Context.A low-or intermediate-mass star is believed to maintain a spherical shape throughout the evolution from the main sequence to the asymptotic giant branch (AGB) phase. However, many post-AGB objects and planetary nebulae exhibit non-spherical symmetry. Several candidates have been suggested as factors that can play a role in this change of morphology, but the problem is still not well understood. Magnetic fields are one of these possible agents. Aims. We aim to detect the magnetic field and infer its properties around four AGB stars using H 2 O maser observations. The sample we observed consists of the following sources: the semi-regular variable RT Vir, and the Mira variables AP Lyn, IK Tau, and IRC+60370. Methods. We observed the 6 1,6 −5 2,3 H 2 O maser rotational transition in full-polarization mode to determine its linear and circular polarization. Based on the Zeeman effect, one can infer the properties of the magnetic field from the maser polarization analysis. Results. We detected a total of 238 maser features in three of the four observed sources. No masers were found toward AP Lyn. The observed masers are all located between 2.4 and 53.0 AU from the stars. Linear and circular polarization was found in 18 and 11 maser features, respectively. Conclusions. We more than doubled the number of AGB stars in which a magnetic field has been detected from H 2 O maser polarization. Our results confirm the presence of fields around IK Tau, RT Vir, and IRC+60370. The strength of the field along the line of sight is found to be between 47 and 331 mG in the H 2 O maser region. Extrapolating this result to the surface of the stars, assuming a toroidal field (∝r −1 ), we find magnetic fields of 0.3−6.9 G on the stellar surfaces. If, instead of a toroidal field, we assume a poloidal field (∝r −2 ), then the extrapolated magnetic field strength on the stellar surfaces are in the range between 2.2 and ∼115 G. Finally, if a dipole field (∝r −3 ) is assumed, the field strength on the surface of the star is found to be between 15.8 and ∼1945 G. The magnetic energy of our sources is higher than the thermal and kinetic energy in the H 2 O maser region of this class of objects. This leads us to conclude that, indeed, magnetic fields probably play an important role in shaping the outflows of evolved stars.

Context. The majority of the observed planetary nebulae exhibit elliptical or bipolar structures. Recent observations have shown that asymmetries already start during the last stages of the AGB phase. Theoretical modeling has indicated that magnetically collimated jets may be responsible for the formation of the non-spherical planetary nebulae. Direct measurement of the magnetic field of evolved stars is possible using polarization observations of different maser species occurring in the circumstellar envelopes around these stars. Aims. The aim of this project is to measure the Zeeman splitting caused by the magnetic field in the OH and H 2 O maser regions occurring in the circumstellar envelope and bipolar outflow of the evolved star W43A. We compare the magnetic field obtained in the OH maser region with the one measured in the H 2 O maser jet. Methods. We used the UK Multi-Element Radio Linked Interferometer Network (MERLIN) to observe the polarization of the OH masers in the circumstellar envelope of W43A. Likewise, we used the Green Bank Telescope (GBT) observations to measure the magnetic field strength obtained previously in the H 2 O maser jet. Results. We report a measured magnetic field of approximately 100 μG in the OH maser region of the circumstellar envelope around W43A. The GBT observations reveal a magnetic field strength B || of ∼30 mG changing sign across the H 2 O masers at the tip of the red-shifted lobe of the bipolar outflow. We also find that the OH maser shell shows no sign of non-spherical expansion and that it probably has an expansion velocity that is typical for the shells of regular OH/IR stars. Conclusions. The GBT observations confirm that the magnetic field collimates the H 2 O maser jet, while the OH maser observations show that a strong large-scale magnetic field is present in the envelope surrounding the W43A central star. The magnetic field in the OH maser envelope is consistent with the one extrapolated from the H 2 O measurements, confirming that magnetic fields play an important role in the entire circumstellar environment of W43A.

