Magnetorotational Instability Research Articles

Magnetic outflows from turbulent accretion disks

Context. Astrophysical disks are likely embedded in an ambient vertical magnetic field generated by its environment. This ambient field is known to drive magneto-rotational turbulence in the disk bulk, but it is also responsible for launching magnetised outflows at the origin of astrophysical jets. Yet, the interplay between turbulence and outflows is not understood. In particular, the vertical structure and long-term (secular) evolution of such a system lack quantitative predictions. It is, nevertheless, this secular evolution which is proposed to explain time variability in many accreting systems such as FuOr, X-ray binaries, and novae like systems. Aims. We seek to constraint the structure and long-term evolution of turbulent astrophysical disks subject to magnetised outflows in the non-relativistic regime. More specifically we aim to characterise the mechanism driving accretion, the dynamics of the disk atmosphere, the role played by the outflow, and the long-term evolution of mass and magnetic flux distributions. Methods. We computed and analysed global 3D ideal magnetohydrynamic (MHD) simulations of an accretion disk threaded by a large-scale magnetic field. We measured the turbulent state of the system by Reynolds averaging the ideal MHD equations and evaluate the role of the turbulent terms in the equilibrium of the system. We then computed the transport of mass, angular momentum, and magnetic fields in the disk to characterise its secular evolution. Finally, we performed a parameter exploration survey in order to characterise how the transport properties depend on the disk properties. Results. We find that weakly magnetised disks drive jets that carry a small fraction of the disk angular momentum away. The mass-weighted accretion speed remains subsonic, although there is always an upper turbulent atmospheric region where transsonic accretion takes place. We show that this turbulence is driven by a strongly magnetised version of the magneto-rotational instability. The internal disk structure therefore appears drastically different from the conventional hydrostatic picture. We expect that the turbulent atmosphere region will lead to non-thermal features in the emission spectra from compact objects. In addition, we show that the disk is subject to a secular viscous-type instability, which leads to the formation of long-lived ring-like structures in the disk surface density distribution. This instability is likely connected to the magnetic field transport. Finally, we show that for all of the parameters explored, the ambient magnetic field is always dragged inward in the disk at a velocity which increases with the disk magnetisation. Beyond a threshold on the latter, the disk undergoes a profound radial readjustment. It leads to the formation of an inner accretion-ejection region with a supersonic mass-weighted accretion speed and where the magnetic field distribution becomes steady and reaches a magnitude near equipartition with the thermal pressure. This inner structure shares many properties with the jet emitting disk model. Overall, these results pave the way for quantitative self-consistent secular disk models.

Formation of intermediate-mass planets via magnetically controlled disk fragmentation

Intermediate mass planets, from Super-Earth to Neptune-sized bodies, are the most common type of planets in the galaxy. The prevailing theory of planet formation, core-accretion, predicts significantly fewer intermediate-mass giant planets than observed. The competing mechanism for planet formation, disk instability, can produce massive gas giant planets on wide-orbits, such as HR8799, by direct fragmentation of the protoplanetary disk. Previously, fragmentation in magnetized protoplanetary disks has only been considered when the magneto-rotational instability is the driving mechanism for magnetic field growth. Yet, this instability is naturally superseded by the spiral-driven dynamo when more realistic, non-ideal MHD conditions are considered. Here we report on MHD simulations of disk fragmentation in the presence of a spiral-driven dynamo. Fragmentation leads to the formation of long-lived bound protoplanets with masses that are at least one order of magnitude smaller than in conventional disk instability models. These light clumps survive shear and do not grow further due to the shielding effect of the magnetic field, whereby magnetic pressure stifles local inflow of matter. The outcome is a population of gaseous-rich planets with intermediate masses, while gas giants are found to be rarer, in qualitative agreement with the observed mass distribution of exoplanets.

