Pollux: A weak dynamo-driven dipolar magnetic field and implications for its probable planet

• Published in: Astronomy & Astrophysics
• Main Authors: Aurière, M., Petit, P., Mathias, P., Konstantinova-Antova, R., Charbonnel, Corinne, Donati, J.-F., Espagnet, O., Folsom, C. P., Roudier, T., Wade, G. A.
• Other Authors: Institut de recherche en astrophysique et planétologie (IRAP), Université Toulouse III - Paul Sabatier (UT3), Université Fédérale Toulouse Midi-Pyrénées-Université Fédérale Toulouse Midi-Pyrénées-Institut national des sciences de l'Univers (INSU - CNRS)-Observatoire Midi-Pyrénées (OMP), Institut de Recherche pour le Développement (IRD)-Université Toulouse III - Paul Sabatier (UT3), Université Fédérale Toulouse Midi-Pyrénées-Université Fédérale Toulouse Midi-Pyrénées-Institut national des sciences de l'Univers (INSU - CNRS)-Centre National d'Études Spatiales Toulouse (CNES)-Centre National de la Recherche Scientifique (CNRS)-Météo-France -Institut de Recherche pour le Développement (IRD)-Institut national des sciences de l'Univers (INSU - CNRS)-Centre National d'Études Spatiales Toulouse (CNES)-Centre National de la Recherche Scientifique (CNRS)-Météo-France -Centre National de la Recherche Scientifique (CNRS), Institute of Astronomy, Bulgarian Academy of Sciences (BAS), Université de Genève = University of Geneva (UNIGE), Royal Military College of Canada
• Format: Article in Journal/Newspaper
• Language: English
• Published: HAL CCSD 2021
• Subjects: stars: individual: Pollux; stars: magnetic field; stars: late giant; stars: rotation; planetary system; [SDU.ASTR.SR]Sciences of the Universe [physics]/Astrophysics [astro-ph]/Solar and Stellar Astrophysics [astro-ph.SR]; Pollux; narval
• Online Access: https://hal.science/hal-03433227; https://hal.science/hal-03433227/document; https://hal.science/hal-03433227/file/2101.02016.pdf; https://doi.org/10.1051/0004-6361/202039573

International audience Context. Pollux is considered as an archetype of a giant star hosting a planet since its radial velocity (RV) presents very stable sinusoidal variations with a period of about 590 d. We then discovered a weak magnetic field at its surface using spectropolarimetry, questioning the planetary hypothesis.Aims. We followed up our investigations on Pollux to characterize its magnetic field and to infer the effects of magnetic activity on the RV variations.Methods. We first used ESPaDOnS at CFHT and then Narval at TBL to obtain Stokes I and Stokes V spectra of Pollux to study their variations for a duration of 4.25 years, that is, for more than two periods of the RV variations. We used the least-squares deconvolution profiles to measure the longitudinal magnetic field and to perform a Zeeman Doppler imaging (ZDI) investigation.Results. The longitudinal magnetic field of Pollux is found to vary with a sinusoidal behavior and a period similar to that of the RV variations. From the ZDI investigation a rotation period of Pollux is determined to be equal to 660 ± 15 days and possibly different than the period of variations of the RV. As to the magnetic topology, the poloidal component is dominant and almost purely dipolar with an inclination of 10.5° of the dipole with respect to the rotation axis. The mean strength of the surface magnetic field is 0.44 G. Pollux is found approximately as active as the Sun observed as a star and this activity could induce moderate RV variations.Conclusions. As to the origin of the magnetic field of Pollux, we favor the hypothesis that it is maintained through contemporaneous dynamo action. Pollux appears as the representative of a class of slowly rotating and weakly magnetic G-K red giants. To explain the sinusoidal RV variations of Pollux, two scenarios are proposed. If the RV period is different from the rotation period, the observed periodic RV variations are due to the hosted planet and the contribution of Pollux magnetic activity is not significantly detected. ...