SiO maser polarization in evolved stars: magnetic field.

We have measured the polarization of the SiO maser emission in a representative sample of evolved stars. We made simultaneous spectroscopic measurement of the 4 Stokes parameters, from which we derive the circular and linear polarization levels. These observations allow us to discuss the existent SiO maser models, to derive the strength of the magnetic field, and thus to determine the real influence of this magnetic field in the life of these evolved objects. The magnetic field strength we derive varies between 0 and 18 Gauss, with a mean value of 3.5 G and may follow a 1/r law throughout the circumstellar envelope. As a consequence, the magnetic field has a real influence on the life of these evolved objects: it definitively plays the role of a collimating, and more generally, of a shaping, agent in evolved objects. © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]