arXiv subjects

Henrik Latter

Publications and source records attributed to Henrik Latter.

At least 19 recordsLinked to original sources

arXiv2026

Magneto-Thermal Instability in Galaxy Clusters -- III. The Limit of Adiabatic Stratification

In the hot and dilute intracluster medium of galaxy clusters, large-scale buoyancy instabilities can develop due to the transport of heat along magnetic field lines. In particular, the peripheries of galaxy clusters are unstable to the magneto-thermal instability (MTI), which may contribute to the observed levels of turbulence. Recent theoretical and numerical work has revealed that the stable background entropy stratification controls the nonlinear saturation of the instability, by setting the strength and the integral scale of the resulting turbulent state. However, observations of the periphery of galaxy clusters show that the radial entropy profiles near the virial radii $R_{500}$ may be flatter than predicted by models of smooth gravitational accretion. This motivates us to investigate the saturation of the MTI in adiabatic (buoyantly neutral) atmospheres, using both phenomenological approaches and Boussinesq numerical simulations, carried out with the pseudospectral code SNOOPY. We find that the adiabatic MTI saturates in a state characterised by the formation of large-scale plumes and their destruction by shear instability, yielding a new scaling law for the saturated turbulent kinetic energy, $\sim$$\chi \omega_T$, as the adiabatic limit is approached, where $\chi$ is the effective thermal diffusivity and $\omega_T$ is the MTI frequency. This predicts that the MTI plumes may achieve near sonic speeds in cluster outskirts, thus providing significant turbulent pressure support, even in the face of suppressed thermal conduction.

astro-ph.CO↗

arXiv2024

Dynamics of cold circumstellar gas in debris disks

Mounting observational evidence indicates that cold circumstellar gas is present in debris disk systems. This work focuses on various dynamical processes that debris-disk gas may undergo. We review five mechanisms that can transport angular momentum and their applications to debris disks. These include molecular viscosity, hydrodynamic turbulence, magnetohydrodynamic turbulence, magnetized disk winds, and laminar magnetic stress. We find that molecular viscosity can result in $α$ as high as $\lesssim 0.1$ for sufficiently low densities, while the Rossby wave instability is a possible source of hydrodynamic turbulence and structure formation. We argue that the vertical shear instability is unlikely due to the long cooling times. The onset of the magnetorotational instability (MRI) is dichotomous: for low density disks the MRI can be excited at the midplane, while for high mass disks it may only be operating at $z>2-3H$, if at all. The MHD wind and laminar magnetic stress mechanisms rely on the configuration and strength of any background large-scale magnetic field, the existence of which is uncertain and possibly unlikely. We conclude that the dominant mechanism and its efficiency in transporting angular momentum varies from one system to the other, depending especially closely on the gas density. More detailed analyses shall be performed in the future focusing on representative, nearby debris disks.

astro-ph.EP↗

arXiv2023

Thermal hysteresis and front propagation in dense planetary rings

Saturn's rings are composed of icy grains, most in the mm to m size ranges, undergoing several collisions per orbit. Their collective behaviour generates a remarkable array of structure over many orders of magnitude, much of it not well understood. On the other hand, the collisional properties and parameters of individual ring particles are poorly constrained; usually N-body simulations and kinetic theory employ hard-sphere models with a coefficient of restitution $ε$ that is constant or a decreasing function of impact speed. Due to plastic deformation of surface regolith, however, it is likely that $ε$ will be more complicated, at the very least a non-monotonic function. We undertake N-body simulations with the REBOUND code with non-monotonic $ε$ laws to approximate surfaces that are friable but not sticking. Our simulations reveal that such ring models can support two thermally stable steady states for the same (dynamical) optical depth: a cold and a warm state. If the ring breaks up into radial bands of one or the other state, we find that warmer states tend to migrate into the colder states via a coherent travelling front. We also find stationary `viscous' fronts, which connect states of different optical depth, but the same angular momentum flux. We discuss these preliminary results and speculate on their implications for structure formation in Saturn's B and C-rings, especially with respect to structures that appear in Cassini images but not in occultations.

