The connection between stellar activity cycles and magnetic field topology

See, Victor; Jardine, M.; Vidotto, A. A.; Donati, J.‐F.; Saikia, S. Boro; Bouvier, J.; Farès, R.; Folsom, C. P.; Gregory, S. G.; Hussain, G. A. J.; Jeffers, S. V.; Marsden, S. C.; Morin, J.; Moutou, C.; Nascimento, J.-D. do; Petit, P.; Waite, I. A.

doi:10.1093/mnras/stw2010

Cited by 81 publications

(82 citation statements)

References 61 publications

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“…Dotted vertical line connected stars with 2 identified cycles. Image reproduced by permission from See et al (2016), copyright by the authors debated; plausibly, it could arise either due to the coverage by many or large starspots on the stellar surface, or through the field amplitude itself through a saturation of the dynamo mechanism, or both (Gondoin 2012;Reiners et al 2014;Brun et al 2015b).…”

Section: Main Sequence Solar-like Starsmentioning

confidence: 99%

Magnetism, dynamo action and the solar-stellar connection

Brun

Browning

2017

Living Rev Sol Phys

232

182

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The Sun and other stars are magnetic: magnetism pervades their interiors and affects their evolution in a variety of ways. In the Sun, both the fields themselves and their influence on other phenomena can be uncovered in exquisite detail, but these observations sample only a moment in a single star's life. By turning to observations of other stars, and to theory and simulation, we may infer other aspects of the magnetism-e.g., its dependence on stellar age, mass, or rotation rate-that would be invisible from close study of the Sun alone. Here, we review observations and theory of magnetism in the Sun and other stars, with a partial focus on the "Solar-stellar connection": i.e., ways in which studies of other stars have influenced our understanding of the Sun and vice versa. We briefly review techniques by which magnetic fields can be measured (or their presence otherwise inferred) in stars, and then highlight some key observational findings uncovered by such measurements, focusing (in many cases) on those that offer particularly direct constraints on theories of how the fields are built and maintained. We turn then to a discussion of how the fields arise in different objects: first, we summarize some essential elements of convection and dynamo theory, including a very brief discussion of mean-field theory and related concepts. Next we turn to simulations of convection and magnetism in stellar interiors, highlighting both some peculiarities of field generation in different types of stars and some unifying physical processes that likely influence dynamo action in general. We conclude with a brief

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Section: Main Sequence Solar-like Starsmentioning

confidence: 99%

Magnetism, dynamo action and the solar-stellar connection

Brun

Browning

2017

Living Rev Sol Phys

232

182

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“…It may be also noted that F-stars of our sample are younger, and they rotate faster compared to cooler stars. Zeeman-Doppler imaging of See et al (2016) have shown that complexity of the field topology increases for smaller age or higher rotation rate.…”

Section: Marginal Stellar Dynamosmentioning

confidence: 99%

How supercritical are stellar dynamos, or why do old main-sequence dwarfs not obey gyrochronology?

Kitchatinov

Nepomnyashchikh

2017

Monthly Notices of the Royal Astronomical Society

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Asteroseismological determinations of stellar ages have shown that old main-sequence dwarfs do not obey gyrochronology. Their rotation is slow compared to young stars but faster than gyrochronology predicts. This can be explained by the presence of a maximum rotation period beyond which the large-scale dynamo switches off and stops providing global magnetic fields necessary for stellar spindown. Assuming this explanation, the excess of stellar dynamo parameters over their marginal values can be estimated for given spectral type and rotation rate. The estimation gives the dynamo number for the Sun about 10% above its critical value. The corresponding dynamo model provides -though with some further tuning -reasonable results for the Sun. Following the same approach, the differential rotation and marginal dynamo modes are computed for stars between 0.7 and 1.2 solar masses. With increasing stellar mass, the differential rotation and the ratio of toroidal-to-poloidal field are predicted to increase while the field topology changes from dipolar to mixed quadrupolar-dipolar parity.

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“…See et al 2016;Oláh et al 2016;García et al 2014, and references therein). These relationships are important to probe the theoretical models developed to simulate the magnetic dynamos.…”

Section: Introductionmentioning

confidence: 99%

Activity cycles in members of young loose stellar associations

Distefano

Lanzafame

Lanza

et al. 2017

A&A

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Context. Magnetic cycles analogous to the solar one have been detected in tens of solar-like stars by analyzing long-term time-series of different magnetic activity indexes. The relationship between the cycle properties and global stellar parameters is not fully understood yet. One reason for this is the sparseness of the data. Aims. In the present paper we searched for activity cycles in a sample of 90 young solar-like stars with ages between 4 and 95 Myr with the aim to investigate the properties of activity cycles in this age range. Methods. We measured the length P cyc of a given cycle by analyzing the long-term time-series of three different activity indexes: the period of rotational modulation, the amplitude of the rotational modulation and the median magnitude in the V band. For each star, we computed also the global magnetic activity index that is proportional to the amplitude of the rotational modulation and can be regarded as a proxy of the mean level of the surface magnetic activity. Results. We detected activity cycles in 67 stars. Secondary cycles were also detected in 32 stars of the sample. The lack of correlation between P cyc and P rot and the position of our targets in the P cyc /P rot − Ro −1 diagram suggest that these stars belong to the so-called Transitional Branch and that the dynamo acting in these stars is different from the solar one and from that acting in the older Mt. Wilson stars. This statement is also supported by the analysis of the butterfly diagrams whose patterns are very different from those seen in the solar case. We computed the Spearman correlation coefficient r S between P cyc , and different stellar parameters. We found that P cyc in our sample is uncorrelated with all the investigated parameters. The index is positively correlated with the convective turn-over time-scale, the magnetic diffusivity time-scale τ diff , and the dynamo number D N , whereas it is anti-correlated with the effective temperature T eff , the photometric shear ∆Ω phot and the radius R C at which the convective zone is located. We investigated how P cyc and evolve with the stellar age. We found that P cyc is about constant and that decreases with the stellare age in the range 4-95 Myr. Finally we investigated the magnetic activity of the star AB Dor A by merging ASAS time-series with previous long-term photometric data. We estimated the length of the AB Dor A primary cycle as P cyc = 16.78 ± 2yr and we found also shorter secondary cycles with lengths of 400 d, 190 d and 90 d respectively.

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The connection between stellar activity cycles and magnetic field topology

Cited by 81 publications

References 61 publications

Magnetism, dynamo action and the solar-stellar connection

Magnetism, dynamo action and the solar-stellar connection

How supercritical are stellar dynamos, or why do old main-sequence dwarfs not obey gyrochronology?

Activity cycles in members of young loose stellar associations

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