Astrophysical wake acceleration driven by relativistic Alfvenic pulse emitted from bursting accretion disk

We consider that electromagnetic pulses produced in the jets of the innermost part of the accretion disk accelerate charged particles (protons, ions, electrons) to very high energies including energies above 1020 eV for the case of protons and nucleus and 1012-15 eV for electrons by electromagnetic wave-particle interaction. The episodic eruptive accretion in the disk by the magneto-rotational instability gives rise to strong electro-magnetic pulses, which act as the driver of the collective accelerating ponderomotive force. The accelerated hadrons (protons and nuclei) are released to the intergalactic space to be ultra-high energy cosmic rays. Some of them collide with the protons in the interstellar medium to produce secondary particles, such as neutrinos and gamma-ray photons. The high-energy electrons, on the other hand, emit photons in the collisions with electromagnetic disturbances to produce various non-thermal emissions (radio, IR, visible, UV, and gamma-rays) of active galactic nuclei. Applying the bow wakefield acceleration theory to eight galaxies (M82, NGC0253, NGC4945, NGC1068, NGC6814, Cen A, M87, and For A), we find two starburst galaxies (M82 and NGC0253), one Seyfert galaxy (NGC4945), and one radio galaxy (Cen A) are promising as UHECR (Ultra-High Energy Cosmic-ray) sources and can form hot spots around them. The sky map of these four galaxies is consistent to the arrival direction analysis of the UHECRs. In particular, M82 seams responsible to form the northern hot spot suggested by TA group.

Viscoelastic Taylor–Couette instability in the Keplerian regime

Instability modes of viscoelastic Taylor–Couette flow in the Keplerian regime are investigated using both linear stability analysis and experimental detection of critical states. A generalised Rayleigh criterion has been derived and it allows to separate the zone of potential purely elastic instability modes and the zone of stability. The analogy between the instability of viscoelastic Taylor–Couette flow in the Keplerian regime and the magnetorotational instability (MRI) of conducting magnetic fluids is established. Viscoelastic stress tensor can be represented by a tensor product of magnetic-like vectors. This allows to use the Velikhov-Chandrasekhar criterion of the MRI to predict the elasto-rotational instability (ERI). ERI modes obtained in linear stability and in the experiments are assumed to represent the MRI analogue modes as they are observed in the stable zone according to generalised Rayleigh discriminant.

Resolved molecular line observations reveal an inherited molecular layer in the young disk around TMC1A

Context.Physical processes that govern the star and planet formation sequence influence the chemical composition and evolution of protoplanetary disks. Recent studies allude to an early start to planet formation already during the formation of a disk. To understand the chemical composition of protoplanets, we need to constrain the composition and structure of the disks from whence they are formed.Aims.We aim to determine the molecular abundance structure of the young disk around the TMC1A protostar on au scales in order to understand its chemical structure and any possible implications for disk formation.Methods.We present spatially resolved Atacama Large Millimeter/submillimeter Array observations of CO, HCO+, HCN, DCN, and SO line emission, as well as dust continuum emission, in the vicinity of TMC1A. Molecular column densities are estimated both under the assumption of optically thin emission from molecules in local thermodynamical equilibrium (LTE) as well as through more detailed non-LTE radiative transfer calculations.Results.Resolved dust continuum emission from the disk is detected between 220 and 260 GHz. Rotational transitions from HCO+, HCN, and SO are also detected from the inner 100 au region. We further report on upper limits to vibrational HCNυ2= 1, DCN, and N2D+lines. The HCO+emission appears to trace both the Keplerian disk and the surrounding infalling rotating envelope. HCN emission peaks toward the outflow cavity region connected with the CO disk wind and toward the red-shifted part of the Keplerian disk. From the derived HCO+abundance, we estimate the ionization fraction of the disk surface, and find values that imply that the accretion process is not driven by the magneto-rotational instability. The molecular abundances averaged over the TMC1A disk are similar to its protostellar envelope and other, older Class II disks. We meanwhile find a discrepancy between the young disk’s molecular abundances relative to Solar System objects.Conclusions.Abundance comparisons between the disk and its surrounding envelope for several molecular species reveal that the bulk of planet-forming material enters the disk unaltered. Differences in HCN and H2O molecular abundances between the disk around TMC1A, Class II disks, and Solar System objects trace the chemical evolution during disk and planet formation.

First Detection of Interaction between a Magnetic Disk Wind and an Episodic Jet in a Protostellar System

Abstract Rotating outflows from protostellar disks might trace extended magnetohydrodynamic (MHD) disk winds (DWs), providing a solution to the angular momentum problem in disk accretion for star formation. In the jet system HH 212, a rotating outflow was detected in SO around an episodic jet detected in SiO. Here we spatially resolve this SO outflow into three components: a collimated jet aligned with the SiO jet, the wide-angle disk outflow, and an evacuated cavity in between created by a large jet-driven bow shock. Although it was theoretically predicted, this is the first time that such a jet–DW interaction has been directly observed and resolved, and it is crucial for the proper interpretation and modeling of non-resolved DW candidates. The resolved kinematics and brightness distribution both support the wide-angle outflow to be an extended MHD DW dominating the local angular momentum extraction out to 40 au, but with an inner launching radius truncated to ≳4 au. Inside 4 au, where the DW may not exist, the magnetorotational instability might be transporting angular momentum outward. The jet–DW interaction in HH 212, potentially present in other similar systems, opens an entirely new avenue to probe the large-scale magnetic field in protostellar disks.