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arXiv2022

Gravitoturbulent dynamo in global simulations of gaseous disks

The turbulence driven by gravitational instabilities (GIs) can amplify magnetic fields in massive gaseous disks. This GI dynamo may appear in young circumstellar disks, whose weak ionization challenges other amplification routes, as well as in active galactic nuclei. Although regarded as a large-scale dynamo, only local simulations have so far described its kinematic regime. We study the GI dynamo in global magnetohydrodynamic (MHD) models of accretion disks, focusing on its kinematic phase. We perform resistive MHD simulations with the Pluto code for different radiative cooling times and electrical resistivities. A weak magnetic field seeds the dynamo, and we adopt mean-field and heuristic models to capture its essence. We recover the same induction process leading to magnetic field amplification as previously identified in local simulations. The dynamo is, however, global in nature, connecting distant annuli of the disk via a large-scale dynamo mode of a fixed growth rate. This large-scale amplification can be described by a mean-field model that does not rely on conventional alpha-Omega effects. When varying the disk parameters we find an optimal resistivity that facilitates magnetic amplification, whose magnetic Reynolds number, Rm < 10, is substantially smaller than in local simulations. Unlike local simulations, we find an optimal cooling rate and the existence of global oscillating dynamo modes. The nonlinear saturation of the dynamo puts the disk in a strongly magnetized turbulent state on the margins of the effective range of GI. In our simulations, the accretion power eventually exceeds the threshold required by local thermal balance against cooling, leaving the long-term nonlinear outcome of the GI dynamo uncertain.

astro-ph.SR↗

arXiv2022

The saturation of the VSI in protoplanetary disks via parametric instability

The vertical shear instability (VSI) is a robust and potentially important phenomenon in irradiated protoplanetary disks (PPDs), yet the mechanism by which it saturates remains poorly understood. Global simulations suggest that the non-linear evolution of the VSI is dominated by radially propagating inertial wavetrains (called `body modes'), but these are known to be susceptible to a parametric instability. In this paper, we propose that the global VSI saturates via this secondary instability, which initiates a redistribution of energy from the large scales to smaller-scale inertial waves, and finally into a turbulent cascade. We present an analytic theory of the instability in a simple idealised model that captures the main physical and mathematical details of the problem. In addition, we conduct numerical simulations with the SNOOPY code to consolidate the theory. Once the parametric instability prevails, the VSI is likely far more disordered and incoherent than current global simulations suggest. We also argue that it is challenging to capture parametric instability in global simulations unless the radial resolution is very fine, possibly $\sim 300$ grid cells per scale height in radius.

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arXiv2021

Magneto-Thermal Instability In Galaxy Clusters II: Three-Dimensional Simulations

In the intracluster medium (ICM) of galaxies, exchanges of heat across magnetic field lines are strongly suppressed. This anisotropic heat conduction, in the presence of a large-scale temperature gradient, destabilizes the outskirts of galaxy clusters via the magneto-thermal instability (MTI), and thus supplies a source of observed ICM turbulence. In this paper we continue our investigation of the MTI with 3D simulations using the Boussinesq code SNOOPY. We redress two issues intrinsic to our previous 2D study: an inverse energy cascade and the impossibility of dynamo action. Contrary to 2D simulations, we find inconsequential transport of energy across scales (most energy is dissipated at the same scale as its injection), and that turbulent eddies are vertically elongated at or below the thermal conduction length, but relatively isotropic on larger scales. Similar to 2D, however, the saturated turbulent energy levels and the integral scale follow clear power-laws that depend on the thermal diffusivity, temperature gradient, and buoyancy frequency. We also show that the MTI amplifies magnetic fields, through a fluctuation dynamo, to equipartition strengths provided that the integral scale of MTI turbulence is larger than the viscous dissipation scale. Finally, we show that our scaling laws are consistent with extant observations of ICM turbulence if the thermal conductivity is reduced by a factor of $\sim 10$ from its Spitzer value, and that on global cluster scales the stable stratification significantly reduces the vertical elongation of MTI motions.