Context. SiO maser emission occurs in the extended atmosphere of evolved stars and can be studied at high angular resolution. As compact, high-brightness components they can be used as important tracers of the dynamics at distances close to the central star. The masers also serve as probes of the evolutionary path from spherically symmetric AGB stars to aspherical PNe. Very long baseline interferometry (VLBI) observations of Mira variables indicate that SiO masers are significantly linearly polarized with linear polarization fractions up to 100%. However, no information is available at high angular resolution for SiO masers in higher mass loss OH/IR stars. Theory indicates a different SiO pumping mechanism in higher mass loss evolved stars. Aims. We extend the VLBI SiO maser studies to OH/IR stars. The observations enable us to understand the SiO pumping mechanisms in higher mass-loss evolved objects and to compare them with Mira variables. Additionally, polarimetric observations of SiO masers help us to understand the magnetic field strength and morphology and to distinguish between conflicting polarization theories. Methods. The 43 GHz SiO maser observations of the OH/IR star OH 44.8-2.3 were performed with the VLBA in full polarization spectral line mode. Auxiliary EVLA observations were performed to allow for the absolute calibration of the polarization angle. The Zeeman splitting was measured by cross correlating the right and left circular polarization spectra, as well as the S-curve fitting. Additionally, we analyzed the 1612 MHz OH maser observations of OH 44.8-2.3 from the VLA archive. Results. The SiO masers of OH 44.8-2.2 form a ring located at ∼5.4 AU around the star. The masers appear to be highly linearly polarized with fractional linear polarization up to 100%. The linear polarization vectors are consistent with a dipole field morphology in this star. We report a tentative detection of circular polarization of ∼0.7% for the brightest maser feature. The magnetic field measured for this feature corresponds to 1.5 ± 0.3 G. Additionally, the distribution of the 1612 MHz OH maser emission could indicate an elongated morphology. Conclusions. The SiO masers in OH 44.8-2.3 exhibit a ring morphology. Even though the central AGB star of OH 44.8-2.3 is expected to be larger than typical Mira variables, the SiO masers occur at a similar distance from the stellar photosphere as Mira variables. The SiO masers and the 1612 MHz OH maser emission suggest a mildly preferred direction of the outflow in the CSE of this star. Significant linear polarization is measured for the SiO region of this star, which could develop from either collisional or radiative pumping. In either case, the observed polarization is also consistent with magnetic field structures along the preferred outflow direction. This could reflect the possible role of the magnetic field in shaping the circumstellar environment of this object. Although we cannot firmly distinguish between the different polarization theories, the derived magnetic...

Context. Most of the planetary nebulae (PNe) observed are not spherical. The loss of spherical symmetry occurs somewhere between the asymptotic giant branch (AGB) phase and the PNe phase. The cause of this change of morphology is not yet well understood, but magnetic fields are one of the possible agents. The origin of the magnetic field remains to be determined, and potentially requires the presence of a massive companion to the AGB star. Therefore, further detections of the magnetic field around evolved stars, and in particular those thought to be part of a binary system, are crucial to improve our understanding of the origin and role of magnetism during the late stages of stellar evolution. One such binary is the pre-PN OH231.8+4.2, around which a magnetic field has previously been detected in the OH maser region of the outer circumstellar envelope. Aims. We aim to detect and infer the properties of the magnetic field of the pre-PN OH231.8+4.2 in the H 2 O maser region that probes the region close to the central star. This source is a confirmed binary with collimated outflows and an envelope containing several maser species. Methods. In this work we observed the 6 1,6 −5 2,3 H 2 O maser rotational transition to determine its linear and circular polarization. As a result of Zeeman splitting, the properties of the magnetic field can be derived from maser polarization analysis. The H 2 O maser emissions of OH231.8+4.2 are located within the inner regions of the source (at a few tens of AU). Results. We detected 30 H 2 O maser features around OH231.8+4.2. The masers occur in two distinct regions that are moving apart with a velocity on the sky of 2.3 mas/year. Taking into account the inclination angle of the source with the line of sight, this corresponds to an average separation velocity of 21 km s −1 . Based on the velocity gradient of the maser emission, the masers appear to be dragged along the direction of the nebula jet. Linear polarization is present in three of the features, and circular polarization is detected in the two brightest features. The circular polarization results imply a magnetic field strength of |B || | ∼ 45 mG. Conclusions. We confirm the presence of a magnetic field around OH231.8+4.2, and report the first measurements of its strength within a few tens of AU of the stellar pair. Assuming a toroidal magnetic field, this imples B ∼ 2.5 G on the stellar surface. The morphology of the field is not yet determined, but the high scatter found in the directions of the linear polarization vectors could indicate that the masers occur near the tangent points of a toroidal field.