scholarly journals Non-ideal magnetohydrodynamics versus turbulence – I. Which is the dominant process in protostellar disc formation?

2020 ◽  
Vol 495 (4) ◽  
pp. 3795-3806 ◽  
Author(s):  
James Wurster ◽  
Benjamin T Lewis
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Rotation Axis ◽  
Low Mass Stars ◽  
Rotation Rates ◽  
Ideal Mhd ◽  
Low Mass ◽  
Dominant Process ◽  
Ideal Magnetohydrodynamics ◽  
Disc Formation

ABSTRACT Non-ideal magnetohydrodynamics (MHD) is the dominant process. We investigate the effect of magnetic fields (ideal and non-ideal) and turbulence (sub- and transsonic) on the formation of circumstellar discs that form nearly simultaneously with the formation of the protostar. This is done by modelling the gravitational collapse of a 1 M⊙ gas cloud that is threaded with a magnetic field and imposed with both rotational and turbulent velocities. We investigate magnetic fields that are parallel/antiparallel and perpendicular to the rotation axis, two rotation rates, and four Mach numbers. Disc formation occurs preferentially in the models that include non-ideal MHD where the magnetic field is antiparallel or perpendicular to the rotation axis. This is independent of the initial rotation rate and level of turbulence, suggesting that subsonic turbulence plays a minimal role in influencing the formation of discs. Aside from first core outflows that are influenced by the initial level of turbulence, non-ideal MHD processes are more important than turbulent processes during the formation of discs around low-mass stars.

scholarly journals Non-ideal magnetohydrodynamics versus turbulence II: Which is the dominant process in stellar core formation?

2020 ◽  
Vol 495 (4) ◽  
pp. 3807-3818 ◽  
Author(s):  
James Wurster ◽  
Benjamin T Lewis
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Rotation Axis ◽  
Core Formation ◽  
Low Mass Stars ◽  
Stellar Core ◽  
Ideal Mhd ◽  
Low Mass ◽  
Dominant Process ◽  
Ideal Magnetohydrodynamics

ABSTRACT Non-ideal magnetohydrodynamics (MHD) is the dominant process. We investigate the effect of magnetic fields (ideal and non-ideal) and turbulence (sub- and transsonic) on the formation of protostars by following the gravitational collapse of 1 M⊙ gas clouds through the first hydrostatic core to stellar densities. The clouds are imposed with both rotational and turbulent velocities, and are threaded with a magnetic field that is parallel/antiparallel or perpendicular to the rotation axis; we investigate two rotation rates and four Mach numbers. The initial radius and mass of the stellar core are only weakly dependent on the initial parameters. In the models that include ideal MHD, the magnetic field strength implanted in the protostar at birth is much higher than observed, independent of the initial level of turbulence; only non-ideal MHD can reduce this strength to near or below the observed levels. This suggests that not only is ideal MHD an incomplete picture of star formation, but that the magnetic fields in low mass stars are implanted later in life by a dynamo process. Non-ideal MHD suppresses magnetically launched stellar core outflows, but turbulence permits thermally launched outflows to form a few years after stellar core formation.

scholarly journals Magnetic field observations of low-mass stars

2008 ◽  
Vol 4 (S259) ◽  
pp. 339-344
Author(s):  
Ansgar Reiners
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Magnetic Measurements ◽  
Polarized Light ◽  
M Dwarfs ◽  
Magnetic Field Generation ◽  
Low Mass Stars ◽  
Strong Magnetic Fields ◽  
Direct Measurements ◽  
Low Mass

AbstractDirect measurements of magnetic fields in low-mass stars of spectral class M have become available during the last years. This contribution summarizes the data available on direct magnetic measurements in M dwarfs from Zeeman analysis in integrated and polarized light. Strong magnetic fields at kilo-Gauss strength are found throughout the whole M spectral range, and so far all field M dwarfs of spectral type M6 and later show strong magnetic fields. Zeeman Doppler images from polarized light find weaker fields, which may carry important information on magnetic field generation in partially and fully convective stars.

