Stellar differential rotation can be separated into two main regimes: solar-like when the equator rotates faster than the poles and antisolar when the polar regions rotate faster than the equator. We investigate the transition between these two regimes with 3D numerical simulations of rotating spherical shells. We conduct a systematic parameter study which also includes models from different research groups. We find that the direction of the differential rotation is governed by the contribution of the Coriolis force in the force balance, independently of the model setup (presence of a magnetic field, thickness of the convective layer, density stratification). Rapidly rotating cases with a small Rossby number yield solar-like differential rotation, while weakly rotating models sustain antisolar differential rotation. Close to the transition, the two kinds of differential rotation are two possible bistable states. This study provides theoretical support for the existence of antisolar differential rotation in cool stars with large Rossby numbers.

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 luminosities (or upper limits for undetected sources) and combine these with existing measurements from Wright & Drake (2016). We confirm the existence of fullyconvective stars in the X-ray unsaturated regime and find that these objects follow the same rotation-activity relationship seen for partly-convective stars. We measure the power-law slope of the relationship between Rossby number (the ratio of the rotation period to the convective turnover time) and the fractional X-ray luminosity for X-ray unsaturated fully-convective stars for the first time, and find it to be consistent with that found for partly-convective stars. We discuss this implications of this result for our understanding of stellar magnetic dynamos in fully-and partly-convective stars. Finally, we also use this data to improve empirical estimates of the convective turnover time for fully-convective stars.

M dwarfs are the most numerous stars in our Galaxy with masses between approximately 0.5 and 0.1 solar mass. Many of them show surface activity qualitatively similar to our Sun and generate flares, high X-ray fluxes, and largescale magnetic fields 1-4 . Such activity is driven by a dynamo powered by the convective motions in their interiors 2,5-8 . Understanding properties of stellar magnetic fields in these stars finds a broad application in astrophysics, including, e.g., theory of stellar dynamos and environment conditions around planets that may be orbiting these stars. Most stars with convective envelopes follow a rotation-activity relationship where various activity indicators saturate in stars with rotation periods shorter than a few days 2,6,8 . The activity gradually declines with rotation rate in stars rotating more slowly. It is thought that due to a tight empirical correlation between X-ray and magnetic flux 9 , the stellar magnetic fields will also saturate, to values around ∼ 4 kG 10 . Here we report the detection of magnetic fields above the presumed saturation limit in four fully convective M-dwarfs. By combining results from spectroscopic and polarimetric studies we explain our findings in terms of bistable dynamo models 11,12 : stars with the strongest magnetic fields are those in a dipole dynamo state, while stars in a multipole state cannot generate fields stronger than about four kilogauss. Our study provides observational evidence that dynamo in fully convective M dwarfs generates magnetic fields that can differ not only in the geometry of their large scale component, but also in the total magnetic energy.Our understanding of origin and evolution of the magnetic fields in M dwarfs is based on the models of the rotationally driven convective dynamos. Modern observations provide two important constraints for these models.First, from the analysis of circular polarization in spectral lines we infer that large-scale magnetic fields tend to have simple axisymmetric geometry with dominant poloidal component in stars that are fully convective. In contrast, M dwarfs that are hotter and therefore only 1 arXiv:1801.08571v1 [astro-ph.SR] 25 Jan 2018 partly convective tend to have more complex fields with strong toroidal components 13 . However, there is a number of exceptions when a rapidly-rotating fully convective star generates a large-scale magnetic field with a complex multipole geometry. This dichotomy of magnetic properties in stars that have similar stellar parameters may be explained in terms of dynamo bistability: stars can relax to either dipole or multipole states depending on the geometry and the amplitude of an initial seed magnetic field 11,12 . Note, however, that dynamo bistability was observed only in models of stars with masses M 0.2M .The second observational constraint is the rotation-activity relation 2,8,14,15 . A remarkable feature of this relation is the existence of two branches, a saturated and a non-saturated branch. In the non-saturated branch, the amount of non-thermal (e.g...

Despite the lack of a shear-rich tachocline region low-mass fully convective stars are capable of generating strong magnetic fields, indicating that a dynamo mechanism fundamentally different from the solar dynamo is at work in these objects. We present a self-consistent three dimensional model of magnetic field generation in low-mass fully convective stars. The model utilizes the anelastic magnetohydrodynamic equations to simulate compressible convection in a rotating sphere. A distributed dynamo working in the model spontaneously produces a dipole-dominated surface magnetic field of the observed strength. The interaction of this field with the turbulent convection in outer layers shreds it, producing small-scale fields that carry most of the magnetic flux. The Zeeman-Doppler-Imaging technique applied to synthetic spectropolarimetric data based on our model recovers most of the large-scale field. Our model simultaneously reproduces the morphology and magnitude of the large-scale field as well as the magnitude of the small-scale field observed on low-mass fully convective stars.