Magnetohydrodynamics of protoplanetary discs

Protoplanetary discs are made of gas and dust orbiting a young star. They are also the birth place of planetary systems, which motivates a large amount of observational and theoretical research. In these lecture notes, I present a review of the magnetic mechanisms applied to the outer regions ( $R\gtrsim 1\ \mathrm {AU}$ ) of these discs, which are the planet-formation regions. In contrast to usual astrophysical plasmas, the gas in these regions is noticeably cold ( $T < 300\ \mathrm {K}$ ) and dense, which implies a very low ionisation fraction close to the disc midplane. In these notes, I deliberately ignore the innermost $(R\sim 0.1\ \mathrm {AU})$ region, which is influenced by the star–disc interaction and various radiative effects. I start by presenting a short overview of the observational evidence for the dynamics of these objects. I then introduce the methods and approximations used to model these plasmas, including non-ideal magnetohydrodynamics, and the uncertainties associated with this approach. In this framework, I explain how the global dynamics of these discs is modelled, and I present a stability analysis of this plasma in the local approximation, introducing the non-ideal magneto-rotational instability. Following this mostly analytical part, I discuss numerical models that have been used to describe the saturation mechanisms of this instability, and the formation of large-scale structures by various saturation mechanisms. Finally, I show that local numerical models are insufficient because magnetised winds are also emitted from the surface of these objects. After a short introduction on wind physics, I present global models of protoplanetary discs, including both a large-scale wind and the non-ideal dynamics of the disc.

A global model of the magnetorotational instability in protoneutron stars

Context. Magnetars are isolated neutron stars characterized by their variable high-energy emission, which is powered by the dissipation of enormous internal magnetic fields. The measured spin-down of magnetars constrains the magnetic dipole to be in the range of 1014 − 1015 G. The magnetorotational instability (MRI) is considered to be a promising mechanism to amplify the magnetic field in fast-rotating protoneutron stars and form magnetars. This scenario is supported by many local studies that have shown that magnetic fields could be amplified by the MRI on small scales. However, the efficiency of the MRI at generating a dipole field is still unknown. Aims. To answer this question, we study the MRI dynamo in an idealized global model of a fast rotating protoneutron star with differential rotation. Methods. Using the pseudo-spectral code MagIC, we performed three-dimensional incompressible magnetohydrodynamics simulations in spherical geometry with explicit diffusivities where the differential rotation is forced at the outer boundary. We performed a parameter study in which we varied the initial magnetic field and investigated different magnetic boundary conditions. These simulations were compared to local shearing box simulations performed with the code Snoopy. Results. We obtain a self-sustained turbulent MRI-driven dynamo, whose saturated state is independent of the initial magnetic field. The MRI generates a strong turbulent magnetic field of B ≥ 2 × 1015 G and a nondominant magnetic dipole, which represents systematically about 5% of the averaged magnetic field strength. Interestingly, this dipole is tilted toward the equatorial plane. By comparing these results with shearing box simulations, we find that local models can reproduce fairly well several characteristics of global MRI turbulence such as the kinetic and magnetic spectra. The turbulence is nonetheless more vigorous in the local models than in the global ones. Moreover, overly large boxes allow for elongated structures to develop without any realistic curvature constraint, which may explain why these models tend to overestimate the field amplification. Conclusions. Overall, our results support the ability of the MRI to form magnetar-like large-scale magnetic fields. They furthermore predict the presence of a stronger small-scale magnetic field. The resulting magnetic field could be important to power outstanding stellar explosions, such as superluminous supernovae and gamma-ray bursts.

