VLBA SiO maser observations of the OH/IR star OH 44.8-2.3: magnetic field and morphology

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 similar to 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 similar to 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 field strength assuming standard Zeeman emission is fully consistent with other maser polarization measurements.