arXiv subjects

Chong-Chong He

Publications and source records attributed to Chong-Chong He.

11 recordsLinked to original sources

arXiv2026

Magnetising the quokka code with fast, second-order accurate, error-correcting schemes for magnetohydrodynamics on GPUs

We implement a second-order accurate constrained transport (CT) magnetohydrodynamics (MHD) module with first-order flux correction in the open-source, adaptive mesh refinement (AMR), GPU-accelerated code quokka, supporting both ideal and resistive (constant Ohmic) regimes. For computing and averaging the electromotive forces (EMFs) at cell edges, the basis of any CT-MHD method, we experiment with a wide range of recent, state-of-the-art schemes, together with different reconstruction schemes. Alongside these, we develop a new EMF compute scheme that requires fewer reconstruction steps than existing approaches. We evaluate these scheme combinations on the basis of accuracy, stability, and GPU throughput, and demonstrate that our new scheme achieves accuracy comparable to the best-in-class existing methods, together with greater stability in reconnection-dominated flows and roughly 25--60% higher GPU throughput. Using this scheme, quokka achieves excellent results across a wide range of MHD flow regimes, reaching >50 million cell updates per GPU per second, with >70% parallel efficiency out to >500 GPUs. Finally, we confirm that our CT implementation preserves divergence-free magnetic fields (to machine precision) under AMR.

astro-ph.IM↗

arXiv2026

A novel algorithm for GPU-accelerated particle-mesh interactions implemented in the QUOKKA code

We present a novel, GPU-optimized algorithm for particle-mesh interactions in grid-based hydrodynamics simulations, designed for massively parallel architectures. This approach overcomes the inefficiency of particle neighbour searches or sorts across multiple GPU nodes by using a new "particle-mesh-particle" interaction scheme, which extends the particle-mesh method for self-gravity. The algorithm proceeds in two main stages: first, quantities exchanged between particles and the mesh -- such as mass, energy, and momentum added by stellar feedback or removed by accretion onto a sink -- are deposited into a buffer mesh equipped with ghost zones, where multiple contributions per cell are accumulated using atomic additions and then communicated across distributed memory ranks. In the second stage, the buffer states are applied to real mesh states, incorporating cell-wise limiters to enforce physical constraints such as positive density. We implement this scheme in the GPU-native radiation-magnetohydrodynamics code QUOKKA and validate it through a comprehensive suite of tests, including Bondi and Bondi-Hoyle accretion, and single and multiple supernova remnant evolution at varying spatial resolutions. We show that the algorithm achieves $\approx 50\%$ weak-scaling efficiency running on up to 8192 GPUs on the Frontier supercomputer. This scheme enables efficient, scalable particle-mesh coupling for GPU-optimized simulations of star formation and feedback in galaxies.

astro-ph.IM↗

arXiv2024

A novel numerical method for mixed-frame multigroup radiation-hydrodynamics with GPU acceleration implemented in the QUOKKA code

Mixed-frame formulations of radiation-hydrodynamics (RHD), where the radiation quantities are computed in an inertial frame but matter quantities are in a comoving frame, are advantageous because they admit algorithms that conserve energy and momentum to machine precision and combine more naturally with adaptive mesh techniques, since unlike pure comoving-frame methods they do not face the problem that radiation quantities must change frame every time a cell is refined or coarsened. However, implementing multigroup RHD in a mixed-frame formulation presents challenges due to the complexity of handling frequency-dependent interactions and the Doppler shift of radiation boundaries. In this paper, we introduce a novel method for multigroup RHD that integrates a mixed-frame formulation with a piecewise powerlaw approximation for frequency dependence within groups. This approach ensures the exact conservation of total energy and momentum while effectively managing the Lorentz transformation of group boundaries and evaluation of group-averaged opacities. Our method takes advantage of the locality of matter-radiation coupling, allowing the source term for $N_g$ frequency groups to be handled with simple equations with a sparse Jacobian matrix of size $N_g + 1$, which can be inverted with $O(N_g)$ complexity. This results in a computational complexity that scales linearly with $N_g$ and requires no more communication than a pure hydrodynamics update, making it highly efficient for massively parallel and GPU-based systems. We implement our method in the GPU-accelerated RHD code QUOKKA and demonstrate that it passes a wide range of numerical tests, including preserving the asymptotic diffusion limit. We demonstrate that the piecewise powerlaw method shows significant advantages over traditional opacity averaging methods for handling rapidly variable opacities with modest frequency resolution.