Transition Region from Turbulent to Dead Zone in Protoplanetary Disks: Local Shearing Box Simulations

Abstract The dynamical evolution of protoplanetary disks is of key interest for building a comprehensive theory of planet formation and to explain the observational properties of these objects. Using the magnetohydrodynamics code Athena++, with an isothermal shearing box setup, we study the boundary between the active and dead zone, where the accretion rate changes and mass can accumulate. We quantify how the turbulence level is affected by the presence of a non-uniform Ohmic resistivity in the radial x direction that leads to a region of inhibited turbulence (or dead zone). Comparing the turbulent activity to that of ideal simulations, the turbulence-inhibited area shows density fluctuations and magnetic activity at its boundaries, driven by energy injection from the active (ideal) zone boundaries. We find magnetic dissipation to be significantly stronger in the ideal regions, and the turbulence penetration through the boundary of the dead zone is determined by the value of the resistivity itself, through the Ohmic dissipation process, though the thickness of the transition does not play a significant role in changing the dissipation. We investigate the 1D spectra along the shearing direction: magnetic spectra appear flat at large scales both in ideal as well as resistive simulations, though a Kolmogorov scaling over more than one decade persists in the dead zone, suggesting the turbulent cascade is determined by the hydrodynamics of the system: magnetorotational instability dynamo action is inhibited where sufficiently high resistivity is present.

Accretion bursts in magnetized gas-dust protoplanetary disks

Aims. Accretion bursts triggered by the magnetorotational instability (MRI) in the innermost disk regions were studied for protoplanetary gas-dust disks that formed from prestellar cores of a various mass Mcore and mass-to-magnetic flux ratio λ. Methods. Numerical magnetohydrodynamics simulations in the thin-disk limit were employed to study the long-term (~1.0 Myr) evolution of protoplanetary disks with an adaptive turbulent α-parameter, which explicitly depends on the strength of the magnetic field and ionization fraction in the disk. The numerical models also feature the co-evolution of gas and dust, including the back-reaction of dust on gas and dust growth. Results. A dead zone with a low ionization fraction of x≲10−13 and temperature on the order of several hundred Kelvin forms in the inner disk soon after its formation, extending from several to several tens of astronomical units depending on the model. The dead zone features pronounced dust rings that are formed due to the concentration of grown dust particles in the local pressure maxima. Thermal ionization of alkaline metals in the dead zone trigger the MRI and associated accretion burst, which is characterized by a sharp rise, small-scale variability in the active phase, and fast decline once the inner MRI-active region is depleted of matter. The burst occurrence frequency is highest in the initial stages of disk formation and is driven by gravitational instability (GI), but it declines with diminishing disk mass-loading from the infalling envelope. There is a causal link between the initial burst activity and the strength of GI in the disk fueled by mass infall from the envelope. We find that the MRI-driven burst phenomenon occurs for λ = 2–10, but diminishes in models with Mcore ≲ M⊙, suggesting a lower limit on the stellar mass for which the MRI-triggered burst can occur. Conclusions. The MRI-triggered bursts occur for a wide range of mass-to-magnetic flux ratios and initial cloud core masses. The burst occurrence frequency is highest in the initial disk formation stage and reduces as the disk evolves from a gravitationally unstable to a viscous-dominated state. The MRI-triggered bursts are intrinsically connected with the dust rings in the inner disk regions, and both can be a manifestation of the same phenomenon, that is to say the formation of a dead zone.

Stratified Global MHD Models of Accretion Disks in Semidetached Binaries

Abstract We present results of the first global magnetohydrodynamic simulations of accretion disks fed by Roche-lobe overflow, including vertical stratification, in order to investigate the roles of spiral shocks, magnetorotational instability (MRI), and the accretion stream in disk structure and evolution. Our models include a simple treatment of gas thermodynamics, with orbital Mach numbers at the inner edge of the disk M in of 5 and 10. We find mass accretion rates to vary considerably on all timescales, with only the Mach 5 model reaching a clear quasi-stationary state. For Mach 10, the model undergoes an outside-in, magnetically driven accretion event occurring on a timescale of ∼10 orbital periods of the binary. Both models exhibit spiral shocks inclined with respect to the binary plane, with their position and inclination changing rapidly. However, the time-averaged location of these shocks in the equatorial plane is well fit by simple linear models. MRI turbulence in the disk generates toroidal magnetic field patterns (butterfly diagrams) that are in some cases irregular, perhaps due to interaction with the spiral structure. While many of our results are in good agreement with local studies, we find some features (most notably those related to spiral shocks) can only be captured in global models such as studied here. Thus, while global studies remain computationally expensive—even as idealized models—they are essential (along with more sophisticated treatment of radiation transport and disk thermodynamics) for furthering our understanding of accretion in binary systems.