scholarly journals Rotation and Magnetic Fields of Solar-Like Stars

2004 ◽  
Vol 215 ◽  
pp. 258-267 ◽  
Author(s):  
J.-F. Donati
Keyword(s):  
Magnetic Fields ◽  
Large Scale ◽  
Current Knowledge ◽  
Global Dynamics ◽  
Low Mass Stars ◽  
Feedback Effects ◽  
Rotation Rates ◽  
Magnetic Imaging ◽  
Low Mass ◽  
New Discovery

In this paper, I review most of the current knowledge about the strong correlation observed between the rotation rates and the magnetic fields of cool active stars. I concentrate in particular on the most recent observational results, derived from studies on dynamo field generation in stellar convective envelopes from magnetic imaging of very active rapidly rotating low mass stars. I also mention the latest attempts at investigating the large scale magnetospheric structure of these objects from extrapolations of the measured photospheric field maps, as well its dependence/influence on rotation. I finally discuss the new discovery of the feedback effects that dynamo magnetic fields generate on the global dynamics of stellar convective envelopes.

On the mutual interaction between rotation and magnetic fields for axisymmetric bodies

1979 ◽  
Vol 368 (1732) ◽  
pp. 75-97 ◽  
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Field Strength ◽  
Magnetic Field Strength ◽  
Rotation Rate ◽  
Rotation Axis ◽  
Magnetic Torque ◽  
Rotation Rates ◽  
Driving Torque ◽  
Stable Rotation

As a simple example of the mutual interaction between magnetic fields and material motions, the rotation of an electrically conducting cylinder of solid material in a transverse magnetic field has been investigated. An applied driving torque produces the rotation, which is opposed by friction and the induced magnetic torque. It is well known that when the field is transverse to the rotation axis the magnetic torque rises from zero as the rotation rate Ω is increased, reaches a maximum and tends to zero as Ω → ∞, and the magnetic flux is expelled. We may consider B 0 (the applied magnetic field strength) and Ω 0 (the rotation rate at which the drive is just balanced by friction alone) as control parameters of the system. For sufficiently strong driving torques, the equilibrium surface Ω ( Ω 0 , B 0 ) develops a fold and consists of two branches - ‘ fast friction-dominated ’ and ‘ slow magnetically dominated ’ stable rotation rates. These solutions embrace an unstable intermediate equilibrium, and the system exhibits hysteresis depending on the manner in which the fold is approached. A ‘potential’ function can be introduced in terms of which the equilibria and stability can be analysed, and this potential function indicates that the equilibrium Ω -surfaces display the characteristics of the cusp catastrophe of Thom. One consequence of this folded structure is the existence of a forbidden band of rotation rates for a given driving torque irrespective of magnetic field strength. Similar properties can be shown for spheres, and we speculate that the general features - fold, upper and lower stable branches, forbidden band of stable rotation rates - are generic to all axisymmetric solid bodies and shells rotating about their axes of symmetry in the presence of a magnetic field with a transverse component. These features are absent if the magnetic field is aligned with the rotation axis. The hysteresis should be observable in the laboratory and experimentally verifiable.

scholarly journals There is no magnetic braking catastrophe: low-mass star cluster and protostellar disc formation with non-ideal magnetohydrodynamics

2019 ◽  
Vol 489 (2) ◽  
pp. 1719-1741 ◽  
Author(s):  
James Wurster ◽  
Matthew R Bate ◽  
Daniel J Price
Keyword(s):  
Magnetic Field ◽  
Field Strength ◽  
Magnetic Field Strength ◽  
Large Scale ◽  
Star Cluster ◽  
Mass Star ◽  
Low Mass ◽  
Ideal Magnetohydrodynamics ◽  
Protostellar Discs ◽  
Disc Formation