astro-ph.GA↗

arXiv2024

An Asymptotically-Correct Implicit-Explicit Time Integration Scheme for Finite Volume Radiation-Hydrodynamics

Numerical radiation-hydrodynamics (RHD) for non-relativistic flows is a challenging problem because it encompasses processes acting over a very broad range of timescales, and where the relative importance of these processes often varies by orders of magnitude across the computational domain. Here we present a new implicit-explicit (IMEX) method for numerical RHD that has a number of desirable properties that have not previously been combined in a single method. Our scheme is based on moments and allows machine-precision conservation of energy and momentum, making it highly suitable for adaptive mesh refinement applications; it requires no more communication than hydrodynamics and includes no non-local iterative steps, making it highly suitable for massively parallel and GPU-based systems where communication is a bottleneck; and we show that it is asymptotically-accurate in the streaming, static diffusion, and dynamic diffusion limits, including in the so-called asymptotic diffusion regime where the computational grid does not resolve the photon mean free path. We implement our method in the GPU-accelerated RHD code QUOKKA and show that it passes a wide range of numerical tests.

astro-ph.IM↗

arXiv2024

Formation of Large Circumstellar Discs in Multi-scale, ideal-MHD Simulations of Magnetically Critical Pre-stellar Cores

The formation of circumstellar discs is a critical step in the formation of stars and planets. Magnetic fields can strongly affect the evolution of angular momentum during prestellar core collapse, potentially leading to the failure of protostellar disc formation. This phenomenon, known as the magnetic braking catastrophe, has been observed in ideal-MHD simulations. In this work, we present results from ideal-MHD simulations of circumstellar disc formation from realistic initial conditions of strongly magnetised, massive cores with masses between $30 ~{\rm M}_\odot$ and $300 ~{\rm M}_\odot$ resolved by zooming into Giant Molecular Clouds with masses $\sim 10^4 \ {\rm M}_\odot$ and initial mass-to-flux ratios $0.6 \le μ_0 \le 3$. Due to the large turbulence caused by the non-axisymmetric gravitational collapse of the gas, the dominant vertical support of discs is turbulent motion, while magnetic and turbulent pressures contribute equally in the outer toroid. The magnetic field topology is extremely turbulent and incoherent, reducing the effect of magnetic braking by roughly one order of magnitude and leading to the formation of large Keplerian discs even in magnetically critical or near-critical cores. Only cores in GMCs with $μ_0 < 1$ fail to form discs. Instead, they collapse into a sheet-like structure and produce numerous low-mass stars. We also discuss a universal $B-ρ$ relation valid over a large range of scales from the GMC to massive cores, irrespective of the GMC magnetisation. This study differs from the vast literature on this topic which typically focus on smaller mass discs with idealised initial and boundary conditions, therefore providing insights into the initial conditions of massive prestellar core collapse and disc formation.

astro-ph.GA↗

arXiv2023

Bimodal Star Formation in Simulations of Strongly Magnetized Giant Molecular Clouds

We present the results of a set of radiation magnetohydrodynamic simulations of turbulent molecular clouds in which we vary the initial strength of the magnetic field within a range ($1 \lesssim μ\lesssim 5$) consistent with observations of local giant molecular clouds (GMCs). We find that as we increase the strength of the magnetic field, star formation transitions from unimodal (the baseline case, $μ=5$, with a single burst of star formation and Salpeter IMF) to bimodal. This effect is clearest in the most strongly magnetized GMCs ($μ=1$): a first burst of star formation with duration, intensity and IMF comparable to the baseline case, is followed by a second star formation episode in which only low-mass stars are formed. Overall, due to the second burst of star formation, the strongly magnetized case results in a longer star formation period and a higher efficiency of star formation. The second burst is produced by gas that is not expelled by radiative feedback, instead remaining trapped in the GMC by the large-scale B-field, producing a nearly one-dimensional flow of gas along the field lines. The trapped gas has a turbulent and magnetic topology that differs from that of the first phase and strongly suppresses gas accretion onto protostellar cores, reducing their masses. We speculate that this star formation bimodality may be an important ingredient to understand the origin of multiple stellar populations observed in massive globular clusters.

