arXiv subjects

Oliver Gressel

Publications and source records attributed to Oliver Gressel.

At least 19 recordsLinked to original sources

arXiv2026

A fast spectral-multigrid Poisson solver in non-Cartesian geometries

Accurate and efficient computation of self-gravity is essential in astrophysical fluid dynamics, particularly in spherical and cylindrical geometries where large radial dynamic ranges and non-axisymmetric structures arise. Poisson solvers in such settings must simultaneously achieve high accuracy, scalability, and flexibility across a wide range of grid configurations and physical regimes. We present a robust and scalable Poisson solver for three-dimensional non-Cartesian geometries, supporting both spherical and cylindrical coordinates with either uniform or logarithmic radial discretizations. The method employs azimuthal Fourier decomposition to transform the 3D Poisson equation into a set of independent 2D Helmholtz equations. These are solved using a geometrically consistent multigrid algorithm that preserves second-order accuracy on both uniform and non-uniform grids. Vacuum boundary conditions are implemented through a screening-mass approach, enabling accurate solutions in domains with open boundaries, inner cavities, and strongly non-axisymmetric mass distributions. Owing to the differing convergence rates of Fourier modes -- where higher-order modes converge more rapidly -- the solver allows efficient mode-by-mode treatment. The combination of spectral decomposition and multigrid acceleration provides an efficient and flexible computational framework. The solver is implemented in the PLUTO code and validated against both analytical solutions and dynamical test problems in spherical and cylindrical geometries. Results demonstrate second-order convergence and excellent agreement with reference solutions. Weak-scaling tests up to 4096 cores show strong parallel performance, with the Poisson solve remaining subdominant to magnetohydrodynamic update cost. This makes the method well suited for large-scale simulations of star formation, accretion disks, and gravitational instabilities.

astro-ph.IM↗

arXiv2026

Formation of spirals in early stage protoplanetary discs

Class II protoplanetary discs feature numerous non-axisymmetric substructures like spirals and the underlying mechanisms for their formation are still highly debated. Coincidentally, early stage, massive discs are subject to the gravitational instability that causes them to collapse into denser substructures. However, like for most instabilities, real systems usually remain marginally stable, here with Toomre parameter $Q \gtrsim 1$. We study how the self-gravity of the gas triggers the growth of spiral structures in the disc. We specifically focus on discs that are considered stable, that is, with respect to the gravitational instability (with $Q > 1$), as these discs remain unstable to non-axisymmetric perturbations like spirals. After a linear stability analysis, we produce high-resolution 2D shearing sheet simulations with the GPU-accelerated code \idefix of self-gravitating discs. We probe different initial densities and thermodynamical models of Toomre-stable discs. The initial transient growth of the spiral wave matches the linear theory provided we take into account the time dependency of the amplification. The spirals are then rapidly non-linearly amplified with growth rate $\approx 10$ orbital time scale. After this time spiral large scale mode are amplified up to 1000 times more than linear theory predicts. At later times, low density discs reach a weak gravito-turbulent state with $α\approx 10^{-3}$ and discs with higher density undergo runaway collapse of the spiral arms. All discs exhibit dominant large-scale spirals.

astro-ph.SR↗

arXiv2025

From thermal to magnetic driving: spectral diagnostics of simulation-based magnetothermal disc wind models

Disc winds driven by thermal and magnetic processes are thought to play a critical role in protoplanetary disc evolution. However, the relative contribution of each mechanism remains uncertain, particularly in light of their observational signatures. We investigate whether spatially resolved emission and synthetic spectral line profiles can distinguish between thermally and magnetically driven winds in protoplanetary discs. We modelled three disc wind scenarios with different levels of magnetisation: a relatively strongly magnetised wind ($β$4), a rather weakly magnetised wind ($β$6), and a purely photoevaporative wind (PE). Using radiative transfer post-processing, we generated synthetic emission maps and line profiles for [OI] 6300 Å, [NeII] 12.81 $\mathrmμ$m, and o-H2 2.12 $\mathrmμ$m, and compared them with observations. The $β$4 model generally produces broader and more blueshifted low-velocity components across all tracers, consistent with compact emission regions and steep velocity gradients. The $β$6 and PE models yield narrower profiles with smaller blueshifts, in better agreement with most observed narrow low-velocity components (NLVCs). We also find that some line profile diagnostics, such as the inclination at maximum centroid velocity, are not robust discriminants. However, the overall blueshift and full-width at half-maximum (FWHM) of the low-velocity components provide reliable constraints. The $β$4 model reproduces the most extreme blueshifted NLVCs in observations, while most observed winds are more consistent with the $β$6 and PE models. Our findings reinforce previous conclusions that most observed NLVCs are compatible with weakly magnetised or purely photoevaporative flows. The combination of line kinematics and emission morphology offers meaningful constraints on wind-driving physics.

