Résumé
Context. Magnetic fields, photospheric and atmospheric dynamics can be involved in triggering the high mass loss observed in evolved cool stars. Previous works have revealed that the magnetic field of these objects extends beyond their surface. The origin of this magnetic field is still debated. The possible mechanisms include a turbulent dynamo, convection, stellar pulsation, and cool spots.Aims. Our goal is to estimate the magnetic field strength in the inner circumstellar envelope of six evolved cool stars (five Miras and one red supergiant). Combining this work with previous studies, we tentatively constrain the global magnetic field type and shed light on the mechanisms that cause it.Methods. Using the XPOL polarimeter installed at the IRAM-30 m telescope, we observed the 28SiO v = 1, J = 2 − 1 maser line emission and obtained simultaneous spectroscopic measurements of the four Stokes parameters. Applying a careful calibration method for Stokes Q, U, and V, we derived estimates of the magnetic field strength from the circular and linear polarization fractions considering the saturated and unsaturated maser cases under the Zeeman hypothesis.Results. Magnetic field strengths from several Gauss up to several dozen Gauss are derived. These new and more accurate measurements constrain the field strength in the region of 2–5 stellar radii better than previous studies and appear to exclude a global poloidal magnetic field type. The combination of a toroidal and poloidal field is not excluded, however. A variation in the magnetic field strength over a two-month timescale is observed in one Mira star, which suggests a possible link to the stellar phase, that is, a link with pulsation and photospheric activity.