The magnetic activity of late-type stars is correlated with their rotation rates. Up to a certain limit, stars with smaller Rossby numbers, defined as the rotation period divided by the convective turnover time, have higher activity. A more detailed look at this rotation-activity relation reveals that, rather than being a simple power law relation, the activity scaling has a shallower slope for the low-Rossby stars than for the high-Rossby ones. We find that, for the chromospheric CaII H&K activity, this scaling relation is well modelled by a broken two-piece power law. Furthermore, the knee-point of the relation coincides with the axisymmetry to non-axisymmetry transition seen in both the spot activity and surface magnetic field configuration of active stars. We interpret this knee-point as a dynamo transition between dominating axi- and non-axisymmetric dynamo regimes with a different dependence on rotation and discuss this hypothesis in the light of current numerical dynamo models.
We analyze the X-ray emission and chromospheric activity of Sun-like stars of F, G and K spectral classes (late-type stars) studied in the framework of the HK project. More powerful coronas are possessed by stars displaying irregular variations of their chromospheric emission, while stars with regular cyclic activity are characterized by comparatively modest X-ray luminosities and ratios of the X-ray to bolometric luminosity. This indicates that the nature of processes associated with magnetic-field amplification on the convective envelope changes appreciably in the transition from small to la…
The duration of activity growths in solar cycles is on average shorter than the duration of its declines. This asymmetry can result from fluctuations in dynamo parameters. A solar dynamo model with fluctuations in the $α$-effect shows the statistical asymmetry which increases with both fluctuation amplitude and coherence time. An interpretation for the asymmetry origin is suggested, which predicts a correlation between the asymmetry measure and delay of the polar field reversals relative to the activity maxima. Data on the twelve latest solar cycles confirm such a correlation.
One interpretation of the activity and magnetism of late-type stars is that these both intensify with decreasing Rossby number up to a saturation level, suggesting that stellar dynamos depend on both rotation and convective turbulence. Some studies have claimed, however, that rotation alone suffices to parametrise this scaling adequately. Here, we tackle the question of the relevance of turbulence to stellar dynamos by including evolved, post main sequence stars in the analysis of the rotation-activity relation. These stars rotate very slowly compared with main sequence stars, but exhibit simi…
The Rotation-Activity Correlations in K and M dwarfs. II. New constraints on the dynamo mechanisms in late-K and M dwarfs before and at the transition to complete convection
Auroral radio emission from late L and T dwarfs: A new constraint on dynamo theory in the substellar regime
我们用卡尔G. Jansky甚大阵列（VLA），以调查与大规模极光流相关的高度圆偏振无线电发射的存在。以前的调查涵盖了约60个L 6或更高的目标，在这个光谱范围内，只有一个检测。我们的样本包括先前检测到的T6.5矮星2 MASS 10475385+2124234以及5个新的目标，这些目标是由于H-α发射或光学/红外光度变化的存在而选择的，这些都是极光活动的可能表现。我们检测到2 MASS 10475385+2124234，以及我们的偏置样本中的5个目标中的4个，包括强IR变量SIMP J 01365662 +0933473和明亮的H-α发射体2 MASS 12373919+6526148，加强了这些不同波长的活动相关的可能性。无线电发射频率对应于每个矮星表面附近的下限磁场强度的精确测定，这个新的样本提供了强大的限制发电机理论在低质量的褐矮星制度。确认5/6个靶的磁场>2.5 kG。我们的研究结果提供了初步的证据表明，发电机在这种质量制度可能是不一致的预测值从最近提出的模型。在更高的射频上进行进一步的观测对于验证这一论断是必不可少的。
In recent years, observers have found that the fraction of M-stars demonstrating significant magnetic activity transitions sharply from roughly $10\%$ for main-sequence stars earlier (more massive) than spectral type M3.5 (0.35 M$_\odot$) to nearly $90\%$ for stars later than M3.5. The latter are typically rotating quite rapidly, suggesting differing spin-down histories. Tantalizingly, it is also later than M3.5 at which main-sequence stars become fully convective, and may no longer contain a tachocline, a layer of rotational shear revealed by helioseismology to separate the radiative zone (RZ…
In 1844 Schwabe discovered that the number of sunspots increased and decreased over a period of about 11 years, that variation became known as the sunspot cycle. Almost eighty years later, Hale described the nature of the Sun's magnetic field, identifying that it takes about 22 years for the Sun's magnetic polarity to cycle. It was also identified that the latitudinal distribution of sunspots resembles the wings of a butterfly showing migration of sunspots in each hemisphere that abruptly start at mid-latitudes towards the Sun's equator over the next 11 years. These sunspot patterns were shown…
The $Λ$ effect describes a rotation-induced non-diffusive contribution to the Reynolds stress. It is commonly held responsible for maintaining the observed differential rotation of the Sun and other late-type stars. Here the sensitivity of the $Λ$ effect to small-scale magnetic fields and compressibility is studied by means of forced turbulence simulations either with anisotropic forcing in fully periodic cubes or in density-stratified domains with isotropic forcing. Effects of small-scale magnetic fields are studied in cases where the magnetic fields are self-consistently generated by a small…
Recent advances in mean-field theory are reviewed and applications to the Sun, late-type stars, accretion disks, galaxies, and the early Universe are discussed. We focus particularly on aspects of spatio-temporal nonlocality, which is one of the main insights that emerged from applying the test-field method to magnetic fields of different length and timescales. We also review the status of nonlinear quenching and the relation to magnetic helicity, which is an important observational diagnostic of modern solar dynamo theory. Both solar and some stellar dynamos seem to operate in an intermediate…