Earth sustains its magnetic field by a dynamo process driven by convection in the liquid outer core. Geodynamo simulations have been successful in reproducing many observed properties of the geomagnetic field. However, although theoretical considerations suggest that flow in the core is governed by a balance between Lorentz force, rotational force, and buoyancy (called MAC balance for Magnetic, Archimedean, Coriolis) with only minute roles for viscous and inertial forces, dynamo simulations must use viscosity values that are many orders of magnitude larger than in the core, due to computational constraints. In typical geodynamo models, viscous and inertial forces are not much smaller than the Coriolis force, and the Lorentz force plays a subdominant role; this has led to conclusions that these simulations are viscously controlled and do not represent the physics of the geodynamo. Here we show, by a direct analysis of the relevant forces, that a MAC balance can be achieved when the viscosity is reduced to values close to the current practical limit. Lorentz force, buoyancy, and the uncompensated (by pressure) part of the Coriolis force are of very similar strength, whereas viscous and inertial forces are smaller by a factor of at least 20 in the bulk of the fluid volume. Compared with nonmagnetic convection at otherwise identical parameters, the dynamo flow is of larger scale and is less invariant parallel to the rotation axis (less geostrophic), and convection transports twice as much heat, all of which is expected when the Lorentz force strongly influences the convection properties.geodynamo | magnetohydrodynamics | planetary dynamos | turbulence | rotating convection S ustained magnetism in astrophysical objects is due to the dynamo mechanism, which relies on the generation of electrical currents by fluid motion (1). The secular cooling of Earth's interior and the release of light elements at the boundary of the solid inner core provide buoyancy sources that drive convection, leading to the generation of electrical currents (2). It has been more than two decades since the idea of modeling the geomagnetic field using computer simulations was successfully demonstrated (3, 4). These pioneering simulations were able to reproduce the dipole-dominant nature of the geomagnetic field and showed reversals of the geomagnetic dipole. Since then, computer simulations have become a primary tool for studying the properties of the geomagnetic field (5-9).The range of flow length scales present in the liquid outer core is enormous due to the very small viscosity of the fluid. To model this aspect in geodynamo simulations would require tremendous computing power that is not available even in the foreseeable future. Therefore, all geodynamo simulations must use unrealistically large viscosity to reduce the level of turbulence. One quantity that epitomizes this discrepancy is the Ekman number E = νΩ −1 D −2 (ν is the viscosity, Ω is Earth's rotation rate, and D is the thickness of the liquid outer core), which roughly quantifies...

The recent discovery of an Earth-like exoplanet around Proxima Centauri has shined a spot light on slowly rotating fully convective M-stars. When such stars rotate rapidly (period 20 days), they are known to generate very high levels of activity that is powered by a magnetic field much stronger than the solar magnetic field. Recent theoretical efforts are beginning to understand the dynamo process that generates such strong magnetic fields. However, the observational and theoretical landscape remains relatively uncharted for fully convective M-stars that rotate slowly. Here, we present an anelastic dynamo simulation designed to mimic some of the physical characteristics of Proxima Centauri, a representative case for slowly rotating fully convective M-stars. The rotating convection spontaneously generates differential rotation in the convection zone that drives coherent magnetic cycles where the axisymmetric magnetic field repeatedly changes polarity at all latitudes as time progress. The typical length of the "activity" cycle in the simulation is about nine years, in good agreement with the recently proposed activity cycle length of about seven years for Proxima Centauri. Comparing our results with earlier work, we hypothesis that the dynamo mechanism undergoes a fundamental change in nature as fully convective stars spin down with age.

Numerical simulations of convection driven rotating spherical shell dynamos have often been performed with rigid boundary conditions, as is appropriate for the metallic cores of terrestrial planets. Free-slip boundaries are more appropriate for dynamos in other astrophysical objects, such as gas-giants or stars. Using a set of 57 direct numerical simulations, we investigate the effect of free-slip boundary conditions on the scaling properties of heat flow, flow velocity and magnetic field strength and compare it with earlier results for rigid boundaries. We find that the nature of the mechanical boundary condition has only a minor influence on the scaling laws. We also find that although dipolar and multipolar dynamos exhibit approximately the same scaling exponents, there is an offset in the scaling pre-factors for velocity and magnetic field strength. We argue that the offset can be attributed to the differences in the zonal flow contribution between dipolar and multipolar dynamos.

A large fraction of known Jupiter like exoplanets are inflated as compared to Jupiter. These "hot" Jupiters orbit close to their parent star and are bombarded with intense starlight. Many theories have been proposed to explain their radius inflation and several suggest that a small fraction of the incident starlight is injected in to the planetary interior which helps to puff up the planet. How will such energy injection affect the planetary dynamo? In this Letter, we estimate the surface magnetic field strength of hot Jupiters using scaling arguments that relate energy available in planetary interiors to the dynamo generated magnetic fields. We find that if we take into account the energy injected in the planetary interior that is sufficient to inflate hot Jupiters to observed radii, then the resulting dynamo should be able generate magnetic fields that are more than an order of magnitude stronger than the Jovian values. Our analysis highlights the potential fundamental role of the stellar light in setting the field strength in hot Jupiters.