astro-ph.CO↗

arXiv2021

Magneto-Thermal Instability In Galaxy Clusters I: Theory and Two-Dimensional Simulations

Determining the origin of turbulence in galaxy clusters, and quantifying its transport of heat, is an outstanding problem, with implications for our understanding of their thermodynamic history and structure. As the dilute plasma of the intracluster medium (ICM) is magnetized, heat and momentum travel preferentially along magnetic field lines. This anisotropy triggers a class of buoyancy instabilities that destabilize the ICM, and whose turbulent motions can augment or impede heat transport. We focus on the magneto-thermal instability (MTI), which may be active in the periphery of galaxy clusters. We aim to take a fresh look at the problem and construct a general theory that explains the MTI saturation mechanism and provides scalings and estimates for the turbulent kinetic energy, magnetic energy, and heat flux. We simulate MTI turbulence with a Boussinesq code, SNOOPY, which, in contrast to previous work, allows us to perform an extensive sampling of the parameter space. In two dimensions the saturation mechanism involves an inverse cascade carrying kinetic energy from the short MTI injection scales to larger scales, where it is arrested by the stable entropy stratification; at a characteristic "buoyancy scale", the energy is dumped into large-scale g-modes, which subsequently dissipate. Consequently, the entropy stratification sets an upper limit on the size and strength of turbulent eddies. Meanwhile, the MTI conveys a substantial fraction of heat, despite the tangled geometry of the magnetic field. In a companion paper, these results are extended to three-dimensional flows, and compared to real cluster observations.

astro-ph.CO↗

arXiv2020

Electric heating and angular momentum transport in laminar models of protoplanetary disks

The vertical temperature structure of a protoplanetary disk bears on several processes relevant to planet formation, such as gas and dust grain chemistry, ice lines and convection. The temperature profile is controlled by irradiation from the central star and by any internal source of heat such as might arise from gas accretion. We investigate the heat and angular momentum transport generated by the resistive dissipation of magnetic fields in laminar disks. We use local one-dimensional simulations to obtain vertical temperature profiles for typical conditions in the inner disk (0.5 to 4 au). Using simple assumptions for the gas ionization and opacity, the heating and cooling rates are computed self-consistently in the framework of radiative non-ideal magnetohydrodynamics. We characterize steady solutions that are symmetric about the midplane and which may be associated with saturated Hall-shear unstable modes. We also examine the dissipation of electric currents driven by global accretion-ejection structures. In both cases we obtain significant heating for a sufficiently high opacity. Strong magnetic fields can induce an order-unity temperature increase in the disk midplane, a convectively unstable entropy profile, and a surface emissivity equivalent to a viscous heating of $α\sim 10^{-2}$. These results show how magnetic fields may drive efficient accretion and heating in weakly ionized disks where turbulence might be inefficient, at least for a range of radii and ages of the disk.

astro-ph.EP↗

arXiv2020

Global simulations of self-gravitating magnetized protoplanetary disks

In the early stages of a protoplanetary disk, when its mass is a significant fraction of its star's, turbulence generated by gravitational instability (GI) should feature significantly in the disk's evolution. At the same time, the disk may be sufficiently ionised for magnetic fields to play some role in the dynamics. Though usually neglected, the impact of magnetism on the GI may be critical, with consequences for several processes: the efficiency of accretion, spiral structure formation, fragmentation, and the dynamics of solids. In this paper, we report on global three-dimensional magnetohydrodynamical simulations of a self-gravitating protoplanetary disk using the meshless finite mass (MFM) Lagrangian technique. We confirm that GI spiral waves trigger a dynamo that amplifies an initial magnetic field to nearly thermal amplitudes (plasma beta < 10), an order of magnitude greater than that generated by the magneto-rotational instability alone. We also determine the dynamo's nonlinear back reaction on the gravitoturbulent flow: the saturated state is substantially hotter, with an associated larger Toomre parameter and weaker, more 'flocculent' spirals. But perhaps of greater import is the dynamo's boosting of accretion via a significant Maxwell stress; mass accretion is enhanced by factors of several relative to either pure GI or pure MRI. Our simulations use ideal MHD, an admittedly poor approximation in protoplanetary disks, and thus future studies should explore the full gamut of non-ideal MHD. In preparation for that, we exhibit a small number of Ohmic runs that reveal that the dynamo, if anything, is stronger in a non-ideal environment. This work confirms that magnetic fields are a potentially critical ingredient in gravitoturbulent young disks, possibly controlling their evolution, especially via their enhancement of (potentially episodic) accretion.