astro-ph.GA↗

arXiv2022

Massive Prestellar Cores in Radiation-magneto-turbulent Simulations of Molecular Clouds

We simulate the formation and collapse of prestellar cores at few-AU resolution in a set of radiation-magneto-hydrodynamic simulations of giant molecular clouds (GMCs) using the grid-based code RAMSES-RT. We adopt, for the first time to our best knowledge, realistic initial/boundary conditions by zooming-in onto individual massive prestellar cores within the GMC. We identify two distinct modes of fragmentation: "quasi-spherical" and "filamentary". In both modes the fragments eventually become embedded in a quasi-steady accretion disk or toroid with radii ~ 500-5000 AU and opening angles $H/R \sim 0.5-1$. The disks/toroids are Toomre stable but the accreted pre-existing fragments are found orbiting the outer disk, appearing as disk fragmentation. Each core converts nearly 100 percent of the gas mass into a few massive stars forming near the disk center. Large and massive disks around high-mass stars are supported by magnetic pressure in the outer disk, at radii >200-1000 AU, and turbulent pressure in the inner disk. The most massive core accretes several times more mass than its initial mass, forming a (proto)star cluster of 8 massive stars enshrouded by a toroid, suggesting a competitive accretion scenario for ultra-high-mass star formation. We also find that the HII regions produced by a single massive star remain trapped in the dense circumstellar disks for a few hundred kiloyears, while the dynamic motions of massive stars in wide binaries or multiple systems displace the stars from the densest parts of the disk, allowing UV radiation to escape producing steady or pulsating bipolar HII regions.

astro-ph.GA↗

arXiv2021

A Fast and Accurate Analytic Method of Calculating Galaxy Two-point Correlation Functions

We have developed a new analytic method to calculate the galaxy two-point correlation functions (TPCFs) accurately and efficiently, applicable to surveys with finite, regular, and mask-free geometries. We have derived simple, accurate formulas of the normalized random-random pair counts $RR$ as functions of the survey area dimensions. We have also suggested algorithms to compute the normalized data-random pair counts $DR$ analytically. With all edge corrections fully accounted for analytically, our method computes $RR$ and $DR$ with perfect accuracy and zero variance in $O(1)$ and $O(N_{\rm g})$ time, respectively. We test our method on a galaxy catalogue from the EAGLE simulation. Our method calculates $RR+DR$ at a speed 3 to 6 orders of magnitude faster than the brute-force Monte Carlo method and 2.5 orders of magnitude faster than tree-based algorithms. For a galaxy catalogue with 10 million data points in a cube, this reduces the computation time to under 1 minute on a laptop. Our analytic method is favored over the traditional Monte Carlo method whenever applicable. Some applications in the study of correlation functions and power spectra in cosmological simulations and galaxy surveys are discussed. However, we recognize that its applicability is very limited for realistic surveys with masks, irregular shapes, and/or weighted patterns.

astro-ph.GA↗

arXiv2020

Simulating Star Clusters Across Cosmic Time: II. Escape Fraction of Ionizing Photons from Molecular Clouds

We calculate the hydrogen and helium-ionizing radiation escaping star-forming molecular clouds, as a function of the star cluster mass and compactness, using a set of high-resolution radiation-magneto-hydrodynamic simulations of star formation in self-gravitating, turbulent molecular clouds. In these simulations, presented in He, Ricotti and Geen (2019), the formation of individual massive stars are well resolved, and their UV radiation feedback and lifetime on the main sequence are modeled self-consistently. We find that the escape fraction of ionizing radiation from molecular clouds, $\langle f_{\rm esc}^{\scriptscriptstyle \rm MC}\rangle$, decreases with increasing mass of the star cluster and with decreasing compactness. Molecular clouds with densities typically found in the local Universe have negligible $\langle f_{\rm esc}^{\scriptscriptstyle \rm MC}\rangle$, ranging between $0.5\%$ to $5\%$. Ten times denser molecular clouds have $\langle f_{\rm esc}^{\scriptscriptstyle \rm MC}\rangle \approx 10\%-20\%$, while $100\times$ denser clouds, which produce globular cluster progenitors, have $\langle f_{\rm esc}^{\scriptscriptstyle \rm MC}\rangle \approx 20\%-60\%$. We find that $\langle f_{\rm esc}^{\scriptscriptstyle \rm MC}\rangle$ increases with decreasing gas metallicity, even when ignoring dust extinction, due to stronger radiation feedback. However, the total number of escaping ionizing photons decreases with decreasing metallicity because the star formation efficiency is reduced. We conclude that the sources of reionization at $z>6$ must have been very compact star clusters forming in molecular clouds about $100\times$ denser than in today's Universe, which leads to a significant production of old globular clusters progenitors.

