Recent instrumental advances on the Very Long Baseline Array, combined with new techniques for millimeter-wavelength polarimetry over the past decade, have opened a science window on the near-circumstellar environment of evolved stars unmatched by any contemporary telescope. The technique employed here allows the astrophysics of this region to be probed by the polarization properties of the silicon monoxide maser components located in the extended atmospheres via synoptic imaging campaigns across complete pulsation periods of the central star. Here Very Long Baseline Array observations of maser emissions in the near-circumstellar regions of asymptotic giant branch (evolved) stars will be carried out. The goal is to provide constraints on various astrophysical models by: 1) measuring the magnitudes and orientation of magnetic fields in the extended atmospheres, 2) determining whether asymmetric mass loss begins at an early phase of asymptotic giant branch evolution, and 3) exploring the possible connection between the central stars and their close circumstellar environments. A series of specific research tasks (and associated sub-tasks) are outlined which employ synoptic maser observations over entire pulsation cycles to (among other goals) map kinematics and shocks, to use polarization and population inversion to infer magnetic fields and turbulence, and to discriminate between various models for polarized maser propagation and silicon oxide maser pumping. The latter studies are vital in order to provide unambiguous estimates of the surface magnetic fields in these stars. The results of this work are expected to have an impact on studies of the physics of maser polarization, models of evolved stars and the process of planetary nebula formation.
This work will help to train graduate students in the data reduction methods and analysis. In addition, the animated movies of maser emission toward late-type stars which are a by-product of this research, will be disseminated to the public through various outreach efforts. The final steps necessary to automate fully the existing calibration and imaging pipeline, which will be developed here, will also be available to the community. This will sharply lower access barriers to the Very Long Baseline Array.
