Time evolution of magnetic activity cycles in young suns: The curious case of Kappa Ceti

AbstractThe existence of starspots on late-type giant stars in close binary systems, that exhibit rapid rotation due to tidal locking, has been known for more than five decades. Photometric monitoring spanning decades has allowed studying the long-term magnetic activity in these stars revealing complicated activity cycles. The development of observing and analysis techniques that has occurred during the past two decades has also enabled us to study the detailed starspot and magnetic field configurations on these active giants. In the recent years magnetic fields have also been detected on slowly rotating giants and supergiant stars. In this paper I review what is known of the surface magnetism in the cool giant and supergiant stars.

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Stellar Chromospheric Variability

Universe ◽

10.3390/universe7110440 ◽

2021 ◽

Vol 7 (11) ◽

pp. 440

Author(s):

Richard de de Grijs ◽

Devika Kamath

Keyword(s):

Large Scale ◽

Binary Systems ◽

Magnetic Activity ◽

Close Binary ◽

Physical Parameters ◽

Stellar Rotation ◽

Chromospheric Activity ◽

Recent Developments ◽

Hα Emission ◽

Activity Cycles

Cool stars with convective envelopes of spectral types F and later tend to exhibit magnetic activity throughout their atmospheres. The presence of strong and variable magnetic fields is evidenced by photospheric starspots, chromospheric plages and coronal flares, as well as by strong Ca ii H+K and Hα emission, combined with the presence of ultraviolet resonance lines. We review the drivers of stellar chromospheric activity and the resulting physical parameters implied by the observational diagnostics. At a basic level, we explore the importance of stellar dynamos and their activity cycles for a range of stellar types across the Hertzsprung–Russell diagram. We focus, in particular, on recent developments pertaining to stellar rotation properties, including the putative Vaughan–Preston gap. We also pay specific attention to magnetic variability associated with close binary systems, including RS Canum Venaticorum, BY Draconis, W Ursae Majoris and Algol binaries. At the present time, large-scale photometric and spectroscopic surveys are becoming generally available, thus leading to a resurgence of research into chromospheric activity. This opens up promising prospects to gain a much improved understanding of chromospheric physics and its wide-ranging impact.

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Time Evolution of the Magnetic Activity Cycle Period. II. Results for an Expanded Stellar Sample

The Astrophysical Journal ◽

10.1086/307794 ◽

1999 ◽

Vol 524 (1) ◽

pp. 295-310 ◽

Cited By ~ 212

Author(s):

Steven H. Saar ◽

Axel Brandenburg

Keyword(s):

Time Evolution ◽

Activity Cycle ◽

Magnetic Activity ◽

Cycle Period

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Transition from spot to faculae domination

Astronomy and Astrophysics ◽

10.1051/0004-6361/201833754 ◽

2018 ◽

Vol 621 ◽

pp. A21 ◽

Cited By ~ 16

Author(s):

Timo Reinhold ◽

Keaton J. Bell ◽

James Kuszlewicz ◽

Saskia Hekker ◽

Alexander I. Shapiro

Keyword(s):

Time Series ◽

Phase Difference ◽

Process Model ◽

Activity Cycle ◽

Magnetic Activity ◽

Cycle Period ◽

Stellar Activity ◽

Rotation Periods ◽

Active Stars ◽

Activity Cycles

Context. The study of stellar activity cycles is crucial to understand the underlying dynamo and how it causes magnetic activity signatures such as dark spots and bright faculae. Having knowledge about the dominant source of surface activity might allow us to draw conclusions about the stellar age and magnetic field topology, and to put the solar cycle in context. Aims. We investigate the underlying process that causes magnetic activity by studying the appearance of activity signatures in contemporaneous photometric and chromospheric time series. Methods. Lomb-Scargle periodograms are used to search for cycle periods present in the photometric and chromospheric time series. To emphasize the signature of the activity cycle we account for rotation-induced scatter in both data sets by fitting a quasi-periodic Gaussian process model to each observing season. After subtracting the rotational variability, cycle amplitudes and the phase difference between the two time series are obtained by fitting both time series simultaneously using the same cycle period. Results. We find cycle periods in 27 of the 30 stars in our sample. The phase difference between the two time series reveals that the variability in fast-rotating active stars is usually in anti-phase, while the variability of slowly rotating inactive stars is in phase. The photometric cycle amplitudes are on average six times larger for the active stars. The phase and amplitude information demonstrates that active stars are dominated by dark spots, whereas less-active stars are dominated by bright faculae. We find the transition from spot to faculae domination to be at the Vaughan–Preston gap, and around a Rossby number equal to one. Conclusions. We conclude that faculae are the dominant ingredient of stellar activity cycles at ages ≳2.55 Gyr. The data further suggest that the Vaughan–Preston gap cannot explain the previously detected dearth of Kepler rotation periods between 15 and 25 days. Nevertheless, our results led us to propose an explanation for the lack of rotation periods to be due to the non-detection of periodicity caused by the cancelation of dark spots and bright faculae at ∼800 Myr.