astro-ph.EP↗

arXiv2025

Global Hall-magnetohydrodynamic simulations of transition disks

Context. Transition disks (TDs) are a type of protoplanetary disk characterized by a central dust and gas cavity. The processes behind how these cavities are formed and maintained, along with their observed high accretion rates of $10^{-8} -10^{-7} \, M_{\odot} \, \mathrm{yr}^{-1}$, continue to be subjects of active research. Aims. This work aims to investigate how the inclusion of the Hall effect (HE) alongside Ohmic resistivity (OR) and ambipolar diffusion (AD) affects the structure of the TD. Of key interest is the dynamical evolution of the cavity and whether it can indeed produce transonic accretion, as predicted by theoretical models in order to account for the observed high accretion rates despite the inner disk's low density. Methods. We present our results of 2D axisymmetric global radiation magnetohydrodynamic (MHD) simulations of TDs for which all three non-ideal MHD effects are accounted. We used the NIRVANA-III fluid code and initialized our model with a disk cavity reaching up to $R=8~\mathrm{au}$ with a density contrast of $10^5$. We performed three runs, one with only OR and AD, and one for each of the two configurations that arise when additionally including the HE, that is, with the field aligned (anti-aligned) with respect to the rotation axis. Results. For all three runs, our models maintain an intact inner cavity and an outer standard disk. MHD winds are launched both from the cavity and from the disk. Notably, when the HE is included, ring-like structures develop within the cavity. We moreover obtain accretion rates of $3 - 8 \times 10^{-8} \, M_{\odot} \, \mathrm{yr}^{-1}$, comparable to typical values seen in full disks. Importantly, we clearly observe transonic accretion ($v_{\mathrm{acc}} \gtrsim c_{s}$) in the cavity. Additionally, outward magnetic flux transport occurs in all three runs.

astro-ph.EP↗

arXiv2024

Toward an efficient second-order method for computing the surface gravitational potential on spherical-polar meshes

Astrophysical accretion discs that carry a significant mass compared with their central object are subject to the effect of self-gravity. In the context of circumstellar discs, this can, for instance, cause fragmentation of the disc gas, and -- under suitable conditions -- lead to the direct formation of gas-giant planets. If one wants to study these phenomena, the disc's gravitational potential needs to be obtained by solving the Poisson equation. This requires to specify suitable boundary conditions. In the case of a spherical-polar computational mesh, a standard multipole expansion for obtaining boundary values is not practicable. We hence compare two alternative methods for overcoming this limitation. The first method is based on a known Green's function expansion (termed "CCGF") of the potential, while the second (termed "James' method") uses a surface screening mass approach with a suitable discrete Green's function. We demonstrate second-order convergence for both methods and test the weak scaling behaviour when using thousands of computational cores. Overall, James' method is found superior owing to its favourable algorithmic complexity of $\sim \mathcal{O}(n^3)$ compared with the $\sim\mathcal{O}(n^4)$ scaling of the CCGF method.

astro-ph.EP↗

arXiv2023

Hall-magnetohydrodynamic simulations of X-ray photoevaporative protoplanetary disc winds

