Precessing planetary magnetospheres in SiO stars ? First detection of quasi - periodic polarization fluctuations in R Leo and V Cam

Context. The origin of magnetism around asymptotic giant branch (AGB) stars is still uncertain. These stars may drive an important dynamo, but if the magnetic energy dissipates all in X-rays, the observed X-ray luminosities are too low for a strong, dynamically important global field. Other explanations for the circular polarization of SiO masers in the AGB atmospheres may thus be needed. Aims. The interaction of the AGB wind with previously ejected matter and with planets is expected to bear complex magnetohydrodynamic phenomena on a short time scale, such that strong magnetic fields can be maintained locally. Here we provide observational evidence for the corresponding magnetic fluctuations. Methods. We use the circular polarization of the v = 1, J = 2− 1 SiO masers as tracer for magnetic activity. A correlation polarimeter allows us to simultaneously record all Stokes parameters. An SiO maser survey of 80 AGB stars was performed, out of which eight sources with the strongest circular polarization were selected for further monitoring. Results. In two AGB stars, V Cam and R Leo, we find evidence of pseudo-periodic fluctuations of the fractional circular polarization on a timescale of a few hours, from which we infer magnetic fluctuations of ∼ 1 G. The phenomenon is rare and, if detected in an SiO star, restricted to a narrow range of velocities. It seems to be associated with planetary wake flows suggested by VLBI maps. Conclusions. While scenarios involving magnetic activity in the extended stellar atmosphere have problems to explain all observed features, precessing Jovian magnetospheres predict all of them without difficulty. For the case of R Leo, we constrain the orbit of the planet (estimated period 5.2 years), derive a stellar mass estimate of 0.7 M⊙ from it, and discuss the impact of planetary magnetism on the survival of planets. Smooth velocity variations of the fluctuating circular polarization feature are predicted as the planet moves along its orbit.