arXiv subjects

Oliver Zier

Publications and source records attributed to Oliver Zier.

40 records · Page 3Linked to original sources

arXiv2023

Gravito-turbulence in local disk simulations with an adaptive moving mesh

Self-gravity plays an important role in the evolution of rotationally supported systems such as protoplanetary disks, accretion disks around black holes, or galactic disks, as it can both feed turbulence or lead to gravitational fragmentation. While such systems can be studied in the shearing box approximation with high local resolution, the large density contrasts that are possible in the case of fragmentation still limit the utility of Eulerian codes with constant spatial resolution. In this paper, we present a novel self-gravity solver for the shearing box based on the TreePM method of the moving-mesh code AREPO. The spatial gravitational resolution is adaptive which is important to make full use of the quasi-Lagrangian hydrodynamical resolution of the code. We apply our new implementation to two- and three-dimensional, self-gravitating disks combined with a simple $β$-cooling prescription. For weak cooling we find a steady, gravito-turbulent state, while for strong cooling the formation of fragments is inevitable. To reach convergence for the critical cooling efficiency above which fragmentation occurs, we require a smoothing of the gravitational force in the two dimensional case that mimics the stratification of the three-dimensional simulations. The critical cooling efficiency we find, $β\approx 3$, as well as box-averaged quantities characterizing the gravito-turbulent state, agree well with various previous results in the literature. Interestingly, we observe stochastic fragmentation for $β> 3$, which slightly decreases the cooling efficiency required to observe fragmentation over the lifetime of a protoplanetary disk. The numerical method outlined here appears well suited to study the problem of galactic disks as well as magnetized, self-gravitating disks.

astro-ph.EP↗

arXiv2023

Non-ideal magnetohydrodynamics on a moving mesh I: Ohmic and ambipolar diffusion

Especially in cold and high-density regions, the assumptions of ideal magnetohydrodynamics (MHD) can break down, making first order non-ideal terms such as Ohmic and ambipolar diffusion as well as the Hall effect important. In this study we present a new numerical scheme for the first two resistive terms, which we implement in the moving-mesh code AREPO using the single-fluid approximation combined with a new gradient estimation technique based on a least-squares fit per interface. Through various test calculations including the diffusion of a magnetic peak, the structure of a magnetic C-shock, and the damping of an Alfvén wave, we show that we can achieve an accuracy comparable to the state-of-the-art code ATHENA++. We apply the scheme to the linear growth of the magnetorotational instability and find good agreement with the analytical growth rates. By simulating the collapse of a magnetised cloud with constant magnetic diffusion, we show that the new scheme is stable even for large density contrasts. Thanks to the Lagrangian nature of the moving mesh method the new scheme is thus well suited for intended future applications where a high resolution in the dense cores of collapsing protostellar clouds needs to be achieved. In a forthcoming work we will extend the scheme to the Hall effect.

astro-ph.EP↗

arXiv2021

Simulating cosmic structure formation with the GADGET-4 code

Numerical methods have become a powerful tool for research in astrophysics, but their utility depends critically on the availability of suitable simulation codes. This calls for continuous efforts in code development, which is necessitated also by the rapidly evolving technology underlying today's computing hardware. Here we discuss recent methodological progress in the GADGET code, which has been widely applied in cosmic structure formation over the past two decades. The new version offers improvements in force accuracy, in time-stepping, in adaptivity to a large dynamic range in timescales, in computational efficiency, and in parallel scalability through a special MPI/shared-memory parallelization and communication strategy, and a more-sophisticated domain decomposition algorithm. A manifestly momentum conserving fast multipole method (FMM) can be employed as an alternative to the one-sided TreePM gravity solver introduced in earlier versions. Two different flavours of smoothed particle hydrodynamics, a classic entropy-conserving formulation and a pressure-based approach, are supported for dealing with gaseous flows. The code is able to cope with very large problem sizes, thus allowing accurate predictions for cosmic structure formation in support of future precision tests of cosmology, and at the same time is well adapted to high dynamic range zoom-calculations with extreme variability of the particle number density in the simulated volume. The GADGET-4 code is publicly released to the community and contains infrastructure for on-the-fly group and substructure finding and tracking, as well as merger tree building, a simple model for radiative cooling and star formation, a high dynamic range power spectrum estimator, and an initial conditions generator based on second-order Lagrangian perturbation theory.

astro-ph.IM↗

arXiv2021

On the interaction of a Bonnor-Ebert sphere with a stellar wind

The structure of protostellar cores can often be approximated by isothermal Bonnor-Ebert spheres (BES) which are stabilized by an external pressure. For the typical pressure of $10^4k_B\,\mathrm{K\,cm^{-3}}$ to $10^5k_B\,\mathrm{K\,cm^{-3}}$ found in molecular clouds, cores with masses below $1.5\,{\rm M_\odot}$ are stable against gravitational collapse. In this paper, we analyze the efficiency of triggering a gravitational collapse by a nearby stellar wind, which represents an interesting scenario for triggered low-mass star formation. We derive analytically a new stability criterion for a BES compressed by a stellar wind, which depends on its initial nondimensional radius $ξ_{max}$. If the stability limit is violated the wind triggers a core collapse. Otherwise, the core is destroyed by the wind. We estimate its validity range to $2.5<ξ_{max}<4.2$ and confirm this in simulations with the SPH Code GADGET-3. The efficiency to trigger a gravitational collapse strongly decreases for $ξ_{max}<2.5$ since in this case destruction and acceleration of the whole sphere begin to dominate. We were unable to trigger a collapse for $ξ_{max}<2$, which leads to the conclusion that a stellar wind can move the smallest unstable stellar mass to $0.5\,\mathrm{M_\odot}$ and destabilizing even smaller cores would require an external pressure larger than $10^5k_B\,\mathrm{K\,cm^{-3}}$. For $ξ_{max}>4.2$ the expected wind strength according to our criterion is small enough so that the compression is slower than the sound speed of the BES and sound waves can be triggered. In this case our criterion underestimates somewhat the onset of collapse and detailed numerical analyses are required.

astro-ph.GA↗