Turbulent magnetic-field amplification in the first 10 milliseconds after a binary neutron star merger: Comparing high-resolution and large-eddy simulations

The detection of binary neutron star mergers represents one of the most important and complex astrophysical discoveries of the recent years. One of the unclear aspects of the problem is the turbulent magnetic field amplification, initially triggered by the Kelvin-Helmholtz instability at much smaller scales than any reachable numerical resolution nowadays. Here we present numerical simulations of the first ten milliseconds of a binary neutron star merger. First, we confirm in detail how the simulated amplification depends on the numerical resolution and is distributed on a broad range of scales, as expected from turbulent MHD theory. We find that an initial large-scale magnetic field of $10^{11}\,$G inside each star is amplified in the remnant to root-mean-square values above $10^{16}\,$G within the first $5$ milliseconds for our highest-resolution run. Then, we run large eddy simulations, exploring the performance of the subgrid-scale gradient model, already tested successfully in previous turbulent box simulations. We show that the addition of this model is especially important in the induction equation, since it leads to an amplification of the magnetic field comparable to a higher-resolution run, but with a greatly reduced computational cost. In the first 10 milliseconds, there is no clear hint for an ordered, large-scale magnetic field, which should indeed occur in longer timescales through magnetic winding and the magneto-rotational instability.

The evolution of a circumplanetary disc with a dead zone

ABSTRACT We investigate whether the regular Galilean satellites could have formed in the dead zone of a circumplanetary disc. A dead zone is a region of weak turbulence in which the magnetorotational instability is suppressed, potentially an ideal environment for satellite formation. With the grid-based hydrodynamic code fargo3d, we examine the evolution of a circumplanetary disc model with a dead zone. Material accumulates in the dead zone of the disc leading to a higher total mass and but a similar temperature profile compared to a fully turbulent disc model. The tidal torque increases the rate of mass transport through the dead zone leading to a steady-state disc with a dead zone that does not undergo accretion outbursts. We explore a range of disc, dead zone, and mass inflow parameters and find that the maximum mass of the disc is around $0.001 M_{\rm J}$. Since the total solid mass of such a disc is much lower, we find that there is not sufficient material in the disc for in situ formation of the Galilean satellites and that external supplement is required.

Zero Net Flux MRI Turbulence in Disks: Sustenance Scheme and Magnetic Prandtl Number Dependence

Abstract We investigate sustenance and dependence on magnetic Prandtl number (Pm) for magnetorotational instability (MRI)-driven turbulence in Keplerian disks with zero net magnetic flux using standard shearing box simulations. We focus on the turbulence dynamics in Fourier space, capturing specific/noncanonical anisotropy of nonlinear processes due to disk flow shear. This is a new type of nonlinear redistribution of modes over wavevector orientations in Fourier space—the nonlinear transverse cascade—which is generic to shear flows and fundamentally different from the usual direct/inverse cascade. The zero flux MRI has no exponentially growing modes, so its growth is transient, or nonmodal. Turbulence self-sustenance is governed by constructive cooperation of the transient growth of MRI and the nonlinear transverse cascade. This cooperation takes place at small wavenumbers (on the flow size scales) referred to as the vital area in Fourier space. The direct cascade transfers mode energy from the vital area to larger wavenumbers. At large Pm, the transverse cascade prevails over the direct one, keeping most of modes’ energy contained in small wavenumbers. With decreasing Pm, however, the action of the transverse cascade weakens and can no longer oppose the action of the direct cascade, which more efficiently transfers energy to higher wavenumbers, leading to increased resistive dissipation. This undermines the sustenance scheme, resulting in the turbulence decay. Thus, the decay of zero net flux MRI turbulence with decreasing Pm is attributed to the topological rearrangement of the nonlinear processes when the direct cascade begins to prevail over the transverse cascade.