Weak magnetic fields have recently been detected in a number of A-type stars, including Vega and Sirius. At the same time, space photometry observations of A- and late B-type stars from Kepler and TESS have highlighted the existence of rotational modulation of surface features akin to stellar spots. Here we explore the possibility that surface magnetic spots might be caused by the presence of small envelope convective layers at or just below the stellar surface, caused by recombination of H and He. Using 1D stellar evolution calculations and assuming an equipartition dynamo, we make simple est…
We scrutinize the red dwarf component in the eclipsing binary system V471 Tau in order to unravel relations between different activity layers from the stellar surface through the chromosphere up to the corona. We aim at studying how the magnetic dynamo in the late-type component is affected by the close white dwarf companion. We use space photometry, high resolution spectroscopy and X-ray observations from different space instruments to explore the main characteristics of magnetic activity. From K2 photomery we find that 5-10 per cent of the apparent surface of the red dwarf is covered by cool…
(abridged) Context: Main-sequence late-type stars with masses less than $0.35 M_\odot$ are fully convective. Aims: The goal is to study convection, differential rotation, and dynamos as functions of rotation in fully convective stars. Methods: Three-dimensional hydrodynamic and magnetohydrodynamic numerical simulations with a star-in-a-box model, where a spherical star is immersed inside of a Cartesian cube, are used. The model corresponds to a $0.2M_\odot$ M5 dwarf. Rotation periods ($P_{\rm rot}$) between 4.3 and 430 days are explored. Results: The slowly rotating model with $P_{\rm rot}=430…
We analyze physical characteristics of late-type stars wherein the Kepler mission registered superflares. We use the revised stellar fundamental parameters, i.e. effective temperatures Teff and surface gravitational accelerations log g, from the Kepler archive published in 2017, as compared to previous studies by Balona (2015) based on the release of 2011. Among superflare stars there are both single objects and members of eclipsing binaries. We select the late-type stars (with Teff $< 6500$ K) wherein occured the most powerful flares with the total flare energy $> 10^{35}$ erg and consider th…
The most powerful superflares reaching 10$^{39}$erg bolometric energy are from giant stars. The mechanism behind flaring is supposed to be the magnetic reconnection, which is closely related to magnetic activity including starspots. However, it is poorly understood, how the underlying magnetic dynamo works and how the flare activity is related to the stellar properties which eventually control the dynamo action. We analyse the flaring activity of KIC 2852961, a late-type giant star, in order to understand how the flare statistics are related to that of other stars with flares and superflares a…
The coronal activity-rotation relationship is considered to be a proxy for the underlying stellar dynamo responsible for magnetic activity in solar and late-type stars. While this has been studied in considerable detail for partly-convective stars that are believed to operate an interface dynamo, it is poorly unconstrained in fully-convective stars that lack the necessary shear layer between radiative core and the convective envelope. We present new X-ray observations of 19 slowly-rotating fully-convective stars with rotation periods from the MEarth Project. We use these to calculate X-ray lum…
Gravitational waves provide a unique and powerful opportunity to constrain the dynamics in the interior of proto-neutron stars during core collapse supernovae. Convective motions play an important role in generating neutron stars magnetic fields, which could explain magnetar formation in the presence of fast rotation. We compute the gravitational wave emission from proto-neutron star convection and its associated dynamo, by post-processing three-dimensional MHD simulations of a model restricted to the convective zone in the anelastic approximation. We consider two different proto-neutron star …
The dynamo mechanism is a process of magnetic field self-excitation in a moving electrically conducting fluid. One of the most interesting applications of this mechanism related to the astrophysical systems is the case of a random motion of plasma. For the very first stage of the process, the governing dynamo equation can be reduced to a system of first order ordinary differential equations. For this case we suggest a regular method to calculate the growth rate of magnetic energy. Based on this method we calculate the growth rate for random flow with finite memory time and anisotropic statisti…
We study a turbulent helical dynamo in a periodic domain by solving the ideal magnetohydrodynamic (MHD) equations with the FLASH code using the divergence-cleaning eight-wave method and compare our results with direct numerical simulations (DNS) using the Pencil Code. At low resolution, FLASH reproduces the DNS results qualitatively by developing the large-scale magnetic field expected from DNS, but at higher resolution, no large-scale magnetic field is obtained. In all those cases in which a large-scale magnetic field is generated, the ideal MHD results yield too little power at small scales.…
