scholarly journals Convection and dynamo action in B stars

2010 ◽  
Vol 6 (S271) ◽  
pp. 361-362 ◽  
Author(s):  
Kyle C. Augustson ◽  
Allan S. Brun ◽  
Juri Toomre
Keyword(s):  
Magnetic Fields ◽  
Main Sequence ◽  
Magnetic Energy ◽  
Magnetic Polarity ◽  
Dynamo Action ◽  
B Stars ◽  
Core Convection ◽  
Dependent Behavior ◽  
And Shifts

AbstractMain-sequence massive stars possess convective cores that likely harbor strong dynamo action. To assess the role of core convection in building magnetic fields within these stars, we employ the 3-D anelastic spherical harmonic (ASH) code to model turbulent dynamics within a 10 M⊙ main-sequence (MS) B-type star rotating at 4 Ω⊙. We find that strong (900 kG) magnetic fields arise within the turbulence of the core and penetrate into the stably stratified radiative zone. These fields exhibit complex, time-dependent behavior including reversals in magnetic polarity and shifts between which hemisphere dominates the total magnetic energy.

On the role of rotation in the generation of magnetic fields by fluid motions

1982 ◽  
Vol 306 (1492) ◽  
pp. 223-234 ◽  
Keyword(s):  
Magnetic Fields ◽  
Axial Symmetry ◽  
Fluid Motion ◽  
Liquid Core ◽  
Flow Speed ◽  
Magnetic Reynolds Number ◽  
Dynamo Action ◽  
Coriolis Forces ◽  
Lorentz Forces

It is generally accepted that the magnetic fields of planets and stars are produced by the self-exciting dynamo process (first proposed by Larmor) and that observed near-alignments of magnetic dipole axes with rotation axes are due to the influence of Coriolis forces on underlying fluid motions. The detailed role of rotation in the generation of cosmical magnetic fields has yet to be elucidated but useful insight can be obtained from general considerations of the governing magnetohydrodynamic equations. A magnetic field B cannot be maintained or amplified by fluid motion u against the effects of ohmic decay unless (a) the magnetic Reynolds number R = ULno is sufficiently large, and (b) the patterns of B and u are sufficiently complicated (where U is a characteristic flow speed, a characteristic length and J and o are typical values of the magnetic permeability and electrical conductivity respectively). Axisymmetric magnetic fields will always decay (a result that suggests that palaeomagnetic and archaeomagnetic data might show evidence that departures from axial symmetry in the geomagnetic field are systematically less during the decay phase of a polarity ‘ reversal ’ or * excursion ’ than during the recovery phase). Dynamo action is stimulated by Coriolis forces, which promote departures from axial symmetry in the pattern of uwhen B is weak, and is opposed by Lorentz forces, which increase in influence as B grows in strength. If equilibrium is attained when Coriolis and Lorentz forces are roughly equal in magnitude then the system becomes ‘ magnetostrophic' and the strengths of the internal and external parts of the field, and respectively, satisfy B i < B 8 R 1/2 and B e < B 8 R -1/2 if B 8 = UL~1 *X)/c f)i » (p being the mean density of the fluid and Q the angular speed of rotation). The slow and dispersive ‘magnetohydrodynamic inertial wave’ with a frequency that depends on the square of the Alfven speed [B]/(up) 1/2 and inversely on Q exemplifies magnetostrophic flow. Such waves probably occur in the electrically conducting fluid interiors of planets and stars, where they play an important role in the generation of magnetic fields as well as in other processes, such as the topographic coupling between the Earth’s liquid core and solid mantle.

scholarly journals Direct numerical simulations of helical dynamo action: MHD and beyond

