Abstract
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 StokesIand StokesVspectra 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. In the peculiar case in which the two periods are equal, we cannot discard the possibility that the activity of Pollux could explain the total RV variations and that the planet hypothesis would appear unnecessary. In any case magnetic activity could contribute significantly to RV variations in some intermediate mass G-K red giants hosting planets, particularly those with small amplitude RV variations.
Highlights
Pollux (β Geminorum, HD 62509) is a well-studied K0III giant neighbor of the Sun (e.g., Gray 2014)
Based on observations obtained at the Télescope Bernard Lyot (TBL) at Observatoire du Pic du Midi, CNRS/INSU and Université de Toulouse, France, and the Canada-France-Hawaii Telescope (CFHT), which is operated by the National Research Council of Canada, CNRS/INSU and the University of Hawaii
We have monitored the magnetic field of Pollux for a duration of 4.25 years, that is, more than two times the period of the radial velocity (RV) variations
Summary
Pollux (β Geminorum, HD 62509) is a well-studied K0III giant neighbor of the Sun (e.g., Gray 2014) It is considered the archetype of a giant star hosting a planet since it presents periodic sinusoidal radial velocity (RV) variations of about 590 d period and 40 m s−1 semi-amplitude, which have been stable for more than 25 years (Hatzes et al 2006). Aurière et al (2009) discovered a weak magnetic field at the surface of Pollux, which was the first detection of a class of weakly magnetic G-K giants (Aurière et al 2015, hereafter AKC; Konstantinova-Antova et al 2014).
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