Understanding the complex evolution of protoplanetary disks (PPDs) and their dispersal via energetic stellar radiation are prominent challenges in astrophysics. It has recently been established that specifically the X-ray luminosity from the central protostar can significantly heat the surface of the disk, causing powerful photoevaporative winds that eject a considerable fraction of the disc's mass. Recent work in the field has moreover shown the importance of global PPD simulations that simultaneously take into account non-ideal magnetohydrodynamic (MHD) effects and detailed thermochemistry. Our motivation with the current paper lies in combining these two aspects and figure out how they interact. Focus is put on the Hall Effect (HE) and the impact it has on the overall field topology and mass loss/accretion rates. Utilizing a novel X-ray temperature parametrisation, we perform 2D-axisymmetric MHD simulations with the NIRVANA fluid code, covering all non-ideal effects. We find that, in the aligned orientation, the HE causes prominent inward displacement of the poloidal field lines that can increase the accretion rate through a laminar Maxwell stress. We find that outflows are mainly driven by photoevaporation -- unless the magnetic field strength is considerable (i.e., $β_p\leq 10^{3}$) or the X-ray luminosity low enough (i.e., $\log{L_X}\leq 29.3$). Inferred mass loss rate are in the range of the expected values $10^{-8}$ to $10^{-7}M_{\odot}yr^{-1}$. For comparison, we have also performed pure hydrodynamic (HD) runs and compared them with the equivalent MHD runs. Here we have found that the magnetic field does indeed contribute to the mass loss rate, albeit only discernibly so for low enough $L_X$ (i.e., $\log{L_X}\leq 30.8$). For values higher than that, the wind mass loss predicted from the MHD set converges to the ones predicted from pure HD.

astro-ph.EP↗

arXiv2023

Alpha tensor and dynamo excitation in turbulent fluids with anisotropic conductivity fluctuations

A mean-field theory of the electrodynamics of a turbulent fluid is formulated under the assumption that the molecular electric conductivity is correlated with the turbulent velocity fluctuation in the (radial) direction, $\mathbf{g}$. It is shown that for such homogeneous fluids a strong turbulence-induced field advection anti-parallel to $\mathbf{g}$ arises almost independently of rotation. For rotating fluids, an extra $α$ effect appears with the known symmetries and with the expected maximum at the poles. Fast rotation, however, with Coriolis number exceeding unity suppresses this term. Numerical simulations of forced turbulence using the NIRVANA code demonstrate that the radial advection velocity, $γ$, always dominates the $α$ term. We show finally with simplified models that $α^2$ dynamos are strongly influenced by the radial pumping: for $γ<α$ the solutions become oscillatory, while for $γ>α$ they become highly exotic if they exist at all. In conclusion, dynamo models for slow and fast solid-body rotation on the basis of finite conductivity-velocity correlations are unlikely to work, at least for $α^2Ω$ dynamos without strong shear.

physics.flu-dyn↗

arXiv2022

The Role of Disk Winds in the Evolution and Dispersal of Protoplanetary Disks

The assembly and architecture of planetary systems strongly depend on the physical processes governing the evolution and dispersal of protoplanetary disks. Since Protostars and Planets VI, new observations and theoretical insights favor disk winds as being one of those key processes. This chapter provides a comprehensive review of recent observations probing outflowing gas launched over a range of disk radii for a wide range of evolutionary stages, enabling an empirical understanding of how winds evolve. In parallel, we review theoretical advancements in both magnetohydrodynamic and photoevaporative disk wind models and identify predictions that can be confronted with observations. By linking theory and observations we critically assess the role of disk winds in the evolution and dispersal of protoplanetary disks. Finally, we explore the impact of disk winds on planet formation and evolution and highlight theoretical work, observations, and critical tests for future progress.

astro-ph.EP↗

arXiv2022

Finite-time response of dynamo mean-field effects in magnetorotational turbulence