astro-ph.EP↗

arXiv2019

Local simulations of MRI turbulence with meshless methods

The magneto-rotational instability (MRI) is one of the most important processes in sufficiently ionized astrophysical disks. Grid-based simulations, especially those using the local shearing box approximation, provide a powerful tool to study the ensuing nonlinear turbulence. On the other hand, while meshless methods have been widely used in both cosmology, galactic dynamics, and planet formation they have not been fully deployed on the MRI problem. We present local unstratified and vertically stratified MRI simulations with two meshless MHD schemes: a recent implementation of SPH MHD (Price2012), and a MFM MHD scheme with a constrained gradient divergence cleaning scheme, as implemented in the GIZMO code \citep{Hopkins2017}. Concerning variants of the SPH hydro force formulation we consider both the "vanilla" SPH and the PSPH variant included in GIZMO. We find, as expected, that the numerical noise inherent in these schemes affects turbulence significantly. A high order kernel, free of the pairing instability, is necessary. Both schemes can adequately simulate MRI turbulence in unstratified shearing boxes with net vertical flux. The turbulence, however, dies out in zero-net-flux unstratified boxes, probably due to excessive and numerical dissipation. In zero-net-flux vertically stratified simulations, MFM can reproduce the MRI dynamo and its characteristic butterfly diagram for several tens of orbits before ultimately decaying. In contrast, extremely strong toroidal fields, as opposed to sustained turbulence, develop in equivalent simulations using SPH MHD. This unphysical state in SPH MHD is likely caused by a combination of excessive artificial viscosity, numerical resistivity, and the relatively large residual errors in the divergence of the magnetic field remaining even after cleaning procedures are applied.

astro-ph.EP↗

arXiv2018

On Kelvin-Helmholtz and parametric instabilities driven by coronal waves

The Kelvin-Helmholtz instability has been proposed as a mechanism to extract energy from magnetohydrodynamic (MHD) kink waves in flux tubes, and to drive dissipation of this wave energy through turbulence. It is therefore a potentially important process in heating the solar corona. However, it is unclear how the instability is influenced by the oscillatory shear flow associated with an MHD wave. We investigate the linear stability of a discontinuous oscillatory shear flow in the presence of a horizontal magnetic field within a Cartesian framework that captures the essential features of MHD oscillations in flux tubes. We derive a Mathieu equation for the Lagrangian displacement of the interface and analyse its properties, identifying two different instabilities: a Kelvin-Helmholtz instability and a parametric instability involving resonance between the oscillatory shear flow and two surface Alfvén waves. The latter occurs when the system is Kelvin-Helmholtz stable, thus favouring modes that vary along the flux tube, and as a consequence provides an important and additional mechanism to extract energy. When applied to flows with the characteristic properties of kink waves in the solar corona, both instabilities can grow, with the parametric instability capable of generating smaller scale disturbances along the magnetic field than possible via the Kelvin-Helmholtz instability. The characteristic time-scale for these instabilities is $\sim 100$ s, for wavelengths of $200$ km. The parametric instability is more likely to occur for smaller density contrasts and larger velocity shears, making its development more likely on coronal loops than on prominence threads.