Multiphase Circumnuclear Gas in a Low-β Disk: Turbulence and Magnetic Field Reversals

Abstract We studied the magnetic field structures and dynamics of magnetized multiphase gas on parsec scales around supermassive black holes by using global 3D magnetohydrodynamics (MHD) simulations. We considered the effect of radiative cooling and X-ray heating due to active galactic nuclei (AGNs). The gas disk consists of a multiphase gas with (1) cold (≤103 K) and thin, and (2) warm (∼104 K) and thick components with a wide range of number densities. The turbulent magnetic energy at maximum is comparable to the thermal and turbulent kinetic energies in the turbulent motion. We confirmed that the turbulent velocity of the warm gas in the ambient cold gas is caused by magnetoconvective instability. The turbulent magnetic field due to magnetorotational instability (MRI) is developed in the disk, but the mean toroidal magnetic field dominates and supports in a quasi-steady state, where the plasma-β, the ratio between gas pressure and magnetic pressure, is low (β < 1). As often seen in adiabatic MHD simulations of rotating disks, the direction of the mean toroidal field periodically reverses with time even in multiphase gas structures. The direction reversal is caused by magnetic flux vertically escaping from the disk and by the combination of the MRI and the Parker instability.

Effect of luminosity outbursts on protoplanetary disk dynamics

Aims. The response of a protoplanetary disk to luminosity bursts of various durations is studied with the purpose to determine the effect of the bursts on the strength and sustainability of gravitational instability in the disk. A special emphasis is paid to the spatial distribution of gas and grown dust (from 1 mm to a few centimetres) during and after the burst. Methods. Numerical hydrodynamics simulations were employed to study the dynamics of gas and dust in the thin-disk limit. Dust-to-gas friction, including back reaction and dust growth, were also considered. Bursts of various durations (from 100 yr to 500 yr) were initiated in accordance with a thermally ignited magnetorotational instability. Luminosity curves for constant- and declining-magnitude bursts were adopted to represent two typical limiting cases for FU Orionis-type eruptions. Results. The short-term effect of the burst is to reduce the strength of gravitational instability by heating and expanding the disk. The longest bursts with durations comparable to the revolution period of the spiral can completely dissolve the original two-armed spiral pattern in the gas disk by the end of the burst, while the shortest bursts only weaken the spiral pattern. The reaction of grown dust to the burst is somewhat different. The spiral-like initial distribution with deep cavities in the inter-armed regions transforms into a ring-like distribution with deep gaps. This transformation is mostly expressed for the longest-duration bursts. The long-term effect of the burst depends on the initial disk conditions at the onset of the burst. In some cases, vigorous disk fragmentation sets in several thousands of years after the burst, which was absent in the model without the burst. Several clumps with masses in the giant-planet mass range form in the outer disk regions. After the disk fragmentation phase, the spatial distribution of grown dust is characterized by multiple sharp rings located from tens to hundreds of astronomical units. The arrangement and sharpness of the rings depends on the strength of dust turbulent diffusion. The wide-orbit rings are likely formed as the result of dust-rich clump dispersal in the preceding gravitationally unstable phase. Conclusions. Luminosity bursts similar in magnitude to FU Orionis-type eruptions can have a profound effect on the dynamics of gas and dust in protoplanetary disks if the burst duration is comparable to, or longer than, the dynamical timescales. In this context, the spatial morphology of the gas-dust disk of V883 Ori, a FU Orionis-like object that is thought to be in the outburst phase for more than a century with an unknown onset date, may be used as test case for the burst models considered in this work. The potential relation of the obtained ring structures to a variety of gaps and rings observed in T Tauri disks remains to be established.

A Magnetar Engine for Short GRBs and Kilonovae

Abstract We investigate the influence of magnetic fields on the evolution of binary neutron star (BNS) merger remnants via three-dimensional (3D) dynamical-spacetime general-relativistic magnetohydrodynamic (MHD) simulations. We evolve a post-merger remnant with an initial poloidal magnetic field, resolve the magnetoturbulence driven by shear flows, and include a microphysical finite-temperature equation of state. A neutrino leakage scheme that captures the overall energetics and lepton number exchange is also included. We find that turbulence induced by the magnetorotational instability in the hypermassive neutron star (HMNS) amplifies magnetic field to beyond magnetar strength (1015 G). The ultra-strong toroidal field is able to launch a relativistic jet from the HMNS. We also find a magnetized wind that ejects neutron-rich material with a rate of . The total ejecta mass in our simulation is 5 × 10−3 M ⊙. This makes the ejecta from the HMNS an important component in BNS mergers and a promising source of r-process elements that can power a kilonova. The jet from the HMNS reaches a terminal Lorentz factor of ∼5 in our highest-resolution simulation. The formation of this jet is aided by neutrino cooling preventing the accretion disk from protruding into the polar region. As neutrino pair-annihilation and radiative processes in the jet (which were not included in the simulations) will boost the Lorentz factor in the jet further, our simulations demonstrate that magnetars formed in BNS mergers are a viable engine for short gamma-ray bursts.