Accretion disc turbulence along with its effect on large-scale magnetic fields plays an important role in understanding disc evolution in general, and the launching of astrophysical jets in particular. Motivated by enabling a comprehensive sub-grid description for global long-term simulations of accretions discs, we aim to further characterize the transport coefficients emerging in local simulations of magnetorotational disc turbulence. For the current investigation, we leverage a time-dependent version of the test-field method, which is sensitive to the turbulent electromotive force (EMF) generated as a response to a set of pulsating background fields. We obtain Fourier spectra of the transport coefficients as a function of oscillation frequency. These are well approximated by a simple response function, describing a finite-time build-up of the EMF as a result of a time-variable mean magnetic field. For intermediate timescales (i.e., slightly above the orbital frequency), we observe a significant phase lag of the EMF compared to the causing field. Augmented with our previous result on a non-local closure relation in space, and incorporated into a suitable mean-field description that we briefly sketch out here, the new framework will allow to drop the restrictive assumption of scale separation.

astro-ph.HE↗

arXiv2020

On the Combined Role of Cosmic Rays and Supernova-Driven Turbulence for Galactic Dynamos

Large-scale coherent magnetic fields observed in the nearby galaxies are thought to originate by a mean-field dynamo. This is governed via the turbulent electromotive force (EMF, $\overline{\mathcal{E}} $) generated by the helical turbulence driven by supernova (SN) explosions in the differentially rotating interstellar medium (ISM). In this paper we aim to investigate the possibility of dynamo action by the virtue of buoyancy due to a cosmic ray (CR) component injected through the SN explosions. We do this by analysing the magnetohydrodynamic simulations of local shearing box of ISM, in which the turbulence is driven via random SN explosions and the energy of the explosion is distributed in the CR and/or thermal energy components. We use the magnetic field aligned diffusion prescription for the propagation of CR. We compare the evolution of magnetic fields in the models with the CR component to our previous models that did not involve the CR. We demonstrate that the inclusion of CR component enhances the growth of dynamo slightly. We further compute the underlying dynamo coefficients using the test-fields method, and argue that the entire evolution of the large scale mean magnetic field can be reproduced with an $α-Ω$ dynamo model. We also show that the inclusion of CR component leads to an unbalanced turbulent pumping between magnetic field components and additional dynamo action by the Rädler effect.

astro-ph.GA↗

arXiv2020

Global Hydromagnetic Simulations of Protoplanetary Disks with Stellar Irradiation and Simplified Thermochemistry

Outflows driven by large-scale magnetic fields likely play an important role in the evolution and dispersal of protoplanetary disks, and in setting the conditions for planet formation. We extend our 2-D axisymmetric non-ideal MHD model of these outflows by incorporating radiative transfer and simplified thermochemistry, with the twin aims of exploring how heating influences wind launching, and illustrating how such models can be tested through observations of diagnostic spectral lines. Our model disks launch magnetocentrifugal outflows primarily through magnetic tension forces, so the mass-loss rate increases only moderately when thermochemical effects are switched on. For typical field strengths, thermochemical and irradiation heating are more important than magnetic dissipation. We furthermore find that the entrained vertical magnetic flux diffuses out of the disk on secular timescales as a result of non-ideal MHD. Through post-processing line radiative transfer, we demonstrate that spectral line intensities and moment-1 maps of atomic oxygen, the HCN molecule, and other species show potentially observable differences between a model with a magnetically driven outflow and one with a weaker, photoevaporative outflow. In particular, the line shapes and velocity asymmetries in the moment-1 maps could enable the identification of outflows emanating from the disk surface.

astro-ph.EP↗

arXiv2020

Low-mass planet migration in three dimensional wind-driven inviscid discs: a negative corotation torque

We present simulations of low-mass planet--disc interactions in inviscid three-dimensional discs. We show that a wind-driven laminar accretion flow through the surface layers of the disc does not significantly modify the migration torque experienced by embedded planets. More importantly, we find that 3D effects lead to a dramatic change in the behaviour of the dynamical corotation torque compared to earlier 2D theory and simulations. Although it was previously shown that the dynamical corotation torque could act to slow and essentially stall the inward migration of a low-mass planet, our results in 3D show that the dynamical corotation torque has the complete opposite effect and speeds up inward migration. Our numerical experiments implicate buoyancy resonances as the cause. These have two effects: (i) they exert a direct torque on the planet, whose magnitude relative to the Lindblad torque is measured in our simulations to be small; (ii) they torque the gas librating on horseshoe orbits in the corotation region and drive evolution of its vortensity, leading to the negative dynamical corotation torque. This indicates that at low turbulent viscosity, the detailed vertical thermal structure of the protoplanetary disc plays an important role in determining the migration behaviour of embedded planets. If this result holds up under a more refined treatment of disc thermal evolution, then it has important implications for understanding the formation and early evolution of planetary systems.

