Analysis of the MOST light curve of the heavily spotted K2IV component of the single-line spectroscopic binary II Pegasi★

Lindborg

et al. 2012

A&A

Aims. We study the spot activity of II Peg during the years 2004-2010 to determine long-and short-term changes in the magnetic activity. In a previous study, we detected a persistent active longitude, as well as major changes in the spot configuration occurring on a timescale of shorter than a year. The main objective of this study is to determine whether the same phenomena persist in the star during these six years of spectroscopic monitoring. Methods. The observations were collected with the high-resolution SOFIN spectrograph at the Nordic Optical Telescope. The temperature maps were calculated using a Doppler imaging code based on Tikhonov regularization. Results. We present 12 new temperature maps that show spots distributed mainly over high and intermediate latitudes. In each image, 1-3 main active regions can be identified. The activity level of the star is clearly lower than during our previous study for the years 1994-2002. In contrast to the previous observations, we detect no clear drift of the active regions with respect to the rotation of the star. Conclusions. Having shown a systematic longitudinal drift of the spot-generating mechanism during 1994-2002, the star has clearly switched to a low-activity state for 2004-2010, during which the spot locations appear more random over phase space. It could be that the star is near to a minimum of its activity cycle.

Section: Our 14 Doppler Images Of IImentioning

confidence: 54%

Section: Introductionmentioning

confidence: 99%

Doppler images of II Pegasi for 2004–2010

Hackman

Lindborg

et al. 2012

A&A

“…Many studies examined photometric variations of this star (e.g. Siwak et al 2010;Roettenbacher et al 2011, and references therein), aiming to explore long-term activity cycles and, in particular, to investigate the role of active longitudes (Berdyugina et al 1999;Rodonò et al 2000).…”

Section: Introductionmentioning

confidence: 99%

“…This effect was ignored by previous spectroscopic DI investigations of II Peg and was theoretically predicted to be negligible in the rapidly rotating stars with deep convective envelopes (Kitchatinov & Rüdiger 1999). On the other hand, photometric analyses occasionally claimed detection of the differential rotation in II Peg (Henry et al 1995;Siwak et al 2010;Roettenbacher et al 2011), yet giving contradictory results, mainly because of the difficulty of constraining the spot latitudes in light curve modelling. However, even the largest value of the differential rotation coefficient reported in the literature k = 0.0245 (Siwak et al 2010) is substantially smaller than the solar value of k = 0.19 and corresponds to an insignificant surface shearing for the time span of most of our SOFIN data sets.…”

Section: Stellar Parametersmentioning

confidence: 99%

Magnetic field topology of the RS CVn star II Pegasi

Kochukhov

Hackman

et al. 2013

A&A

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.

“…The star's photometric light curves have also been intensively studied with time-series analysis and spot-modeling methods (see e.g. Henry et al 1995;Rodonò et al 2000;Siwak et al 2010;Hackman et al 2011;Roettenbacher et al 2011). All longerterm light curve variations are commonly interpreted as being due to starspots (see e.g.…”

Section: Introductionmentioning

confidence: 99%

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

Lindborg

Olspert

et al. 2013

A&A

Aims. According to previously published Doppler images of the magnetically active primary giant component of the RS CVn binary II Peg, the surface of the star was dominated by one single active longitude that was clearly drifting in the rotational frame of the binary system during 1994-2002; later imaging for 2004-2010, however, showed decreased and chaotic spot activity, with no signs of the drift pattern. Here we set out to investigate from a more extensive photometric dataset whether this drift is a persistent phenomenon, in which case it could be caused either by an azimuthal dynamo wave or be an indication that the binary system's orbital synchronization is still incomplete. On a differentially rotating stellar surface, spot structures preferentially on a certain latitude band could also cause such a drift, the disruption of which could arise from the change of the preferred spot latitude. Methods. We analyzed the datasets using the carrier fit (CF) method, which is especially suitable for analyzing time series in which a fast clocking frequency (such as the rotation of the star) is modulated with a slower process (such as the stellar activity cycle).Results. We combined all collected photometric data into one single data set and analyzed it with the CF method. We confirm the previously published results that the spot activity has been dominated by one primary spotted region almost through the entire data set and also confirm a persistent, nearly linear drift. Disruptions of the linear trend and complicated phase behavior are also seen, but the period analysis reveals a rather stable periodicity with P spot = 6. d 71054 ± 0. d 00005. After removing the linear trend from the data, we identified several abrupt phase jumps, three of which are analyzed in more detail with the CF method. These phase jumps closely resemble what is called a flip-flop event, but the new spot configurations do not persist for longer than a few months in most cases. Conclusions. There is some evidence that the regular drift without phase jumps is related to the high state, while the complex phase behavior and disrupted drift pattern are related to the low state of magnetic activity. The most natural explanation of the drift is weak anti-solar (pole rotating faster than the equator) differential rotation with a coefficient k ≈ 0.002 combined with the preferred latitude of the spot structure.