Interplay between magnetic fields and differential rotation in a stably stratified stellar radiative zone

Context. The interactions between magnetic fields and differential rotation in stellar radiative interiors could play a major role in achieving an understanding of the magnetism of intermediate-mass and massive stars and of the differential rotation profile observed in red-giant stars. Aims. The present study is aimed at studying the flow and field produced by a stellar radiative zone which is initially made to rotate differentially in the presence of a large-scale poloidal magnetic field threading the whole domain. We focus both on the axisymmetric configurations produced by the initial winding-up of the magnetic field lines and on the possible instabilities of those configurations. We investigate in detail the effects of the stable stratification and thermal diffusion and we aim, in particular, to assess the role of the stratification at stabilising the system. Methods. We performed 2D and 3D global Boussinesq numerical simulations started from an initial radial or cylindrical differential rotation and a large-scale poloidal magnetic field. Under the conditions of a large rotation frequency compared to the Alfvén frequency, we built a magnetic configuration strongly dominated by its toroidal component. We then perturbed this configuration to observe the development of non-axisymmetric instabilities. Results. The parameters of the simulations were chosen to respect the ordering of time scales of a typical stellar radiative zone. In this framework, the axisymmetric evolution is studied by varying the relative effects of the thermal diffusion, the Brunt-Väisälä frequency, the rotation, and the initial poloidal field strength. After a transient time and using a suitable adimensionalisation, we find that the axisymmetric state only depends on tes/tAp the ratio between the Eddington–Sweet circulation time scale and the Alfvén time scale. A scale analysis of the Boussinesq magnetohydrodynamical equations allows us to recover this result. In the cylindrical case, a magneto-rotational instability develops when the thermal diffusivity is sufficiently high to enable the favored wavenumbers to be insensitive to the effects of the stable stratification. In the radial case, the magneto-rotational instability is driven by the latitudinal shear created by the back-reaction of the Lorentz force on the flow. Increasing the level of stratification then leaves the growth rate of the instability mainly unaffected while its horizontal length scale grows. Conclusions. Non-axisymmetric instabilities are likely to exist in stellar radiative zones despite the stable stratification. They could be at the origin of the magnetic dichotomy observed in intermediate-mass and massive stars. They are also unavoidable candidates for the transport of angular momentum in red giant stars.

Convection-caused symmetry breaking of azimuthal magnetorotational instability in a liquid metal Taylor--Couette flow

We report the results of a liquid metal Taylor-Couette experiment in the Rayleigh-stable regime under the influence of an azimuthal magnetic field. We observe that the resulting azimuthal magnetorotational instability (AMRI) from our experimental setup is significantly influenced by the thermal boundary conditions. Even a minimal radial heat flux leads to a symmetry breaking, which results in the AMRI waves traveling either upwards or downwards. We identify the thermal radiation by the central axial current as the heat source responsible for vertical convection in the liquid. Preliminary numerical investigations point towards an interaction between AMRI and thermal convection, which supports our experimental findings.

Parameter space mapping of the Princeton magnetorotational instability experiment.

Extensive simulations of the Princeton Magnetorotational Instability (MRI) Experiment with the Spectral/Finite Element code for Maxwell and Navier-Stokes Equations (SFEMaNS) have been performed to map the MRI-unstable region as a function of inner cylinder angular velocity and applied vertical magnetic field. The angular velocities of the outer cylinder and the end-cap rings follow the inner cylinder in fixed ratios optimized for MRI. We first confirm the exponential growth of the MRI linear phase using idealized conducting vertical boundaries (end caps) rotating differentially with a Taylor-Couette profile. Subsequently, we run a multitude of simulations to scan the experimental parameter space and find that the normalized volume-averaged mean-square radial magnetic field, our main instability indicator, rises significantly where MRI is expected. At various locations, the local radial components of fluid velocity and generated magnetic field are well correlated with the volume-averaged indicator. Based on this correlation, a diagnostic system that will measure the radial magnetic field at several locations on the inner cylinder is proposed as the main comparison between simulation and experiment. A detailed analysis of poloidal mode structures in the SFEMaNS code indicates that MRI, rather than Ekman circulation or Rayleigh instability, dominates the fluid behavior in the region where MRI is expected.