2004 ◽  
Vol 11 (5/6) ◽  
pp. 619-629 ◽  
Author(s):  
D. O. Gómez ◽  
P. D. Mininni
Keyword(s):  
Magnetic Fields ◽  
Numerical Simulations ◽  
Periodic Boundary ◽  
Incompressible Flows ◽  
Magnetic Energy ◽  
Periodic Boundary Conditions ◽  
Direct Numerical Simulations ◽  
Dynamo Action ◽  
Spatial Fluctuations ◽  
Kinetic Effects

Abstract. Magnetohydrodynamic dynamo action is often invoked to explain the existence of magnetic fields in several astronomical objects. In this work, we present direct numerical simulations of MHD helical dynamos, to study the exponential growth and saturation of magnetic fields. Simulations are made within the framework of incompressible flows and using periodic boundary conditions. The statistical properties of the flow are studied, and it is found that its helicity displays strong spatial fluctuations. Regions with large kinetic helicity are also strongly concentrated in space, forming elongated structures. In dynamo simulations using these flows, we found that the growth rate and the saturation level of magnetic energy and magnetic helicity reach an asymptotic value as the Reynolds number is increased. Finally, extensions of the MHD theory to include kinetic effects relevant in astrophysical environments are discussed.

scholarly journals The source of magnetic fields in (neutron-) stars

2008 ◽  
Vol 4 (S259) ◽  
pp. 61-74 ◽  
Author(s):  
Hendrik C. Spruit
Keyword(s):  
Magnetic Fields ◽  
Neutron Stars ◽  
Core Collapse ◽  
Magnetorotational Instability ◽  
Dynamo Action ◽  
Field Amplification ◽  
Equilibrium Configurations ◽  
Puzzling Problem ◽  
Collapse Process

AbstractSome arguments, none entirely conclusive, are reviewed about the origin of magnetic fields in neutron stars, with emphasis of processes during and following core collapse in supernovae. Possible origins of the magnetic fields of neutron stars include inheritance from the main sequence progenitor and dynamo action at some stage of evolution of progenitor. Inheritance is not sufficient to explain the fields of magnetars. Energetic considerations point to differential rotation in the final stages of core collapse process as the most likely source of field generation, at least for magnetars. A runaway phase of exponential growth is needed to achieve sufficient field amplification during relevant phase of core collapse; it can probably be provided by a some form of magnetorotational instability. Once formed in core collapse, the field is in danger of decaying again by magnetic instabilities. The evolution of a magnetic field in a newly formed neutron star is discussed, with emphasis on the existence of stable equilibrium configurations as end products of this evolution, and the role of magnetic helicity in their existence. A particularly puzzling problem is the large range of field strengths observed in neutron stars (as well as in A stars and white dwarfs). It implies that a single, deterministic process is insufficient to explain the origin of the magnetic fields in these stars.

Magnetohydrodynamics: Overview

2020 ◽  
Author(s):  
E.R. Priest
Keyword(s):  
Magnetic Fields ◽  
Magnetic Energy ◽  
Liquid Metals ◽  
Plasma Heating ◽  
The Sun ◽  
Pressure Gradients ◽  
Dynamo Action ◽  
Electrically Conducting ◽  
Stellar Flares ◽  
Outer Atmosphere

Magnetohydrodynamics is sometimes called magneto-fluid dynamics or hydromagnetics and is referred to as MHD for short. It is the unification of two fields that were completely independent in the 19th, and first half of the 20th, century, namely, electromagnetism and fluid mechanics. It describes the subtle and complex nonlinear interaction between magnetic fields and electrically conducting fluids, which include liquid metals as well as the ionized gases or plasmas that comprise most of the universe. In places such as the Earth’s magnetosphere or the Sun’s outer atmosphere (the corona) where the magnetic field provides an important component of the free energy, MHD effects are responsible for much of the observed dynamic behavior, such as geomagnetic substorms, solar flares and huge eruptions from the Sun that dominate the Earth’s space weather. However, MHD is also of great importance in astrophysics, since many of the MHD processes that are observed in the laboratory or in the Sun and the magnetosphere also take place under different parameter regimes in more exotic cosmical objects such as active stars, accretion discs, and black holes. The different aspects of MHD include determining the nature of: magnetic equilibria under a balance between magnetic forces, pressure gradients and gravity; MHD wave motions; magnetic instabilities; and the important process of magnetic reconnection for converting magnetic energy into other forms. In turn, these aspects play key roles in the fundamental astrophysical processes of magnetoconvection, magnetic flux emergence, star spots, plasma heating, stellar wind acceleration, stellar flares and eruptions, and the generation of magnetic fields by dynamo action.

