ORCID Profile
0000-0003-2309-8963
Current Organisation
University of Duisburg-Essen
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Publisher: Cambridge University Press (CUP)
Date: 06-2020
DOI: 10.1017/S1743921322001429
Abstract: We present a new algorithm to solve the equations of radiation hydrodynamics (RHD) in a frequency-integrated, two-moment formulation. Novel features of the algorithm include i) the adoption of a non-local Variable Eddington Tensor (VET) closure for the radiation moment equations, computed with a ray-tracing method, ii) support for adaptive mesh refinement (AMR), iii) use of a time-implicit Godunov method for the hyperbolic transport of radiation, and iv) a fixed-point Picard iteration scheme to accurately handle the stiff nonlinear gas-radiation energy exchange. Tests demonstrate that our scheme works correctly, yields accurate rates of energy and momentum transfer between gas and radiation, and obtains the correct radiation field distribution even in situations where more commonly used – but less accurate – closure relations like the Flux-limited Diffusion and Moment-1 approximations fail. Our scheme presents an important step towards performing RHD simulations with increasing spatial and directional accuracy, effectively improving their predictive capabilities.
Publisher: American Astronomical Society
Date: 15-11-2016
Publisher: American Astronomical Society
Date: 12-02-2015
Publisher: Oxford University Press (OUP)
Date: 23-10-2020
Abstract: The ionizing radiation of massive stars sculpts the surrounding neutral gas into pillar-like structures. Direct signatures of star formation through outflows and jets are observed in these structures, typically at their tips. Recent numerical simulations have suggested that this star formation could potentially be triggered by photoionizing radiation, driving compressive modes of turbulence in the pillars. In this study, we use recent high-resolution ALMA observations of 12CO, 13CO, and C18O, J = 2 − 1 emission to test this hypothesis for pillars in the Carina Nebula. We analyse column density and intensity-weighted velocity maps, and subtract any large-scale bulk motions in the plane of the sky to isolate the turbulent motions. We then reconstruct the dominant turbulence driving mode in the pillars, by computing the turbulence driving parameter b, characterized by the relation $\\sigma _{\\rho /\\rho _0} = b \\mathcal {M}$ between the standard deviation of the density contrast $\\sigma _{\\rho /\\rho _0}$ (with gas density ρ and its average ρ0) and the turbulent Mach number $\\mathcal {M}$. We find values of b ∼ 0.7–1.0 for most of the pillars, suggesting that predominantly compressive modes of turbulence are driven in the pillars by the ionizing radiation from nearby massive stars. We find that this range of b values can produce star formation rates in the pillars that are a factor ∼3 greater than with b ∼ 0.5, a typical average value of b for spiral-arm molecular clouds. Our results provide further evidence for the potential triggering of star formation in pillars through compressive turbulent motions.
Publisher: Oxford University Press (OUP)
Date: 28-02-2020
Abstract: We investigate the turbulence driving mode of ionizing radiation from massive stars on the surrounding interstellar medium. We run hydrodynamical simulations of a turbulent cloud impinged by a plane-parallel ionization front. We find that the ionizing radiation forms pillars of neutral gas reminiscent of those seen in observations. We quantify the driving mode of the turbulence in the neutral gas by calculating the driving parameter b, which is characterized by the relation $\\sigma _s^2 = \\ln ({1+b^2\\mathcal {M}^2})$ between the variance of the logarithmic density contrast $\\sigma _s^2$ [where s = ln (ρ/ρ0) with the gas density ρ and its average ρ0], and the turbulent Mach number $\\mathcal {M}$. Previous works have shown that b ∼ 1/3 indicates solenoidal ( ergence-free) driving and b ∼ 1 indicates compressive (curl-free) driving, with b ∼ 1 producing up to ten times higher star formation rates than b ∼ 1/3. The time variation of b in our study allows us to infer that ionizing radiation is inherently a compressive turbulence driving source, with a time-averaged b ∼ 0.76 ± 0.08. We also investigate the value of b of the pillars, where star formation is expected to occur, and find that the pillars are characterized by a natural mixture of both solenoidal and compressive turbulent modes (b ∼ 0.4) when they form, and later evolve into a more compressive turbulent state with b ∼ 0.5–0.6. A virial parameter analysis of the pillar regions supports this conclusion. This indicates that ionizing radiation from massive stars may be able to trigger star formation by producing predominately compressive turbulent gas in the pillars.
Publisher: Oxford University Press (OUP)
Date: 22-02-2022
Abstract: We present Variable Eddington Tensor (VET)-closed Transport on Adaptive Meshes (VETTAM), a new algorithm to solve the equations of radiation hydrodynamics (RHD) with support for adaptive mesh refinement (AMR) in a frequency-integrated, two-moment formulation. The method is based on a non-local VET closure computed with a hybrid characteristics scheme for ray tracing. We use a Godunov method for the hyperbolic transport of radiation with an implicit backwards-Euler temporal update to avoid the explicit time-step constraint imposed by the light-crossing time, and a fixed-point Picard iteration scheme to handle the nonlinear gas-radiation exchange term, with the two implicit update stages jointly iterated to convergence. We also develop a modified wave-speed correction method for AMR, which we find to be crucial for obtaining accurate results in the diffusion regime. We demonstrate the robustness of our scheme with a suite of pure radiation and RHD tests, and show that it successfully captures the streaming, static diffusion, and dynamic diffusion regimes and the spatial transitions between them, casts sharp shadows, and yields accurate results for rates of momentum and energy exchange between radiation and gas. A comparison between different closures for the radiation moment equations, with the Eddington approximation (0th-moment closure) and the M1 approximation (1st-moment closure), demonstrates the advantages of the VET method (2nd-moment closure) over the simpler closure schemes. VETTAM has been coupled to the AMR FLASH (magneto-)hydrodynamics code and we summarize by reporting performance features and bottlenecks of our implementation.
No related grants have been discovered for Rolf Kuiper.