Magnetic field topology of the RS CVn star II Pegasi

Abstract

Context. The dynamo processes in cool active stars generate complex magnetic fields responsible for prominent surface stellar activity and variability at different time scales. For a small number of cool stars magnetic field topologies were reconstructed from the time series of spectropolarimetric observations using the Zeeman Doppler imaging (ZDI) method, often yielding surprising and controversial results. Aims. In this study we follow a long-term evolution of the magnetic field topology of the RS CVn binary star II Peg using a more self-consistent and physically more meaningful modelling approach compared to previous ZDI studies. Methods. We collected high-resolution circular polarisation observations of II Peg using the SOFIN spectropolarimeter at the Nordic Optical Telescope. These data cover 12 epochs spread over 7 years, comprising one of the most comprehensive spectropolarimetric data sets acquired for a cool active star. A multi-line diagnostic technique in combination with a new ZDI code is applied to interpret these observations. Results. We have succeeded in detecting clear magnetic field signatures in average Stokes V profiles for all 12 data sets. These profiles typically have complex shapes and amplitudes of ∼10 −3 of the unpolarised continuum, corresponding to mean longitudinal fields of 50-100 G. Magnetic inversions using these data reveals evolving magnetic fields with typical local strengths of 0.5-1.0 kG and complex topologies. Despite using a self-consistent magnetic and temperature mapping technique, we do not find a clear correlation between magnetic and temperature features in the ZDI maps. Neither do we confirm the presence of persistent azimuthal field rings found in other RS CVn stars. Reconstruction of the magnetic field topology of II Peg reveals significant evolution of both the surface magnetic field structure and the extended magnetospheric field geometry on the time scale covered by our observations. From 2004 to 2010 the total field energy drastically declined and the field became less axisymmetric. This also coincided with the transition from predominantly poloidal to mainly toroidal field topology. Conclusions. A qualitative comparison of the ZDI maps of II Peg with the prediction of dynamo theory suggests that the magnetic field in this star is produced mainly by the turbulent α 2 dynamo rather than the solar αΩ dynamo. Our results do not show a clear active longitude system, nor is there evidence of the presence of an azimuthal dynamo wave.

Multiperiodicity, modulations and flip-flops in variable star light curves

“…From spectroscopy (covering the time 1994-2010Lindborg et al 2011Hackman et al 2012) and spectropolarimetry (Kochukhov et al 2013(Kochukhov et al , covering the time 2004(Kochukhov et al -2010 there is some evidence of the star's magnetic field strength weakning and the spots are more randomly distributed across the phase during the latest epoch of disturbed phase activity (2004)(2005)(2006)(2007)(2008)(2009)(2010). The chosen CF visualization scheme of normalizing each stripe with its own extrema acts to hide possible amplitude variations in our photometric analysis.…”

“…If the drift was due to an azimuthal dynamo wave or orbital de-synchronization, we should be able to find a persistent drift period. For differential rotation, this could occur only if the spots were to persistently occur around the same latitude, which seems somewhat unlikely, although no systematic latitudinal variation has been reported yet in the object according to Lindborg et al (2011), Hackman et al (2012 and Kochukhov et al (2013). Nevertheless, differential rotation is the only scenario that would naturally allow for changing drift periods associated with changes in the spot latitudes.…”

“…Berdyugina et al 1998aBerdyugina et al , 1999Gu et al 2003;Lindborg et al 2011;Hackman et al 2012) and magnetic field maps have been published (Kochukhov et al 2013;Carroll et al 2007Carroll et al , 2009). The star's photometric light curves have also been intensively studied with time-series analysis and spot-modeling methods (see e.g.…”

“…Intriguingly, the drift was no longer visible in the Doppler (DI) and Zeeman-Doppler images (ZDI) of Hackman et al (2012) and Kochukhov et al (2013), who also reported on the clear diminishing trend in the spottedness and magnetic field strength of the object. This was interpreted as the star's activity declining toward its magnetic minimum, due to which the spot distribution on the stellar surface was postulated to be more stochastic than during the high-activity state, veiling the possible drift due to the mechanism producing it underneath.…”

Diagnostic of stellar magnetic fields with cumulative circular polarisation profiles

“…Such Stokes V profiles cannot be meaningfully characterised with integral magnetic observables because their low-order moments tend to be close to zero even when strong polarisation signatures are evident in the data (e.g. Kochukhov et al 2013). In other words, the surface magnetic field distribution is so complex that vector averaging over the visible stellar disk leads to a substantial cancellation of the Stokes V signal corresponding to different A&A 580, A39 (2015) field polarities.…”