Abstract We present results from the first radiation non-ideal magnetohydrodynamics (MHD) simulations of low-mass star cluster formation that resolve the fragmentation process down to the opacity limit. We model 50 M⊙ turbulent clouds initially threaded by a uniform magnetic field with strengths of 3, 5 10, and 20 times the critical mass-to-magnetic flux ratio, and at each strength, we model both an ideal and non-ideal (including Ohmic resistivity, ambipolar diffusion, and the Hall effect) MHD cloud. Turbulence and magnetic fields shape the large-scale structure of the cloud, and similar structures form regardless of whether ideal or non-ideal MHD is employed. At high densities (106 ≲ nH ≲ 1011 cm−3), all models have a similar magnetic field strength versus density relation, suggesting that the field strength in dense cores is independent of the large-scale environment. Albeit with limited statistics, we find no evidence for the dependence of the initial mass function on the initial magnetic field strength, however, the star formation rate decreases for models with increasing initial field strengths; the exception is the strongest field case where collapse occurs primarily along field lines. Protostellar discs with radii ≳ 20 au form in all models, suggesting that disc formation is dependent on the gas turbulence rather than on magnetic field strength. We find no evidence for the magnetic braking catastrophe, and find that magnetic fields do not hinder the formation of protostellar discs.

scholarly journals Infrared Measurements of Stellar Magnetic Fields

1994 ◽  
Vol 154 ◽  
pp. 437-447 ◽  
Author(s):  
Steven H. Saar
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Pressure Support ◽  
Field Research ◽  
Future Directions ◽  
Rotation Rates ◽  
Infrared Measurements ◽  
Cool Stars ◽  
Rs Cvn Variables ◽  
Stellar Magnetic Fields

I review the advantages, techniques, and results of measurement of magnetic fields on cool stars in the infrared (IR). These measurements have generated several important results, including the following: the first data on the magnetic parameters of dMe and RS CVn variables; evidence for field strength confinement by photospheric gas pressure; support for the correlation between magnetic flux and rotation, with possible saturation at high rotation rates; indications of horizontal and/or vertical magnetic field structure; and evidence of spatial variations in B over a stellar surface. I discuss these results in detail, and suggest future directions for IR magnetic field research.

scholarly journals Submillimeter Array Observations of Magnetic Fields in Star Forming Regions

2010 ◽  
Vol 6 (S270) ◽  
pp. 103-106
Author(s):  
R. Rao ◽  
J.-M. Girart ◽  
D. P. Marrone
Keyword(s):  
Magnetic Field ◽  
Star Formation ◽  
Magnetic Fields ◽  
Computational Models ◽  
Dominant Role ◽  
Theoretical Models ◽  
Mass Star ◽  
Star Forming ◽  
Star Forming Regions ◽  
Low Mass

AbstractThere have been a number of theoretical and computational models which state that magnetic fields play an important role in the process of star formation. Competing theories instead postulate that it is turbulence which is dominant and magnetic fields are weak. The recent installation of a polarimetry system at the Submillimeter Array (SMA) has enabled us to conduct observations that could potentially distinguish between the two theories. Some of the nearby low mass star forming regions show hour-glass shaped magnetic field structures that are consistent with theoretical models in which the magnetic field plays a dominant role. However, there are other similar regions where no significant polarization is detected. Future polarimetry observations made by the Submillimeter Array should be able to increase the sample of observed regions. These measurements will allow us to address observationally the important question of the role of magnetic fields and/or turbulence in the process of star formation.

The impact of magnetism on tidal dynamics in the convective envelope of low-mass stars

2019 ◽  
Vol 15 (S354) ◽  
pp. 195-199
Author(s):  
A. Astoul ◽  
S. Mathis ◽  
C. Baruteau ◽  
F. Gallet ◽  
A. Strugarek ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Orbital Evolution ◽  
Magnetic Contribution ◽  
Low Mass Stars ◽  
Convective Envelope ◽  
Tidal Waves ◽  
Tidal Interactions ◽  
Rotational Evolution ◽  
Low Mass ◽  
The Impact