Magnetohydrodynamic Turbulence Decay Under the Influence of Uniform or Random Magnetic Fields

2011 ◽  
Vol 133 (8) ◽  
Author(s):  
Jacques C. Richard ◽  
Benjamin M. Riley ◽  
Sharath S. Girimaji
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Lorentz Force ◽  
Magnetic Energy ◽  
Small Scale ◽  
Magnetohydrodynamic Turbulence ◽  
Turbulence Decay ◽  
Structure Orientation ◽  
The Magnetic Field

We perform direct numerical simulations of decaying magnetohydrodynamic turbulence subject to initially uniform or random magnetic fields. We investigate the following features: (i) kinetic–magnetic energy exchange and velocity field anisotropy, (ii) action of Lorentz force, (iii) enstrophy and helicity behavior, and (iv) internal structure of the small scales. While tendency toward kinetic–magnetic energy equi-partition is observed in both uniform and random magnetic field simulations, the manner of approach to that state is very different in the two cases. Overall, the role of the Lorentz force is merely to bring about the equi-partition. No significant variance anisotropy of velocity fluctuations is observed in any of the simulations. The mechanism of enstrophy generation changes with the strength of the magnetic field, and helicity shows no significant growth in any of the cases. The small-scale structure (orientation between vorticity and strain-rate eigenvectors) does not appear to be influenced by the magnetic field.

scholarly journals Magnetic fields in M dwarfs from the CARMENES survey

2019 ◽  
Vol 626 ◽  
pp. A86 ◽  
Author(s):  
D. Shulyak ◽  
A. Reiners ◽  
E. Nagel ◽  
L. Tal-Or ◽  
J. A. Caballero ◽  
...  
Keyword(s):  
Magnetic Fields ◽  
Near Infrared ◽  
Magnetic Energy ◽  
Spectral Lines ◽  
High Spectral Resolution ◽  
Stellar Rotation ◽  
M Dwarfs ◽  
Dynamo Action ◽  
Rotation Periods ◽  
Consistent Picture

Context. M dwarfs are known to generate the strongest magnetic fields among main-sequence stars with convective envelopes, but we are still lacking a consistent picture of the link between the magnetic fields and underlying dynamo mechanisms, rotation, and activity. Aims. In this work we aim to measure magnetic fields from the high-resolution near-infrared spectra taken with the CARMENES radial-velocity planet survey in a sample of 29 active M dwarfs and compare our results against stellar parameters. Methods. We used the state-of-the-art radiative transfer code to measure total magnetic flux densities from the Zeeman broadening of spectral lines and filling factors. Results. We detect strong kG magnetic fields in all our targets. In 16 stars the magnetic fields were measured for the first time. Our measurements are consistent with the magnetic field saturation in stars with rotation periods P < 4 d. The analysis of the magnetic filling factors reveal two different patterns of either very smooth distribution or a more patchy one, which can be connected to the dynamo state of the stars and/or stellar mass. Conclusions. Our measurements extend the list of M dwarfs with strong surface magnetic fields. They also allow us to better constrain the interplay between the magnetic energy, stellar rotation, and underlying dynamo action. The high spectral resolution and observations at near-infrared wavelengths are the beneficial capabilities of the CARMENES instrument that allow us to address important questions about the stellar magnetism.

scholarly journals 2-D and 3-D models of convective turbulence and oscillations in intermediate-mass main-sequence stars

