Recent progress in observational studies of magnetic activity in M dwarfs urgently requires support from ideas of stellar dynamo theory. We propose a strategy to connect observational and theoretical studies. In particular, we suggest four magnetic configurations that appear relevant to dwarfs from the viewpoint of the most conservative version of dynamo theory, and discuss observational tests to identify the configurations observationally. As expected, any such identification contains substantial uncertainties. However the situation in general looks less pessimistic than might be expected. Several identifications between the phenomenology of individual stars and dynamo models are suggested. Remarkably, all models discussed predict substantial surface magnetic activity at rather high stellar latitudes. This prediction looks unexpected from the viewpoint of our experience observing the Sun (which of course differs in some fundamental ways from these late-type dwarfs). We stress that a fuller understanding of the topic requires a long-term (at least 15 years) monitoring of M dwarfs by Zeeman-Doppler imaging.
The origin of strong magnetic fields in white dwarfs has been a puzzle for decades. Recently, a dynamo mechanism operating in rapidly rotating and crystallizing white dwarfs has been suggested to explain the occurrence rates of strong magnetic fields in white dwarfs with close low-mass main sequence star companions. Here we investigate whether the same mechanism may produce strong magnetic fields in close double white dwarfs. The only known strongly magnetic white dwarf that is part of a close double white dwarf system, the magnetic component of NLTT 12758, is rapidly rotating and likely cryst…
M dwarfs are low-mass main-sequence stars, the most numerous type of stars in the solar neighbourhood, which are known to have significant magnetic activity. The aim of this work is to explore the dynamo solutions and magnetic fields of fully convective M dwarfs with varying magnetic Prandtl numbers ${\rm Pr_M}$, and a rotation period (${\rm P_{rot}}$) of 43 days. ${\rm Pr_M}$ is known to play an important role in the dynamo action; dynamos for low-$\rm Pr_M$ and large-$\rm Pr_M$ have very different properties. We performed three-dimensional magnetohydrodynamical (MHD) numerical simulations wi…
It has been recently suggested that white dwarfs generate magnetic fields in a process analogous to the Earth. The crystallization of the core creates a compositional inversion that drives convection, and combined with rotation, this can sustain a magnetic dynamo. We reanalyse the dynamo mechanism, arising from the slow crystallization of the core, and find convective turnover times $t_{\rm conv}$ of weeks to months - longer by orders of magnitude than previously thought. With white dwarf spin periods $P\ll t_{\rm conv}$, crystallization-driven dynamos are almost always in the fast rotating re…
Retarded Stellar Dynamo in Tidally Deformed M Dwarfs
National Astronomical Observatories, Chinese Academy of Sciences（中国科学院国家天文台）；Beijing Normal University（北京师范大学）；Shanghai Astronomical Observatory, Chinese Academy of Sciences（中国科学院上海天文台）；University of Chinese Academy of Sciences（中国科学院大学）；Chongqing University（重庆大学）
Houdebine et al (2017: H17) combined CaII data with projected rotational velocities (v sin i) to construct rotation-activity correlations (RAC) in K-M dwarfs. The RAC slopes were used to argue that a transition between dynamo modes occurs at a spectral type between M2 and M3. H17 suggested that the dynamo transition corresponds to a transition to complete convection (TTCC). An independent study of GAIA data led Jao et al (2018) to suggest that the TTCC sets in near M3.0V, close to the H17 result. However, the changes in a star which cause TTCC signatures in GAIA data might not lead to changes …
M-dwarf stars below a certain mass are convective from their cores to their photospheres. These fully convective objects are extremely numerous, very magnetically active, and the likely hosts of many exoplanets. Here we study, for the first time, dynamo action in simulations of stratified, rotating fully convective M-dwarf stars. Importantly, we use new techniques to capture the correct full ball geometry down to the center of the star. We find surprising dynamo states in these systems, with the global-scale mean fields confined strongly to a single hemisphere, in contrast to prior stellar dyn…
We investigate whether the recently suggested rotation and crystallization driven dynamo can explain the apparent increase of magnetism in old metal polluted white dwarfs. We find that the effective temperature distribution of polluted magnetic white dwarfs is in agreement with most/all of them having a crystallizing core and increased rotational velocities are expected due to accretion of planetary material which is evidenced by the metal absorption lines. We conclude that a rotation and crystallization driven dynamo offers not only an explanation for the different occurrence rates of strongl…
M-dwarf stars are well known for the intense magnetic activity that many of them exhibit. In cool stars with near-surface convection zones, this magnetic activity is thought to be driven largely by the interplay of convection and the large scale differential rotation and circulations it establishes. The highly nonlinear nature of these flows yields a fascinatingly sensitive and diverse parameter space, with a wide range of possible dynamics. We report here on a set of three global MHD simulations of rapidly rotating M2 (0.4 $M_\odot$) stars. Each of these three models established nests of vigo…
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
We investigate the possibility to apply the already suggested by Sarafopoulos (2017, 2019) main concept of dynamo action to red dwarfs and red giant and supergiant stars. Thus, we attempt to establish a unified dynamo action, being potentially at work at widely varying stellar domains. Thus, the powerful, unique and leading entity generating the primary stellar magnetic field remains the so-called Torus structure. Within the Torus the same sign charges are mutually attracted and the Torus could be simulated as a superconductor. An existing gradient of the rotation rate accumulates net charge i…
We have carried out numerical simulations of the convection zone in a K dwarf of 0.7 solar masses rotating at the solar rotation period. We study the convection pattern, the differential rotation and meridional flows, and the dynamo-generated magnetic field. We find that for a star of this type, the solar rotation period represents a case of fairly rapid rotation and the differential is solar-type. A dynamo-generated large scale field appears but it is neither dipolar nor does it show a simple activity cycle.
