanol maser transition at 6.668 GHz and three of the four excited OH maser hyperfine transitions at 6 GHz. Methanol maser emission is found towards both W51 Main and South. We did not detect any emission in the excited OH maser lines at 6.030 and 6.049 GHz down to a 3σ limit of ∼20 mJy beam−1. Excited OH maser emission at 6.035 GHz is only found towards W51 Main. This emission is highly circularly polarized (typically ≥45 per cent and up to 87 per cent). Seven Zeeman pairs were identified in this transition, one of which contains detectable linear polarization. The magnetic field strength derived from these Zeeman pairs ranges from +1.6 to +6.8 mG, consistent with the previously published magnetic field strengths inferred from the OH ground-state lines. The bulk of the methanol maser emission is associated with W51 Main, sampling a total area of ∼3 × 2.2 arcsec2 (i.e. ∼16 200 × 11 900 au), while only two maser components, separated by ∼2.5 arcsec, are found in the W51 South region. The astrometric distributions of both 6.668-GHz methanol and 6.035-GHz excited-OH maser emission in the W51 Main/South region are presented here. The methanol masers in W51 Main show a clear coherent velocity and spatial structure with the bulk of the maser components distributed into two regions showing a similar conical opening angle with a central velocity of ∼+55.5 km s−1 and an expansion velocity of ≤5 km s−1. The mass contained in this structure is estimated to be at least 22 M⊙. The location of maser emission in the two aforementioned lines is compared with that of previously published OH ground-state emission. The association with the ultra-compact H ii regions in the W51 Main/South complex and the relationship of the masers to infall or outflow in the region are discussed. [ABSTRACT FROM AUTHOR]

We present the first view of the magnetic field structure in the OH shell of the extreme OH/infrared star OH 26.5+0.6. Multi-Element Radio-Linked Interferometer Network (MERLIN) interferometric observations of this object were obtained in 1993 December in full polarization, at 1612, 1665 and 1667 MHz. The maser spots show a spheroidal distribution at both 1612 and 1667 MHz, while at 1665 MHz emission from the blueshifted maser peak is concentrated on the stellar position and the redshifted peak emission exhibits a filamentary structure oriented on a south-east–north-west axis. The linear polarization in both main lines is rather faint, ranging from 9 to 20 per cent at 1665 MHz and from 0 to 30 per cent at 1667 MHz. At 1612 MHz, most maser spots exhibit a similar range of linear polarization although those in the outermost parts of the envelope reach values as high as 66 per cent. This is particularly apparent in the southern part of the shell. The detailed distribution of the polarization vectors could only be obtained at 1612 MHz. The polarization vectors show a highly structured distribution indicative of a poloidal magnetic field inclined by 40°–60° to the line of sight. The velocity distribution of the maser spots with respect to the radial distance is well explained by an isotropic outflow at constant velocity in the case of a prolate-shaped spheroid envelope, also tilted about 45°–65° to the line of sight. [ABSTRACT FROM AUTHOR]

Multi-Element Radio-Linked Interferometer Network polarization measurements of the OH 1.7- and 6.0-GHz, and the H2O 22-GHz masers in the massive star-forming region S140-IRS1 are presented. The observations were taken with astrometric accuracy of 15–30 mas. OH maser emission has been detected only in the ground-state 1665-MHz line. The OH masers are located approximately 2.5 arcsec north-west of IRS1. They lie in two distinct regions separated by approximately 1 arcsec along a north-east/south-west axis. Analysis of the OH maser polarization yields a magnetic field of mG, directed towards us. The polarization position angles attest to a magnetic field which is preferentially orientated along the direction of the CO outflow associated with IRS1. The brightest H2O masers coincide in position with IRS1 and show a gradient of increasing Doppler velocity from north-west to south-east. These water masers also have proper motions of ∼18 km s−1 in a similar direction. The closest OH and H2O masers are separated by more than 2 arcsec. H2O masers have also been detected at several locations up to ≈0.5 arcmin distance, where masers have not previously been reported. The relationships between our maser observations, the central engine/object of IRS1 and the multiple outflows detected by observations at other wavelengths are also discussed. [ABSTRACT FROM AUTHOR]

Rapidly-evolving red supergiants (RSG) lose half or more of their mass before ending their lives as supernovae. Masers allow us to study the mass loss from 4 nearby RSG in AU-scale detail using MERLIN and EVN/global VLBI. The water maser clouds are over-dense and over-magnetised with respect to the surrounding wind. In most cases, the brighter an individual maser component is the smaller its apparent (beamed) FWHM appears, as predicted for approximately spherical clouds. Individual water maser features have a typical half-life of 5-10 yr, but comparison with single dish monitoring suggests that the water vapour clouds themselves survive many decades (the water maser shell crossing time), within which the local masers wink on and off. OH mainline masers are found in the tenuous surrounding gas, overlapping the water maser shell, surrounded by OH 1612-MHz masers at a greater distance from the star. [ABSTRACT FROM PUBLISHER]

We present single-baseline Multi-Element Radio-Linked Interferometer Network (MERLIN) measurements of excited OH 6.0-GHz masers and methanol 6.7-GHz masers for the source W3(OH). These allow us to compare the positions of individual maser spots of these two species to∼15 mas accuracy for the first time, and to compare these with previously published positions of ground-state OH masers near 1.7 GHz and excited-state OH masers near 4.7 GHz. There is a strong association between OH 6035-MHz and 1665-MHz masers. OH and methanol have very similar distributions, but associations of individual masers are relatively rare: most methanol 6.7-GHz masers are within 100 mas of OH 6.0-GHz masers, but only four methanol masers are within 15 mas of an OH 6.0-GHz maser. There are no correspondences of either species with excited OH 4.7-GHz masers. Zeeman splitting of the 6.0-GHz OH lines indicates an ordered magnetic field ranging from 3.2 to 14.4 mG. The magnetic fields estimated from co-propagating masers such as 6035 and 1665 MHz are generally in good agreement with each other. [ABSTRACT FROM AUTHOR]