Magnetic field at Pollux (RV correlation?)

Discovery of a weak magnetic field in the photosphere of the single giant Pollux
http://arxiv.org/abs/0907.1423

Abstract wrote:

Aims: We observe the nearby, weakly-active single giant, Pollux, in order to directly study and infer the nature of its magnetic field. Methods: We used the new generation spectropolarimeters ESPaDOnS and NARVAL to observe and detect circular polarization within the photospheric absorption lines of Pollux. Our observations span 18 months from 2007-2009. We treated the spectropolarimetric data using the Least-Squares Deconvolution method to create high signal-to-noise ratio mean Stokes V profiles. We also measured the classical activity indicator S-index for the Ca H&K lines, and the stellar radial velocity (RV). Results: We have unambiguously detected a weak Stokes V signal in the spectral lines of Pollux, and measured the related surface-averaged longitudinal magnetic field Bl. The longitudinal field averaged over the span of the observations is below one gauss. Our data suggest variations of the longitudinal magnetic field, but no significant variation of the S-index. We observe variations of RV which are qualitatively consistent with the published ephemeris for a proposed exoplanet orbiting Pollux. The observed variations of Bl appear to mimic those of RV, but additional data for this relationship to be established. Using evolutionary models including the effects of rotation, we derive the mass of Pollux and we discuss its evolutionary status and the origin of its magnetic field. Conclusions: This work presents the first direct detection of the magnetic field of Pollux, and demonstrates that ESPaDOnS and NARVAL are capable of obtaining sub-G measurements of the surface-averaged longitudinal magnetic field of giant stars, and of directly studying the relationships between magnetic activity, stellar evolution and planet hosting of these stars.

Looks like this magnetic field may be a better explanation for the RV data than a planetary companion.

Pollux: a stable weak dipolar magnetic field but no planet ?
http://arxiv.org/abs/1310.6907

Quote :

As an alternative to the scenario in which Pollux hosts a close-in exoplanet, we suggest that the magnetic dipole of Pollux can be associated with two temperature and macroturbulent velocity spots which could be sufficient to produce the RV variations.

Sounds like weakly-magnetic giant stars are another kind of potential false positive to watch out for in RV planet hunting:

Quote :

In this way Pollux could be the representative of a class of weakly magnetic G K giants, recently discovered (Aurière et al. in preparation, Konstantinova-Antova et al. this symposium). These stars would include bright giants like Alphard and Arcturus, and some of them, like Aldebaran and ε Tau also present stable RV variations and are considered to host planets. Though we cannot exclude completely that neither Pollux hosts a planet in addition to its magnetic field, nor that it is a peculiar very stable magnetic star, our investigation suggests that Pollux could be the archetype of a class of weakly magnetic G K giants.

Hum, should I remouve Pllux b from confirmed planets?

That's what I've done for my personal notes. The paper makes a pretty compelling argument that the RV signal is due to stellar activity.

Wasn't there an astrometric confirmation of Pollux b in 2011?

Stalker wrote:

Yep

Indeed. Doesn't that count?  

I dont know... Astrometric studies of exoplanets are always suspicious. I will wait Gaïa's results, or even juste another study with the old data.

That study only provides a maximum mass (12.8 MJ), so it didn't actually detect an astrometric perturbation. Also, Gaia will not be able to observe a star as bright as Pollux, as it is limited to magnitude ~6 and below.

This is a very interesting discovery. The "planet" distribution around giants has remained dominated by ~zero eccentricity objects, with high eccentricity objects being very unusual (Edasich b, Chi Virginis b, aaand… no others spring to mind), so the sinusoidal RV variations could very well be explained by a stellar effect. But the lack of variation in the classical activity indicators is odd, particularly in the S-index as it is itself a measurement of magnetic activity. There's definitely more to be explained here.

In addition that study fixes the orbital period, eccentricity, argument of periastron, time of periastron and mass function to the RV values, so even if they did detect the companion it would not be independent of the RV detection.

Also, looks like EPE missed this paper, I'm not seeing it listed in their Bibliography section.

http://arxiv.org/abs/1508.04769
Pollux b's optical and infra-red RV amplitudes may be compatible, strengthening the existence of the planet.

Another study, but less than conclusive.

Pollux: A weak dynamo-driven dipolar magnetic field and implications for its probable planet
https://arxiv.org/abs/2101.02016

Quote :

Context: Pollux is considered as an archetype of a giant star hosting a planet. We then discovered a weak magnetic field at its surface using spectropolarimetry. Aims and Methods: We followed up our investigations on Pollux first using ESPaDOnS at CFHT and then Narval at TBL to obtain Stokes I and Stokes V spectra to study their variations for a duration of 4.25 years, that is, for more than two periods of about 590 d of the RV variations. We used the least-squares deconvolution (LSD) 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. 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.