astro-ph.SR↗

arXiv2017

Turbulent fluctuations and the excitation of Z Cam outbursts

Z Cam variables are a subclass of dwarf nova that lie near a global bifurcation between outbursting ("limit cycle") and non-outbursting ("standstill") states. It is believed that variations in the secondary star's mass-injection rate instigate transitions between the two regimes. In this paper we explore an alternative trigger for these transitions: stochastic fluctuations in the disk's turbulent viscosity. We employ simple one-zone and global viscous models which, though inappropriate for detailed matching to observed light curves, clearly indicate that turbulent disk fluctuations induce outbursts when the system is sufficiently close to the global bifurcation point. While the models easily produce the observed "outburst dip" pairs exhibited by Z Cam and Nova-like variables, they struggle to generate long trains of outbursts. We conclude that mass transfer variability is the dominant physical process determining the overall Z Cam standstill outburst pattern, but that viscous stochasticity provides an additional ingredient explaining some of the secondary features observed.

astro-ph.SR↗

arXiv2017

MRI turbulence and thermal instability in accretion disks

A long-standing puzzle in the study of black-hole accretion concerns the presence or not of thermal instability. Classical theory predicts the encircling accretion disk is unstable, as do some self-consistent MHD simulations of the flow. Yet observations of strongly accreting sources generally fail to exhibit cyclic or unstable dynamics on the expected timescales. This paper checks whether turbulent fluctuations impede thermal instability. It also asks if it makes sense to conduct linear stability analyses on a turbulent background. These issues are explored with a set of MRI simulations in thermally unstable local boxes in combination with stochastic equations that approximate the disk energetics. These models show that the disk's thermal behaviour deviates significantly from laminar theory, though ultimately a thermal runaway does occur. We find that the disk temperature evolves as a biased random walk, rather than increasing exponentially, and thus generates a broad spread of outcomes, with instability often delayed for several thermal times. We construct a statistical theory that describes some of this behaviour, emphasising the importance of the `escape time' and its associated probability distribution. In conclusion, turbulent fluctuations on there own cannot stabilise a disk, but they can weaken and delay thermal instability.

astro-ph.HE↗

arXiv2016

On dust-gas gravitational instabilities in protoplanetary discs

In protoplanetary disks the aerodynamical friction between particles and gas induces a variety of instabilities that facilitate planet formation. Of these we examine the so-called `secular gravitational instability' (SGI) in the two-fluid approximation, deriving analytical expressions for its stability criteria and growth rates. Concurrently, we present a physical explanation of the instability that shows how it manifests upon an intermediate range of lengthscales exhibiting geostrophic balance in the gas component. In contrast to a single-fluid treatment, the SGI is quenched within a critical disk radius, as large as 10 AU and 30 AU for cm and mm sized particles respectively, although establishing robust estimates is hampered by uncertainties in the parameters (especially the strength of turbulence) and deficiencies in the razor-thin disk model we employ. It is unlikely, however, that the SGI is relevant for well-coupled dust. We conclude by applying these results to the question of planetesimal formation and the provenance of large-scale dust rings.

astro-ph.EP↗

arXiv2016

The magnetorotational instability in debris-disc gas

Debris discs are commonly swathed in gas which can be observed in UV, in fine structure lines in FIR, and in resolved maps of CO emission. Carbon and oxygen are overabundant in such gas, but it is severely depleted in hydrogen. As a consequence, its ionisation fraction is remarkably high, suggesting magnetohydrodynamic (MHD) processes may be important. In particular, the gas may be subject to the magnetorotational instability (MRI), and indeed recent modelling of $β$ Pictoris requires an anomalous viscosity to explain the gas's observed radial structure. In this paper we explore the possibility that the MRI is active in debris-disc gas and responsible for the observed mass transport. We find that non-ideal MHD and dust-gas interactions play a subdominant role, and that linear instability is viable at certain radii. However, owing to low gas densities, the outer parts of the disc could be stabilised by a weak ambient magnetic field, though it is difficult to constrain such a field. Even if the MRI is stabilised by too strong a field, a magnetocentrifugal wind may be launched in its place and this could lead to equivalent (non-turbulent) transport. Numerical simulations of the vertically stratified MRI in conditions appropriate to the debris disc gas should be able to determine the nature of the characteristic behaviour at different radii, and decide on the importance of the MRI (and MHD more generally) on the evolution of these discs.

