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

Abstract

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...

Linear polarization of submillimetre masers

“…Alves et al 2012;Amiri et al 2012;Leal-Ferreira et al 2012;Vlemmings et al 2011Vlemmings et al , 2006b). Both single-dish and interferometric observations have revealed that silicon monoxide (SiO), water (H 2 O), hydrogen cyanide (HCN), hydroxyl (OH), and methanol (CH 3 OH), among others, can naturally generate polarized maser emission in these enviroments (e.g.…”

Maser polarization with ALMA

“…Observationally, fractional linear polarization of up to 100% for J=1-0 SiO maser transitions (Amiri et al 2012) and values over 33% for SiO molecular transitions involving higher angular momentum states (e.g. Vlemmings et al 2011) have been detected.…”

“…To explain the high levels of fractional linear polarization detected in some cases (specifically SiO masers, e.g. Amiri et al 2012, Vlemmings et al 2011 other physical processes that can produce polarized maser radiation need to be considered. Anisotropic pumping can, for example, enhance the fractional linear polarized maser radiation that is produced due to a magnetic field permeating the masing region, towards SFR or CSE of late-type stars (Vlemmings et al 2011, Nedoluha & Watson 1990.…”

Simultaneous VLBA polarimetric observations of the v = J = 1-0 and v =1, J = 2-1 SiO maser emission towards VY CMa: maser morphology and pumping

“…Similarly, circular polarisation studies of SiO masers have also been used to derive magnetic field estimates in the circumstellar envelopes of evolved stars (e.g. Barvainis et al 1987;Kemball & Diamond 1997;Amiri et al 2012).…”