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…
Kilogauss magnetic fields from simulations of small-scale dynamo action in the convective envelope of a white dwarf
Istituto ricerche solari Aldo e Cele Daccò (IRSOL), Faculty of Informatics, Università della Svizzera italiana (USI)（瑞士意大利大学信息学院太阳物理研究所）；Euler Institute, Università della Svizzera italiana (USI)（瑞士意大利大学欧拉研究所）；Center for Astrophysics, Harvard & Smithsonian（哈佛-史密森尼天体物理中心）；Zentrum für Astronomie der Universität Heidelberg, Landessternwarte（海德堡大学天文中心州立天文台）；Institut für Sonnenphysik (KIS)（太阳物理研究所）；Department of Physics, The University of Warwick（华威大学物理系）
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。
For more than thirty years, the dynamical maintenance of the thin solar tachocline has remained one of the central outstanding problems of stellar astrophysics. Three main theories have been developed to explain the tachocline's thinness, but so far none of them has been shown to work convincingly in the extreme parameter regime of the solar interior. Here, we present a rotating, 3D, spherical-shell simulation of a combined solar-like convection zone and radiative zone that achieves a tachocline built and maintained by convective dynamo action. Because of numerical constraints, the dynamo prev…
The evolution of a star is influenced by its internal rotation dynamics through transport and mixing mechanisms, which are poorly understood. Magnetic fields can play a role in transporting angular momentum and chemical elements, but the origin of magnetism in radiative stellar layers is unclear. Using global numerical simulations, we identify a subcritical transition to turbulence due to the generation of a magnetic dynamo. Our results have many of the properties of the theoretically-proposed Tayler-Spruit dynamo mechanism, which strongly enhances transport of angular momentum in radiative zo…
Recently, Bahramian et al. investigated a large sample of globular clusters (GCs) and found that bright intermediate polars (IPs) are a factor of 10 less frequent in GCs than in the Galactic field. We theoretically investigate here this discrepancy based on GC numerical simulations. We found that, due to disruptive dynamical interaction, there is on average a reduction of only half of bright IP progenitors, which is clearly not enough to explain the observed deficiency. However, if the rotation- and crystallization-driven dynamo scenario recently proposed by Schreiber et al. is incorporated in…
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…
A major outstanding problem in solar physics is the confinement of the solar tachocline, the thin shear layer that separates nearly solid-body rotation in the radiative interior from strong differential rotation in the convection zone. Here, we present the first 3-D, global solar simulation in which a tachocline is confined by a self-excited dynamo. The non-axisymmetric magnetism is initially built in the convection zone and then diffusively imprints downward. Additionally, the field is locally amplified throughout the radiative interior by vigorous horizontal motions that arise from equatoria…
Our understanding of stellar dynamos has largely been driven by the phenomena we have observed of our own Sun. Yet, as we amass longer-term datasets for an increasing number of stars, it is clear that there is a wide variety of stellar behavior. Here we briefly review observed trends that place key constraints on the fundamental dynamo operation of solar-type stars to fully convective M dwarfs, including: starspot and sunspot patterns, various magnetism-rotation correlations, and mean field flows such as differential rotation and meridional circulation. We also comment on the current insight t…
A Comprehensive Study of the Magnetic White Dwarfs in DESI DR1
University of Florida（佛罗里达大学）；Université de Montréal（蒙特利尔大学）；University of Oklahoma（俄克拉荷马大学）；Center for Astrophysics | Harvard & Smithsonian（哈佛-史密森天体物理中心）
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 large-scale magnetic fields. Such activity is driven by a dynamo powered by the convective motions in their interiors. 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 enve…
Intense but Harmless: Exo-Space Weather Around an M Dwarf with a Single-Hemisphere Dynamo
Peking University（北京大学）；National Space Science Center, Chinese Academy of Sciences（中国科学院国家空间科学中心）；Leibniz Institute for Astrophysics Potsdam（波茨坦阿斯特拉诺伊莱布尼茨研究所）；University of Massachusetts Lowell（马萨诸塞大学洛厄尔分校）；Lockheed Martin Solar and Astrophysics Laboratory（洛克希德·马丁太阳与天体物理实验室）；University of Potsdam（波茨坦大学）；Nanjing University（南京大学）
A milestone of dynamo theory is Cowling's theorem, known in its modern form as the impossibility for an axisymmetric velocity field to generate an axisymmetric magnetic field by dynamo action. Using an anisotropic electrical conductivity we show that an axisymmetric dynamo is in fact possible with a motion as simple as solid body rotation. On top of that the instability analysis can be conducted entirely analytically, leading to an explicit expression of the dynamo threshold which is the only example in dynamo theory.