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Magnetic activity cycles in solar-like stars: The cross-correlation technique ofp-mode frequency shifts

Astronomy and Astrophysics ◽

10.1051/0004-6361/201425408 ◽

2016 ◽

Vol 589 ◽

pp. A103 ◽

Cited By ~ 19

Author(s):

C. Régulo ◽

R. A. García ◽

J. Ballot

Keyword(s):

Cross Correlation ◽

Magnetic Activity ◽

Mode Frequency ◽

Correlation Technique ◽

Frequency Shifts ◽

Activity Cycles ◽

The Cross ◽

Cross Correlation Technique

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Solar Rotation, Irradiance Changes and Climate

International Astronomical Union Colloquium ◽

10.1017/s025292110002474x ◽

1994 ◽

Vol 143 ◽

pp. 244-251

Author(s):

Elizabeth Nesme-Ribes ◽

Dmitry Sokoloff ◽

Robert Sadourny

Keyword(s):

Magnetic Fields ◽

Internal Rotation ◽

Solar Rotation ◽

Total Solar Irradiance ◽

Magnetic Activity ◽

Convective Envelope ◽

Surface Rotation ◽

Solar Type ◽

Activity Cycles ◽

The Impact

Magnetic activity cycles for solar-type stars are believed to originate from non-uniform internal rotation. To determine this depthwise angular velocity distribution, helioseismology is a valuable source of information. Surface rotation, as traced by sunspot motion, is a well-observed parameter with data going back to the beginning of the telescopic era. This long sunspot series can be used in understanding the behaviour of the Sun’s surface rotation, the connection with its internal rotation, and thereby its magnetic activity. Apparent solar diameter is another important parameter. This is related to the structure of the convective envelope and how it reacts to the presence of magnetic fields. Both these parameters are related to the solar output, and can provide a surrogate for total solar irradiance, by way of a theoretical modeling of the response of the convective zone to the emergence of periodic magnetic fields. The impact of solar variability on the terrestrial climate is also addressed.

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Stellar Activity and Calcium Emission Variability

International Astronomical Union Colloquium ◽

10.1017/s0252921100096093 ◽

1983 ◽

Vol 71 ◽

pp. 195-199

Author(s):

Sallie L. Baliunas

Keyword(s):

Solar Activity ◽

Magnetic Fields ◽

Magnetic Activity ◽

Small Scale ◽

Physical Parameters ◽

Stellar Activity ◽

Ongoing Work ◽

Chromospheric Emission ◽

Activity Cycles

ABSTRACT.Time series analysis of fluctuations of Ca II H and K chromospheric emission has provided us with much information concerning stellar activity. On all timescales, events which parallel solar behavior can be observed: activity cycles, on timescales of years; rotation of stars and evolution of active areas on timescales of days to weeks; flare-like phenomena on timescales as short as minutes.We expect that the analogues of solar activity exist on other stars . By studying stellar counterparts to solar activity, we can hope to investigate the physical parameters which are thought to influence chromospheric and coronal activity. The stellar surfaces are usually spatially unresolvable; it is thus difficult to measure directly either small-scale surface inhomogeneities or the associated magnetic fields expected from spatially restricted areas.On the Sun, however, areas with strong surface magnetic fields show intense chromospheric Ca II H and K emission (Babcock and Babcock 1955; Skumanich et al 1975). Although indirect, the Ca II H and K features are good indicators of stellar magnetic activity. A major advantage of the Ca II features is their accessibility to ground-based observatories. Long-term synoptic programs are in progress to monitor stellar chromospheric activity, and this paper will highlight ongoing work at Mt. Wilson. Monitoring variations of Ca II H and K chromospheric emission over different timescales can reveal different physical phenomena: (1) Long-term (years) variations corresponding to stellar activity cycles; (2) intermediate term (days-months) variations indicating rotation or evolution of stellar active areas; (3) short-term (minutes-hours) variations resulting from impulsive and flare-like phenomena.