2015 ◽  
Vol 11 (A29B) ◽  
pp. 540-543
Author(s):  
Joyce A. Guzik ◽  
T. H. Morgan ◽  
N. J. Nelson ◽  
C. Lovekin ◽  
K. Kosak ◽  
...  
Keyword(s):  
Main Sequence ◽  
Convection Zone ◽  
Low Frequency ◽  
Main Sequence Stars ◽  
Low Frequencies ◽  
Rotation Rates ◽  
Multidimensional Modeling ◽  
Core Convection ◽  
Radiative Zone

AbstractWe present multidimensional modeling of convection and oscillations in main-sequence stars somewhat more massive than the Sun, using three separate approaches: 1) Using the 3-D planar StellarBox radiation hydrodynamics code to model the envelope convection zone and part of the radiative zone. Our goals are to examine the interaction of stellar pulsations with turbulent convection in the envelope, excitation of acoustic modes, and the role of convective overshooting; 2) Applying the spherical 3-D MHD ASH (Anelastic Spherical Harmonics) code to simulate the core convection and radiative zone. Our goal is to determine whether core convection can excite low-frequency gravity modes, and thereby explain the presence of low frequencies for some hybrid γ Dor/δ Sct variables for which the envelope convection zone is too shallow for the convective blocking mechanism to drive gravity modes; 3) Applying the ROTORC 2-D stellar evolution and dynamics code to calculate evolution with a variety of initial rotation rates and extents of core convective overshooting. The nonradial adiabatic pulsation frequencies of these nonspherical models are calculated using the 2-D pulsation code NRO. We present new insights into pulsations of 1-2 M⊙ stars gained by multidimensional modeling.

scholarly journals Magnetic fields and internal mixing of main sequence B stars

2014 ◽  
Vol 9 (S307) ◽  
pp. 401-403 ◽  
Author(s):  
G. A. Wade ◽  
C. P. Folsom ◽  
J. Grunhut ◽  
J. D. Landstreet ◽  
V. Petit
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Main Sequence ◽  
Field Measurements ◽  
High Quality ◽  
B Stars ◽  
Surface Rotation ◽  
Internal Mixing ◽  
Magnetic Field Measurements

AbstractWe have obtained high-quality magnetic field measurements of 19 sharp-lined B-type stars with precisely-measured N/C abundance ratios (Nieva & Przybilla 2012). Our primary goal is to test the idea (Meynetet al. 2011) that a magnetic field may explain extra drag (through the wind) on the surface rotation, thus producing more internal shear and mixing, and thus could provide an explanation for the appearance of slowly rotating N-rich main sequence B stars.

scholarly journals Magnetism, rotation and accretion in Herbig Ae-Be stars

2007 ◽  
Vol 3 (S243) ◽  
pp. 43-50 ◽  
Author(s):  
E. Alecian ◽  
G.A. Wade ◽  
C. Catala ◽  
C. Folsom ◽  
J. Grunhut ◽  
...  
Keyword(s):  
Magnetic Fields ◽  
Main Sequence ◽  
Be Stars ◽  
Origin And Evolution ◽  
Intermediate Mass Stars ◽  
New Information ◽  
Late Formative ◽  
Stellar Masses ◽  
Late Stages

AbstractStudies of stellar magnetism at the pre-main sequence phase can provide important new insights into the detailed physics of the late stages of star formation, and into the observed properties of main sequence stars. This is especially true at intermediate stellar masses, where magnetic fields are strong and globally organised, and therefore most amenable to direct study. This talk reviews recent high-precision ESPaDOnS observations of pre-main sequence Herbig Ae-Be stars, which are yielding qualitatively new information about intermediate-mass stars: the origin and evolution of their magnetic fields, the role of magnetic fields in generating their spectroscopic activity and in mediating accretion in their late formative stages, and the factors influencing their rotational angular momentum.

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