The magnetic fields at the surface of active single G-K giants

Abstract

Aims. We investigate the magnetic field at the surface of 48 red giants selected as promising for detection of Stokes V Zeeman signatures in their spectral lines. In our sample, 24 stars are identified from the literature as presenting moderate to strong signs of magnetic activity. An additional 7 stars are identified as those in which thermohaline mixing appears not to have occured, which could be due to hosting a strong magnetic field. Finally, we observed 17 additional very bright stars which enable a sensitive search to be performed with the spectropolarimetric technique. Methods. We use the spectropolarimeters Narval and ESPaDOnS to detect circular polarization within the photospheric absorption lines of our targets. We treat the spectropolarimetric data using the least-squares deconvolution method to create high signal-to-noise ratio mean Stokes V profiles. We also measure the classical S -index activity indicator for the Ca ii H&K lines, and the stellar radial velocity. To infer the evolutionary status of our giants and to interpret our results, we use state-of-the-art stellar evolutionary models with predictions of convective turnover times. Results. We unambiguously detect magnetic fields via Zeeman signatures in 29 of the 48 red giants in our sample. Zeeman signatures are found in all but one of the 24 red giants exhibiting signs of activity, as well as 6 out of 17 bright giant stars. However no detections were obtained in the 7 thermohaline deviant giants. The majority of the magnetically detected giants are either in the first dredge up phase or at the beginning of core He burning, i.e. phases when the convective turnover time is at a maximum: this corresponds to a "magnetic strip" for red giants in the Hertzsprung-Russell diagram. A close study of the 16 giants with known rotational periods shows that the measured magnetic field strength is tightly correlated with the rotational properties, namely to the rotational period and to the Rossby number Ro. Our results show that the magnetic fields of these giants are produced by a dynamo, possibly of α-ω origin since Ro is in general smaller than unity. Four stars for which the magnetic field is measured to be outstandingly strong with respect to that expected from the rotational period/magnetic field relation or their evolutionary status are interpreted as being probable descendants of magnetic Ap stars. In addition to the weak-field giant Pollux, 4 bright giants (Aldebaran, Alphard, Arcturus, η Psc) are detected with magnetic field strength at the sub-Gauss level. Besides Arcturus, these stars were not considered to be active giants before this study and are very similar in other respects to ordinary giants, with S -index indicating consistency with basal chromospheric flux.Key words. stars: magnetic field -stars: late-type -stars: evolution -stars: rotation Tables 6-8

Planets around evolved intermediate-mass stars

“…For example, polar cap-like starspots appear to have lifetimes of over a decade in RS CVn binaries as well as in young MS stars (Strassmeier 2009). Nevertheless, a handful of giant stars with slow rotation have been studied in the last decade with the Zeeman Doppler imaging, which allows to obtain the strength and geometry of the surface magnetic field by using new high resolution spectropolarimeters such as Narval, ESPaDOnS or HARPSpol (Aurière et al 2015). These authors found that most of the giant stars with detected magnetic fields (the longitudinal component) are undergoing the first dredge-up (i.e.…”

Time-series Doppler images and surface differential rotation of the effectively single, rapidly rotating K-giant KU Pegasi

“…According to the theoretical estimation by Aurière et al (2015) the maximum convective turnover time of KU Peg should be around τ ≈ 100 d. This would yield a moderate Rossby number of Ro ≈ 0.24, indicating that the star operates an αΩ type dynamo. However, from the time evolution of the spotted surface, especially considering the polar spots between 2006-2011, we were only able to estimate a rough cycle length of a few years.…”

“…Lèbre et al 2009;Aurière et al 2015) we carried out a comparative study by using the spectrum synthesis code SPECTRUM by R. Gray (www.appstate.edu/~grayro/spectrum/spectrum.html). We calculated synthetic spectra from our new parameters (T eff = 4440 K, log g = 2.0, v sin i = 29.4 km s −1 , [Fe/H] = −0.37, and micro-and macroturbulences of ξ mic = 1.8 km s −1 and ξ mac = 3.0 km s −1 ) and from the old parameter set taken from Lèbre et al (2009)…”

Detection of solar-like oscillations in the bright red giant starsγPiscium andθ1 Tauri from a 190-day high-precision spectroscopic multi-site campaign

“…Therefore only the most massive stars are already in more advanced evolutionary stages after the main sequence. Very recently, Auriere et al (2015) reported on the detection of a magnetic field in θ 1 Tau whose unsigned longitudinal component peaked to 3 ± 0.5 G. From their measurements they find indications for a complex surface magnetic field structure.…”