ACES A magnetic field study in the envelope of cool evolved stars - Louise Marinho

Louise Marinho
Laboratoire d’Astrophysique de Bordeaux, France
Evolved cool stars exhibit a high mass loss rate, which contributes to enriching the interstellar
medium and, therefore, to the recycling of the matter in the Universe. Both magnetic fields
and photospheric/atmospheric dynamics can be involved in triggering this important mass loss.
Previous works have revealed that these objects exhibit a magnetic field extending beyond their
surface. The origin of this magnetic field is still under debate with mechanisms involving a
turbulent dynamo, convection, stellar pulsation, and cool spots. These mechanisms can be
constrained thanks to observations of various molecular transitions in the circumstellar envelope.
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 4 Stokes
parameters. Applying a careful calibration method, from the circular and linear polarization
fractions we derive estimates of the magnetic field strength (from several tenths up to several
Gauss) considering the saturated and unsaturated cases under the Zeeman hypothesis. These
new and more accurate measurements constrain the 2-5 stellar radii region better than previous
studies and exclude a global poloidal field. A combination of a toroidal and a poloidal field is
nevertheless not excluded. A variation of the magnetic field strength over a two-month timescale
is observed which suggests a possible link to the stellar phase, i.e. with pulsation/photospheric
activity. I will conclude this talk presenting what are the next steps concerning this magnetic
field investigation.