[en] A method for implementing cylindrical coordinates in the Athena magnetohydrodynamics (MHD) code is described. The extension follows the approach of Athena's original developers and has been designed to alter the existing Cartesian-coordinates code as minimally and transparently as possible. The numerical equations in cylindrical coordinates are formulated to maintain consistency with constrained transport (CT), a central feature of the Athena algorithm, while making use of previously implemented code modules such as the Riemann solvers. Angular momentum transport, which is critical in astrophysical disk systems dominated by rotation, is treated carefully. We describe modifications for cylindrical coordinates of the higher-order spatial reconstruction and characteristic evolution steps as well as the finite-volume and CT updates. Finally, we present a test suite of standard and novel problems in one, two, and three dimensions designed to validate our algorithms and implementation and to be of use to other code developers. The code is suitable for use in a wide variety of astrophysical applications and is freely available for download on the Web.

[en] Radiation feedback from young star clusters embedded in giant molecular clouds (GMCs) is believed to be important to the control of star formation. For the most massive and dense clouds, including those in which super star clusters (SSCs) are born, pressure from reprocessed radiation exerted on dust grains may disperse a significant portion of the cloud mass back into the interstellar medium. Using our radiation hydrodynamics code, Hyperion, we conduct a series of numerical simulations to test this idea. Our models follow the evolution of self-gravitating, strongly turbulent clouds in which collapsing regions are replaced by radiating sink particles representing stellar clusters. We evaluate the dependence of the star formation efficiency (SFE) on the size and mass of the cloud and κ, the opacity of the gas to infrared (IR) radiation. We find that the single most important parameter determining the evolutionary outcome is κ, with $\mathrm{\kappa <!--\; ??\; -->}\gtrsim <!--\; ???\; -->15{\mathrm{c}\mathrm{m}}^2{\mathrm{g}}^{-<!--\; ???\; -->1}$ needed to disrupt clouds. For $\mathrm{\kappa <!--\; ??\; -->}=20-<!--\; ???\; -->40{\mathrm{c}\mathrm{m}}^2{\mathrm{g}}^{-<!--\; ???\; -->1}$, the resulting SFE $=50\mathrm{\%<!--\; \%\; -->}-<!--\; ???\; -->70\mathrm{\%<!--\; \%\; -->}$ is similar to empirical estimates for some SSC-forming clouds. The opacities required for GMC disruption likely apply only in dust-enriched environments. We find that the subgrid model approach of boosting the direct radiation force $L/c$ by a “trapping factor” equal to a cloud’s mean IR optical depth can overestimate the true radiation force by factors of $\sim <!--\; ???\; -->4-<!--\; ???\; -->5$. We conclude that feedback from reprocessed IR radiation alone is unlikely to significantly reduce star formation within GMCs unless their dust abundances or cluster light-to-mass ratios are enhanced.

[en] We describe a module for the Athena code that solves the gray equations of radiation hydrodynamics (RHD), based on the first two moments of the radiative transfer equation. We use a combination of explicit Godunov methods to advance the gas and radiation variables including the non-stiff source terms, and a local implicit method to integrate the stiff source terms. We adopt the M₁ closure relation and include all leading source terms to O(βτ). We employ the reduced speed of light approximation (RSLA) with subcycling of the radiation variables in order to reduce computational costs. Our code is dimensionally unsplit in one, two, and three space dimensions and is parallelized using MPI. The streaming and diffusion limits are well described by the M₁ closure model, and our implementation shows excellent behavior for a problem with a concentrated radiation source containing both regimes simultaneously. Our operator-split method is ideally suited for problems with a slowly varying radiation field and dynamical gas flows, in which the effect of the RSLA is minimal. We present an analysis of the dispersion relation of RHD linear waves highlighting the conditions of applicability for the RSLA. To demonstrate the accuracy of our method, we utilize a suite of radiation and RHD tests covering a broad range of regimes, including RHD waves, shocks, and equilibria, which show second-order convergence in most cases. As an application, we investigate radiation-driven ejection of a dusty, optically thick shell in the ISM. Finally, we compare the timing of our method with other well-known iterative schemes for the RHD equations. Our code implementation, Hyperion, is suitable for a wide variety of astrophysical applications and will be made freely available on the Web.

