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

Kitchatinov, L. L.; Nepomnyashchikh, Alexander

doi:10.1093/mnras/stx1473

Cited by 35 publications

(34 citation statements)

References 50 publications

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“…Stars with very low activity are "superrotating," i.e., they rotate faster than expected based on their activity level. This is again a clear indication that these stars, notably 16 Cyg A and B (blue q and r symbols), α Cen A (blue k symbol), and KIC 8006161 (blue o symbol), have experienced reduced magnetic braking as a consequence of the large-scale cyclic dynamo having started to shut down (Kitchatinov & Nepomnyashchikh 2017). For 16 Cyg A and B (blue q and r symbols in Figure 10), this has already happened.…”

Section: Dependence On Convection Zone Thicknessmentioning

confidence: 86%

See 1 more Smart Citation

Evolution of Co-existing Long and Short Period Stellar Activity Cycles

Brandenburg

Mathur

Metcalfe

2017

ApJ

118

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The magnetic activity of the Sun becomes stronger and weaker over roughly an 11 year cycle, modulating the radiation and charged particle environment experienced by the Earth as "space weather." Decades of observations from the Mount Wilson Observatory have revealed that other stars also show regular activity cycles in their Ca II H+K line emission, and identified two different relationships between the length of the cycle and the rotation rate of the star. Recent observations at higher cadence have allowed the discovery of shorter cycles with periods between 1-3 yr. Some of these shorter cycles coexist with longer cycle periods, suggesting that two underlying dynamos can operate simultaneously. We combine these new observations with previous data, and we show that the longer and shorter cycle periods agree remarkably well with those expected from an earlier analysis based on the mean activity level and the rotation period. The relative turbulent length scales associated with the two branches of cyclic behavior suggest that a near-surface dynamo may be the dominant mechanism that drives cycles in more active stars, whereas a dynamo operating in deeper layers may dominate in less active stars. However, several examples of equally prominent long and short cycles have been found at all levels of activity of stars younger than 2.3 Gyr. Deviations from the expected cycle periods show no dependence on the depth of the convection zone or on the metallicity. For some stars that exhibit longer cycles, we compute the periods of shorter cycles that might be detected with future high-cadence observations.

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Section: Dependence On Convection Zone Thicknessmentioning

confidence: 86%

“…cal once the rotation rate drops below a critical rotation rate (Kitchatinov & Nepomnyashchikh 2017). The BV plot for our sample of F and G dwarfs is shown in Figure 4.…”

Section: F and G Dwarfsmentioning

confidence: 99%

Evolution of Co-existing Long and Short Period Stellar Activity Cycles

Brandenburg

Mathur

Metcalfe

2017

ApJ

118

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“…Scattering in theoretical estimates may be as large as order of magnitudes. The viewpoint of [39] that solar dynamo is just above the excitation threshold is close to our understanding of the situation however it is too far to be unanimously accepted by the community. Of course, some very qualitative scalings for D are possible.…”

Section: Magnetic Configurations Known For Spherical Dynamosmentioning

confidence: 99%

Symmetries of Magnetic Fields Driven by Spherical Dynamos of Exoplanets and Their Host Stars

2020

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Observations of exoplanets open a new area of scientific activity and the structure of exoplanet magnetospheres is an important part of this area. Here we use symmetry arguments and experiences in spherical dynamo modeling to obtain the set of possible magnetic configurations for exoplanets and their corresponding host stars. The main part of our results is that the possible choice is much richer than the basic dipole magnetic field of both exoplanets and stars. Other options, for example, are quadrupole configurations or mixed parity solutions. Expected configurations of current sheets for the above mentioned exoplanet host star systems are presented as well.

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“…Further, there is an upper value in the rotation period for each spectral type (Rengarajan 1984), also see Metcalfe et al (2016). These observations can be explained by the ceased of the large-scale dynamo above a certain rotation period; see discussion in Kitchatinov & Nepomnyashchikh (2017); Cameron & Schüssler (2017). Thus, the solar dynamo is possibly operating not too far from the critical to the dynamo transition.…”

Section: Introductionmentioning

confidence: 80%

“…Three-dimensional numerical simulations (Karak et al 2015b) and mean-field models (Kitchatinov & Olemskoy 2010) showed that a slightly subcritical dynamo is also possible when the initial condition for the magnetic field is strong. On the other hand, explaining the deviation of gyrochronology in dynamo model, Kitchatinov & Nepomnyashchikh (2017) predicted that solar dynamo is about 10% supercritical.…”

Section: Introductionmentioning

confidence: 99%

Supercriticality of the dynamo limits the memory of the polar field to one cycle

Kumar,

Karak,

Vashishth

2021

Preprint

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The polar magnetic field precursor is considered to be the most robust and physics-based method for the prediction of the next solar cycle strength. However, to make a reliable prediction of a cycle, is the polar field at the solar minimum of the previous cycle enough or we need the polar field of many previous cycles? To answer this question, we performed several simulations using Babcock-Leighton type flux transport dynamo models with stochastically forced source for the poloidal field (α term). We show that when the dynamo is operating near the critical dynamo transition or only weakly supercritical, the polar field of cycle n determines the amplitude of the next several cycles (at least three). However, when the dynamo is substantially supercritical, this correlation of the polar field is reduced to one cycle. This change in the memory of the polar field from multi-to one-cycle with the increase of the super-criticality of the dynamo is independent of the importance of various turbulent transport processes in the model. We further show that when the dynamo operates near the critical, it produces frequent extended episodes of weaker activity, resembling the solar grand minima. Occurrence of grand minima is accompanied with the multi-cycle correlation of polar field. The frequency of grand minima decreases with the increase of supercriticality of the dynamo.

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

Cited by 35 publications

References 50 publications

Evolution of Co-existing Long and Short Period Stellar Activity Cycles

Evolution of Co-existing Long and Short Period Stellar Activity Cycles

Symmetries of Magnetic Fields Driven by Spherical Dynamos of Exoplanets and Their Host Stars

Supercriticality of the dynamo limits the memory of the polar field to one cycle

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