astro-ph.GA↗

arXiv2019

Simulating Star Clusters Across Cosmic Time: I. Initial Mass Function, Star Formation Rates and Efficiencies

We present radiation-magneto-hydrodynamic simulations of star formation in self-gravitating, turbulent molecular clouds, modeling the formation of individual massive stars, including their UV radiation feedback. The set of simulations have cloud masses between $m_{\rm gas}=10^3$~M$_\odot$ to $3 \times 10^5$~M$_\odot$ and gas densities typical of clouds in the local universe ($\overline n_{\rm gas} \sim 1.8\times 10^2$~cm$^{-3}$) and 10$\times$ and 100$\times$ denser, expected to exist in high-redshift galaxies. The main results are: {\it i}) The observed Salpeter power-law slope and normalisation of the stellar initial mass function at the high-mass end can be reproduced if we assume that each star-forming gas clump (sink particle) fragments into stars producing on average a maximum stellar mass about $40\%$ of the mass of the sink particle, while the remaining $60\%$ is distributed into smaller mass stars. Assuming that the sinks fragment according to a power-law mass function flatter than Salpeter, with log-slope $0.8$, satisfy this empirical prescription. {\it ii}) The star formation law that best describes our set of simulation is $dρ_*/dt \propto ρ_{gas}^{1.5}$ if $\overline n_{gas}<n_{cri}\approx 10^3$~cm$^{-3}$, and $dρ_*/dt \propto ρ_{\rm gas}^{2.5}$ otherwise. The duration of the star formation episode is roughly $6$ cloud's sound crossing times (with $c_s=10$~km/s). {\it iii}) The total star formation efficiency in the cloud is $f_*=2\% (m_{\rm gas}/10^4~M_\odot)^{0.4}(1+\overline n_{\rm gas}/n_{\rm cri})^{0.91}$, for gas at solar metallicity, while for metallicity $Z<0.1$~Z$_\odot$, based on our limited sample, $f_*$ is reduced by a factor of $\sim 5$. {\it iv)} The most compact and massive clouds appear to form globular cluster progenitors, in the sense that star clusters remain gravitationally bound after the gas has been expelled.

astro-ph.GA↗

arXiv2015

Anisotropy of X-ray bursts from neutron stars with concave accretion disks

Emission from neutron stars and accretion disks in low-mass X-ray binaries is not isotropic. The non-spherical shape of the disk as well as blocking of the neutron star by the disk and vice versa cause the observed flux to depend on the inclination angle of the disk with respect to the line of sight. This is of special importance for the interpretation of Type I X-ray bursts, which are powered by the thermonuclear burning of matter accreted onto the neutron star. Because part of the X-ray burst is reflected off the disk, the observed burst flux depends on the anisotropies for both direct emission from the neutron star and reflection off the disk. This influences measurements of source distance, mass accretion rate, and constraints on the neutron star equation of state. Previous studies made predictions of the anisotropy factor for the total burst flux, assuming a geometrically flat disk. Recently, detailed observations of two exceptionally long bursts (so-called superbursts) allowed for the first time for the direct and the reflected burst flux to each be measured, as opposed to just their sum. The ratio of the reflected and direct flux (the reflection fraction) was much higher than what the anisotropies of a flat disk can account for. We create numerical models to calculate the anisotropy factors for different disk shapes, including concave disks. We present the anisotropy factors of the direct and reflected burst flux separately, as well as the anisotropy of the persistent flux. Reflection fractions substantially larger than unity are produced in case the inner accretion disk steeply increases in height, such that part of the star is blocked from view. Such a geometry could possibly be induced by the X-ray burst, if X-ray heating causes the inner disk to puff up.

astro-ph.HE↗