astro-ph.EP↗

arXiv2016

On the survival of zombie vortices in protoplanetary discs

Recently it has been proposed that the zombie vortex instability (ZVI) could precipitate hydrodynamical activity and angular momentum transport in unmagnetised regions of protoplanetary discs, also known as "dead zones". In this letter we scrutinise, with high resolution 3D spectral simulations, the onset and survival of this instability in the presence of viscous and thermal physics. First, we find that the ZVI is strongly dependent on the nature of the viscous operator. Although the ZVI is easily obtained with hyper-diffusion, it is difficult to sustain with physical (second order) diffusion operators up to Reynolds numbers as high as 10^7. This sensitivity is probably due to the ZVI's reliance on critical layers, whose characteristic lengthscale, structure, and dynamics are controlled by viscous diffusion. Second, we observe that the ZVI is sensitive to radiative processes, and indeed only operates when the Peclet number is greater than a critical value ~10^4, or when the cooling time is longer than ~10 Omega^{-1}. As a consequence, the ZVI struggles to appear at R>0.3 AU in standard 0.01 M_sun T Tauri disc models, though younger more massive disks provide a more hospitable environment. Together these results question the prevalence of the ZVI in protoplanetary discs.

astro-ph.EP↗

arXiv2015

On the convective overstability in protoplanetary discs

This paper explores the driving of low-level hydrodynamical activity in protoplanetary-disc dead zones. A small adverse radial entropy gradient, ordinarily stabilised by rotation, excites oscillatory convection (`convective overstability') when thermal diffusion, or cooling, is neither too strong nor too weak. I revisit the linear theory of the instability, discuss its prevalence in protoplanetary discs, and show that unstable modes are exact nonlinear solutions in the local Boussinesq limit. Overstable modes cannot grow indefinitely, however, as they are subject to a secondary parametric instability that limits their amplitudes to relatively low levels. If parasites set the saturation level of the ensuing turbulence then the convective overstability is probably too weak to drive significant angular momentum transport or to generate vortices. But I also discuss an alternative, and far more vigorous, saturation route that generates radial `layers' or `zonal flows' (witnessed also in semiconvection). Numerical simulations are required to determine which outcome is favoured in realistic discs, and consequently how important the instability is for disc dynamics.

astro-ph.EP↗

arXiv2014

The ballistic transport instability in Saturn's rings III: numerical simulations

Saturn's inner B-ring and its C-ring support wavetrains of contrasting amplitudes but with similar length scales, 100-1000 km. In addition, the inner B-ring is punctuated by two intriguing `flat' regions between radii 93,000 km and 98,000 km in which the waves die out, whereas the C-ring waves coexist with a forest of plateaus, narrow ringlets, and gaps. In both regions the waves are probably generated by a large-scale linear instability whose origin lies in the meteoritic bombardment of the rings: the ballistic transport instability. In this paper, the third in a series, we numerically simulate the long-term nonlinear evolution of this instability in a convenient local model. Our C-ring simulations confirm that the unstable system forms low-amplitude wavetrains possessing a preferred band of wavelengths. B-ring simulations, on the other hand, exhibit localised nonlinear wave `packets' separated by linearly stable flat zones. Wave packets travel slowly while spreading in time, a result that suggests the observed flat regions in Saturn's B-ring are shrinking. Finally, we present exploratory runs of the inner B-ring edge which reproduce earlier numerical results: ballistic transport can maintain the sharpness of a spreading edge while building a `ramp' structure at its base. Moreover, the ballistic transport instability can afflict the ramp region, but only in low-viscosity runs.

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