AbstractFor the shortest period exoplanets, star-planet tidal interactions are likely to have played a major role in the ultimate orbital evolution of the planets and on the spin evolution of the host stars. Although low-mass stars are magnetically active objects, the question of how the star’s magnetic field impacts the excitation, propagation and dissipation of tidal waves remains open. We have derived the magnetic contribution to the tidal interaction and estimated its amplitude throughout the structural and rotational evolution of low-mass stars (from K to F-type). We find that the star’s magnetic field has little influence on the excitation of tidal waves in nearly circular and coplanar Hot-Jupiter systems, but that it has a major impact on the way waves are dissipated.

scholarly journals Angular momentum transport in massive stars and natal neutron star rotation rates

2019 ◽  
Vol 488 (3) ◽  
pp. 4338-4355 ◽  
Author(s):  
Linhao Ma ◽  
Jim Fuller
Keyword(s):  
Angular Momentum ◽  
Massive Stars ◽  
Baroclinic Instability ◽  
Core Collapse ◽  
Slow Rotation ◽  
Low Mass Stars ◽  
Rotation Rates ◽  
Rotation Periods ◽  
Low Mass ◽  
Binary Evolution

Abstract The internal rotational dynamics of massive stars are poorly understood. If angular momentum (AM) transport between the core and the envelope is inefficient, the large core AM upon core-collapse will produce rapidly rotating neutron stars (NSs). However, observations of low-mass stars suggest an efficient AM transport mechanism is at work, which could drastically reduce NS spin rates. Here, we study the effects of the baroclinic instability and the magnetic Tayler instability in differentially rotating radiative zones. Although the baroclinic instability may occur, the Tayler instability is likely to be more effective for AM transport. We implement Tayler torques as prescribed by Fuller, Piro, and Jermyn into models of massive stars, finding they remove the vast majority of the core’s AM as it contracts between the main-sequence and helium-burning phases of evolution. If core AM is conserved during core-collapse, we predict natal NS rotation periods of $P_{\rm NS} \approx 50\!-\!200 \, {\rm ms}$, suggesting these torques help explain the relatively slow rotation rates of most young NSs, and the rarity of rapidly rotating engine-driven supernovae. Stochastic spin-up via waves just before core-collapse, asymmetric explosions, and various binary evolution scenarios may increase the initial rotation rates of many NSs.

Planetary Magnetic Fields and Dynamos

2019 ◽  
Author(s):  
Ulrich R. Christensen
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Field Strength ◽  
Thermal Evolution ◽  
Rotation Axis ◽  
Space Environment ◽  
Conducting Layer ◽  
Iron Core ◽  
Complex Flow ◽  
Dynamo Theory

Since 1973 space missions carrying vector magnetometers have shown that most, but not all, solar system planets have a global magnetic field of internal origin. They have also revealed a surprising diversity in terms of field strength and morphology. While Jupiter’s field, like that of Earth, is dominated by a dipole moderately tilted relative to the planet’s spin axis, the fields of Uranus and Neptune are multipole-dominated, whereas those of Saturn and Mercury are highly symmetric relative to the rotation axis. Planetary magnetism originates from a dynamo process, which requires a fluid and electrically conducting region in the interior with sufficiently rapid and complex flow. The magnetic fields are of interest for three reasons: (i) they provide ground truth for dynamo theory, (ii) the magnetic field controls how the planet interacts with its space environment, for example, the solar wind, and (iii) the existence or nonexistence and the properties of the field enable us to draw inferences on the constitution, dynamics, and thermal evolution of the planet’s interior. Numerical simulations of the geodynamo, in which convective flow in a rapidly rotating spherical shell representing the outer liquid iron core of the Earth leads to induction of electric currents, have successfully reproduced many observed properties of the geomagnetic field. They have also provided guidelines on the factors controlling magnetic field strength and morphology. For numerical reasons the simulations must employ viscosities far greater than those inside planets and it is debatable whether they capture the correct physics of planetary dynamo processes. Nonetheless, such models have been adapted to test concepts for explaining magnetic field properties of other planets. For example, they show that a stable stratified conducting layer above the dynamo region is a plausible cause for the strongly axisymmetric magnetic fields of Mercury or Saturn.

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