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Magnetic activity of the solar-like star HD 140538

Astronomy and Astrophysics ◽

10.1051/0004-6361/201935654 ◽

2019 ◽

Vol 628 ◽

pp. A107 ◽

Cited By ~ 1

Author(s):

M. Mittag ◽

J. H. M. M. Schmitt ◽

T. S. Metcalfe ◽

A. Hempelmann ◽

K.-P. Schröder

Keyword(s):

Rotation Period ◽

Activity Cycle ◽

Magnetic Activity ◽

Main Sequence Stars ◽

Age Relation ◽

Activity Cycles ◽

Index Time Series ◽

Evolution With Time ◽

High Cadence

The periods of rotation and activity cycles are among the most important properties of the magnetic dynamo thought to be operating in late-type, main-sequence stars. In this paper, we present a SMWO-index time series composed from different data sources for the solar-like star HD 140538 and derive a period of 3.88 ± 0.02 yr for its activity cycle. Furthermore, we analyse the high-cadence, seasonal SMWO data taken with the TIGRE telescope and find a rotational period of 20.71 ± 0.32 days. In addition, we estimate the stellar age of HD 140538 as 3.7 Gyrs via a matching evolutionary track. This is slightly older than the ages obtained from gyrochronology based on the above rotation period, as well as the activity-age relation. These results, together with its stellar parameters that are very similar to a younger Sun, make HD 140538 a relevant case study for our understanding of solar activity and its evolution with time.

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Starspot rotation rates versus activity cycle phase: Butterfly diagrams of Kepler stars are unlike that of the Sun

Astronomy and Astrophysics ◽

10.1051/0004-6361/201834373 ◽

2019 ◽

Vol 622 ◽

pp. A85 ◽

Cited By ~ 5

Author(s):

M. B. Nielsen ◽

L. Gizon ◽

R. H. Cameron ◽

M. Miesch

Keyword(s):

Effective Temperature ◽

Rotation Rate ◽

Activity Cycle ◽

Magnetic Activity ◽

Activity Level ◽

The Sun ◽

Rotation Rates ◽

Rotation Periods ◽

Activity Cycles ◽

The Mean

Context. During the solar magnetic activity cycle the emergence latitudes of sunspots change, leading to the well-known butterfly diagram. This phenomenon is poorly understood for other stars since starspot latitudes are generally unknown. The related changes in starspot rotation rates caused by latitudinal differential rotation can, however, be measured. Aims. Using the set of 3093 Kepler stars with measured activity cycles, we aim to study the temporal change in starspot rotation rates over magnetic activity cycles, and how this relates to the activity level, the mean rotation rate of the star, and its effective temperature. Methods. We measured the photometric variability as a proxy for the magnetic activity and the spot rotation rate in each quarter over the duration of the Kepler mission. We phase-folded these measurements with the cycle period. To reduce random errors, we performed averages over stars with comparable mean rotation rates and effective temperature at fixed activity-cycle phases. Results. We detect a clear correlation between the variation of activity level and the variation of the starspot rotation rate. The sign and amplitude of this correlation depends on the mean stellar rotation and – to a lesser extent – on the effective temperature. For slowly rotating stars (rotation periods between 15 − 28 days), the starspot rotation rates are clearly anti-correlated with the level of activity during the activity cycles. A transition is observed around rotation periods of 10 − 15 days, where stars with an effective temperature above 4200 K instead show positive correlation. Conclusions. Our measurements can be interpreted in terms of a stellar “butterfly diagram”, but these appear different from that of the Sun since the starspot rotation rates are either in phase or anti-phase with the activity level. Alternatively, the activity cycle periods observed by Kepler are short (around 2.5 years) and may therefore be secondary cycles, perhaps analogous to the solar quasi-biennial oscillations.

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