astro-ph.EP↗

arXiv2020

On the spatial and temporal non-locality of dynamo mean-field effects in supersonic interstellar turbulence

The interstellar medium of the Milky Way and nearby disk galaxies harbours large-scale coherent magnetic fields of Microgauss strength, that can be explained via the action of a mean-field dynamo. As in our previous work, we aim to quantify dynamo effects that are self-consistently emerging in realistic direct magnetohydrodynamic simulations, but we generalise our approach to the case of a non-local (non-instantaneous) closure relation, described by a convolution integral in space (time). To this end, we leverage our comprehensive simulation framework for the supernova-regulated turbulent multi-phase interstellar medium. By introducing spatially (temporally) modulated mean fields, we extend the previously used test-field method to the spectral realm -- providing the Fourier representation of the convolution kernels. The resulting spectra of the dynamo mean-field coefficients that we obtain broadly match expectations and allow to rigorously constrain the degree of scale separation in the Galactic dynamo. A surprising result is found for the diamagnetic pumping term, which increases in amplitude when going to smaller scales. Our results amount to the most comprehensive description of dynamo mean-field effects in the Galactic context to date. Surveying the relevant parameter space and quenching behaviour, this will ultimately enable the development of assumption-free sub-grid prescriptions for otherwise unresolved global galaxy simulations.

astro-ph.GA↗

arXiv2019

Predicting the observational signature of migrating Neptune-sized planets in low-viscosity disks

The migration of planetary cores embedded in a protoplanetary disk is an important mechanism within planet-formation theory, relevant for the architecture of planetary systems. Consequently, planet migration is actively discussed, yet often results of independent theoretical or numerical studies are unconstrained due to the lack of observational diagnostics designed in light of planet migration. In this work we follow the idea of inferring the migration behavior of embedded planets by means of the characteristic radial structures that they imprint in the disk's dust density distribution. We run hydrodynamical multifluid simulations of gas and several dust species in a locally isothermal $α$-disk in the low-viscosity regime ($α=10^{-5}$) and investigate the obtained dust structures. In this framework, a planet of roughly Neptune mass can create three (or more) rings in which dust accumulates. We find that the relative spacing of these rings depends on the planet's migration speed and direction. By performing subsequent radiative transfer calculations and image synthesis we show that - always under the condition of a near-inviscid disk - different migration scenarios are, in principle, distinguishable by long-baseline, state-of-the-art ALMA observations.

astro-ph.EP↗

arXiv2019

Turbulent transport coefficients in galactic dynamo simulations using singular value decomposition

Coherent magnetic fields in disc galaxies are thought to be generated by a large-scale (or mean-field) dynamo operating in their interstellar medium. A key driver of mean magnetic field growth is the turbulent electromotive force (EMF), which represents the influence of correlated small-scale (or fluctuating) velocity and magnetic fields on the mean field. The EMF is usually expressed as a linear expansion in the mean magnetic field and its derivatives, with the dynamo tensors as expansion coefficients. Here, we adopt the singular value decomposition (SVD) method to directly measure these turbulent transport coefficients in a simulation of the turbulent interstellar medium that realizes a large-scale dynamo. Specifically, the SVD is used to least-square fit the time series data of the EMF with that of the mean field and its derivatives, to determine these coefficients. We demonstrate that the spatial profiles of the EMF reconstructed from the SVD coefficients match well with that taken directly from the simulation. Also, as a direct test, we use the coefficients to simulate a 1-D mean-field dynamo model and find an overall similarity in the evolution of the mean magnetic field between the dynamo model and the direct simulation. We also compare the results with those which arise using simple regression and the ones obtained previously using the test-field (TF) method, to find reasonable qualitative agreement. Overall, the SVD method provides an effective post-processing tool to determine turbulent transport coefficients from simulations.