[en] Radiation feedback from stellar clusters is expected to play a key role in setting the rate and efficiency of star formation in giant molecular clouds. To investigate how radiation forces influence realistic turbulent systems, we have conducted a series of numerical simulations employing the Hyperion radiation hydrodynamics solver, considering the regime that is optically thick to ultraviolet and optically thin to infrared radiation. Our model clouds cover initial surface densities between ${\mathrm{\Sigma <!--\; ??\; -->}}_{\mathrm{c}\mathrm{l},0}\sim <!--\; ???\; -->10\text{--}300M_{\odot <!--\; ???\; -->}{\mathrm{p}\mathrm{c}}^{-<!--\; ???\; -->2}$, with varying initial turbulence. We follow them through turbulent, self-gravitating collapse, star cluster formation, and cloud dispersal by stellar radiation. All our models display a log-normal distribution of gas surface density Σ; for an initial virial parameter ${\mathrm{\alpha <!--\; ??\; -->}}_{\mathrm{v}\mathrm{i}\mathrm{r},0}=2$, the log-normal standard deviation is ${\mathrm{\sigma <!--\; ??\; -->}}_{\mathrm{l}\mathrm{n}\mathrm{\Sigma <!--\; ??\; -->}}=1\text{--}1.5$ and the star formation rate coefficient ${\mathrm{\epsilon <!--\; ??\; -->}}_{\mathrm{f}\mathrm{f},\stackrel{¯<!--\; ??\; -->}{\mathrm{\rho <!--\; ??\; -->}}}=0.3\text{--}0.5$, both of which are sensitive to turbulence but not radiation feedback. The net star formation efficiency (SFE) ${\mathrm{\epsilon <!--\; ??\; -->}}_{\mathrm{f}\mathrm{i}\mathrm{n}\mathrm{a}\mathrm{l}}$ increases with ${\mathrm{\Sigma <!--\; ??\; -->}}_{\mathrm{c}\mathrm{l},0}$ and decreases with ${\mathrm{\alpha <!--\; ??\; -->}}_{\mathrm{v}\mathrm{i}\mathrm{r},0}$. We interpret these results via a simple conceptual framework, whereby steady star formation increases the radiation force, such that local gas patches at successively higher Σ become unbound. Based on this formalism (with fixed ${\mathrm{\sigma <!--\; ??\; -->}}_{\mathrm{l}\mathrm{n}\mathrm{\Sigma <!--\; ??\; -->}}$), we provide an analytic upper bound on ${\mathrm{\epsilon <!--\; ??\; -->}}_{\mathrm{f}\mathrm{i}\mathrm{n}\mathrm{a}\mathrm{l}}$, which is in good agreement with our numerical results. The final SFE depends on the distribution of Eddington ratios in the cloud and is strongly increased by the turbulent compression of gas.

[en] We perform the first self-consistent, time-dependent, multi-group calculations in two dimensions (2D) to address the consequences of using the ray-by-ray+ transport simplification in core-collapse supernova simulations. Such a dimensional reduction is employed by many researchers to facilitate their resource-intensive calculations. Our new code (Fornax) implements multi-D transport, and can, by zeroing out transverse flux terms, emulate the ray-by-ray+ scheme. Using the same microphysics, initial models, resolution, and code, we compare the results of simulating 12, 15, 20, and 25 M _⊙ progenitor models using these two transport methods. Our findings call into question the wisdom of the pervasive use of the ray-by-ray+ approach. Employing it leads to maximum post-bounce/pre-explosion shock radii that are almost universally larger by tens of kilometers than those derived using the more accurate scheme, typically leaving the post-bounce matter less bound and artificially more “explodable.” In fact, for our 25 M _⊙ progenitor, the ray-by-ray+ model explodes, while the corresponding multi-D transport model does not. Therefore, in two dimensions, the combination of ray-by-ray+ with the axial sloshing hydrodynamics that is a feature of 2D supernova dynamics can result in quantitatively, and perhaps qualitatively, incorrect results.

[en] In diverse areas of science and technology, including inertial confinement fusion (ICF), astrophysics, geophysics, and engineering processes, turbulent mixing induced by hydrodynamic instabilities is of scientific interest as well as practical significance. Because of the fundamental roles they often play in ICF and other applications, three classes of hydrodynamic instability-induced turbulent flows—those arising from the Rayleigh-Taylor, Richtmyer-Meshkov, and Kelvin-Helmholtz instabilities—have attracted much attention. ICF implosions, supernova explosions, and other applications illustrate that these phases of instability growth do not occur in isolation, but instead are connected so that growth in one phase feeds through to initiate growth in a later phase. Essentially, a description of these flows must encompass both the temporal and spatial evolution of the flows from their inception. Hydrodynamic instability will usually start from potentially infinitesimal spatial perturbations, will eventually transition to a turbulent flow, and then will reach a final state of a true multiscale problem. Indeed, this change in the spatial scales can be vast, with hydrodynamic instability evolving from just a few microns to thousands of kilometers in geophysical or astrophysical problems. These instabilities will evolve through different stages before transitioning to turbulence, experiencing linear, weakly, and highly nonlinear states. The challenges confronted by researchers are enormous. The inherent difficulties include characterizing the initial conditions of such flows and accurately predicting the transitional flows. Of course, fully developed turbulence, a focus of many studies because of its major impact on the mixing process, is a notoriously difficult problem in its own right. In this pedagogical review, we will survey challenges and progress, and also discuss outstanding issues and future directions.

[en] Here, we present results of 2D axisymmetric core-collapse supernova simulations, employing the FORNAX code, of nine progenitor models spanning 12 to 25 M⊙. Four of the models explode with inelastic scattering off electrons and neutrons as well as the many-body correction to neutrino-nucleon scattering opacities. We show that these four models feature sharp Si–O interfaces in their density profiles, and that the corresponding dip in density reduces the accretion rate around the stalled shock and prompts explosion. The non-exploding models lack such a steep feature, highlighting the Si–O interface as one key to explosion. Furthermore, we show that all of the non-exploding models can be nudged to explosion with modest changes to macrophysical inputs, including moderate rotation and perturbations to infall velocities, as well as to microphysical inputs, including reasonable changes to neutrino-nucleon interaction rates, suggesting that all the models are perhaps close to criticality. Exploding models have energies of a few × 10⁵⁰ erg at the end of our simulation, and are rising, emphasizing the need to continue these simulations over larger grids and for longer times to reproduce the energies seen in nature. Morphology of the explosion contributes to the explosion energy, with more isotropic ejecta producing larger explosion energies. We do not find evidence for the Lepton-number Emission Self-sustained Asymmetry. Finally, we look at proto-neutron star (PNS) properties and explore the role of dimension in our simulations. We find that convection in the PNS produces larger PNS radii as well as greater ‘ν_μ’ luminosities in 2D compared to 1D.