The kinematics and magnetic fields in water-fountain sources based on OH maser observations

Kemball

et al. 2012

A&A

Self Cite

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...

Section: Cse Morphology and Magnetic Fieldmentioning

confidence: 99%

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

Amiri

Kemball

et al. 2012

A&A

Self Cite

“…These studies have revealed that magnetic fields are present throughout the entire envelope. The SiO, H 2 O, and OH maser observations indicate that magnetic fields appear well ordered and the Zeeman splitting indicates the field strengths range from several Gauss close to the stellar surface to several mG at a few thousand AU (e.g., Etoka & Diamond 2004;Vlemmings et al 2005Vlemmings et al , 2011Herpin et al 2006;Kemball et al 2009;Amiri et al 2011). However, masers probe only a limited number of lines of sight through the CSE and in most cases it is thus impossible to fully reconstruct the magnetic field morphology throughout the envelope.…”

Section: Introductionmentioning

confidence: 99%

Mapping the Linearly Polarized Spectral Line Emission Around the Evolved Star Irc+10216

Girart

Patel

et al. 2012

ApJ

We present spectro-polarimetric observations of several molecular lines obtained with the Submillimeter Array toward the carbon-rich asymptotic giant branch star IRC+10216. We have detected and mapped the linear polarization of the CO 3-2, SiS 19-18, and CS 7-6 lines. The polarization arises at a distance of 450 AU from the star and is blueshifted with respect to the Stokes I. The SiS 19-18 polarization pattern appears to be consistent with a locally radial magnetic field configuration. However, the CO 3-2 and CS 7-6 line polarization suggests an overall complex magnetic field morphology within the envelope. This work demonstrates the feasibility of using spectro-polarimetric observations to carry out tomographic imaging of the magnetic field in circumstellar envelopes.

“…In the class of water-fountain sources, a strong toroidal magnetic field has been found for W43A (Vlemmings et al 2006), and the poloidal field component is seen in IRAS 15445-5449 (Pérez-Sánchez et al 2011). While an order of magnitude weaker then in the OH regions of Miras, the magnetic field is also significant in the OH maser region of the water fountains (Amiri et al 2011). The observations of further post-AGB objects have also revealed a large scale magnetic field in the OH maser region of OH 17.7-2.0 (Bains et al 2003) potentially consistent with a stretched dipole field.…”

Section: Current Statusmentioning

confidence: 87%

Magnetic fields during the evolution towards planetary nebulae

2011

Proc. IAU

Abstract. Magnetic fields appear ubiquitous throughout the envelopes of evolved stars. However, their origin and role in the formation of planetary nebulae is still unclear. As observations of magnetic fields are complicated and time consuming, the observed samples of AGB and post-AGB stars and planetary nebulae are still small. Still, magnetic energy seems to dominate the energy budget out to a distance of several tens of AU from the central star and the field morphology often appears to be well ordered. A short summary is given of the current observations and the potential of new instruments such as ALMA is discussed.