astro-ph.GA↗

arXiv2019

Probing the Protosolar Disk Using Dust Filtering at Gaps in the Early Solar System

Jupiter and Saturn formed early, before the gas disk dispersed. The presence of gap-opening planets affects the dynamics of the gas and embedded solids and halts the inward drift of grains above a certain size. A drift barrier can explain the absence of calcium aluminium rich inclusions (CAIs) in chondrites originating from parent bodies that accreted in the inner solar system. Employing an interdisciplinary approach, we use a $μ$-X-Ray-fluorescence scanner to search for large CAIs and a scanning electron microscope to search for small CAIs in the ordinary chondrite NWA 5697. We carry out long-term, two-dimensional simulations including gas, dust, and planets to characterize the transport of grains within the viscous $α$-disk framework exploring the scenarios of a stand-alone Jupiter, Jupiter and Saturn \textit{in situ}, or Jupiter and Saturn in a 3:2 resonance. In each case, we find a critical grain size above which drift is halted as a function of the physical conditions in the disk. From the laboratory search we find four CAIs with a largest size of $\approx$200$\,μ$m. \Combining models and data, we provide an estimate for the upper limit of the $α$-viscosity and the surface density at the location of Jupiter, using reasonable assumptions about the stellar accretion rate during inward transport of CAIs, and assuming angular momentum transport to happen exclusively through viscous effects. Moreover, we find that the compound gap structure in the presence of Saturn in a 3:2 resonance favors inward transport of grains larger than CAIs currently detected in ordinary chondrites.

astro-ph.EP↗

arXiv2019

Streaming Instability for Particle-Size Distributions

The streaming instability is thought to play a central role in the early stages of planet formation by enabling the efficient bypass of a number of barriers hindering the formation of planetesimals. We present the first study exploring the efficiency of the linear streaming instability when a particle-size distribution is considered. We find that, for a given dust-to-gas mass ratio, the multi-species streaming instability grows on timescales much longer than those expected when only one dust species is involved. In particular, distributions that contain close-to-order-unity dust-to-gas mass ratios lead to unstable modes that can grow on timescales comparable, or larger, with those of secular instabilities. We anticipate that processes leading to particle segregation and/or concentration can create favourable conditions for the instability to grow fast. Our findings may have important implications for a large number of processes in protoplanetary disks that rely on the streaming instability as usually envisioned for a unique dust species. Our results suggest that the growth rates of other resonant-drag-instabilities may also decrease considerably when multiple species are considered.

astro-ph.EP↗

arXiv2019

Dust segregation in Hall-dominated turbulent protoplanetary disks

Imaging of the dust continuum emitted from disks around nearby protostars reveals diverse substructure. In recent years, theoretical efforts have been intensified to investigate how far the intrinsic dynamics of protoplanetary disks (PPDs) can lead to such features. Turbulence in the realm of non-ideal magnetohydrodynamics (MHD) is one candidate for explaining the generation of zonal flows which can lead to local dust enhancements. Adopting a radially varying cylindrical disk model, and considering combinations of vertical and azimuthal initial net flux, we perform 3D non-ideal MHD simulations aimed at studying self-organization induced by the Hall effect in turbulent PPDs. To this end, new modules have been incorporated into the NIRVANA-III and FARGO3D MHD codes. We moreover include dust grains, treated in the fluid approximation, in order to study their evolution subject to the emerging zonal flows. In the regime of a dominant Hall effect, we robustly obtain large-scale organized concentrations in the vertical magnetic field that remain stable for hundreds of orbits. For disks with vertical initial net flux alone, we confirm the presence of zonal flows and vortices that introduce regions of super-Keplerian gas flow. Including a moderately strong net-azimuthal magnetic flux can significantly alter the dynamics, partially preventing the self-organization of zonal flows. For plasma beta-parameters smaller than 50, large-scale, near-axisymmetric structures develop in the vertical magnetic flux. In all cases, we demonstrate that the emerging features are capable of accumulating dust grains for a range of Stokes numbers.

astro-ph.EP↗