Precessing planetary magnetospheres in SiO stars?
First detection of quasi-periodic polarization fluctuations in R Leonis and V CamelopardalisH. Wiesemeyer1, C. Thum1, A. Baudry2, 3, and F. Herpin2, 3
1 Institut de Radio Astronomie Millimétrique, 300 rue de la Piscine, 38406 Saint Martin d'Hères, France
2 Université de Bordeaux, Laboratoire d'Astrophysique de Bordeaux, 33000 Bordeaux, France
3 CNRS/INSU, UMR 5804, BP 89, 33270 Floirac, France
Received 28 October 2008 / Accepted 26 January 2009
Context. The origin of magnetism around asymptotic giant branch (AGB) stars remains an uncertainty. These stars may drive an important dynamo, but if the magnetic energy dissipates entirely into X-rays, the observed X-ray luminosities are too low to maintain a strong, dynamically important global field. Other explanations of the circular polarization in SiO masers in AGB atmospheres may thus be required.
Aims. The interaction of the AGB wind with both previously ejected matter and planets is expected to produce complex magnetohydrodynamic phenomena on a short timescale, such that strong magnetic fields can be maintained locally. Here, we provide observational evidence of the corresponding magnetic fluctuations.
Methods. We use the circular polarization of the v=1, J=2–1 SiO masers as a tracer of magnetic activity. A correlation polarimeter allows us to record simultaneously all Stokes parameters. An SiO maser survey of 77 AGB stars was performed from which eight sources of 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 in 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 explaining 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 from it, and discuss the impact of planetary magnetism on the survival of planets. Smooth velocity variations in the fluctuating circular polarization feature are predicted as the planet moves along its orbit.
Key words: masers -- polarization -- stars: AGB and post-AGB -- stars: atmospheres -- stars: magnetic fields -- stars: planetary systems
© ESO 2009