3D simulations of Rayleigh-Taylor mixing in core-collapse SNe with CASTRO

We present multidimensional simulations of the post-explosion hydrodynamics in three different 15 solar mass supernova models with zero, 10^{-4} solar metallicity, and solar metallicities. We follow the growth of the Rayleigh-Taylor instability that mixes together the stellar layers in the wake of the explosion. Models are initialized with spherically symmetric explosions and perturbations are seeded by the grid. Calculations are performed in two-dimensional axisymmetric and three-dimensional Cartesian coordinates using the new Eulerian hydrodynamics code, CASTRO. We find as in previous work, that Rayleigh-Taylor perturbations initially grow faster in 3D than in 2D. As the Rayleigh-Taylor fingers interact with one another, mixing proceeds to a greater degree in 3D than in 2D, reducing the local Atwood number and slowing the growth rate of the instability in 3D relative to 2D. By the time mixing has stopped, the width of the mixed region is similar in 2D and 3D simulations provided the Rayleigh-Taylor fingers show significant interaction. Our results imply that 2D simulations of light curves and nucleosynthesis in supernovae (SNe) that die as red giants may capture the features of an initially spherically symmetric explosion in far less computational time than required by a full 3D simulation. However, capturing large departures from spherical symmetry requires a significantly perturbed explosion. Large scale asymmetries cannot develop through an inverse cascade of merging Rayleigh-Taylor structures; they must arise from asymmetries in the initial explosion.Comment: 12 pages, 5 figures, ApJ accepte

The Early Evolution of Primordial Pair-Instability Supernovae

The observational signatures of the first cosmic explosions and their chemical imprint on second-generation stars both crucially depend on how heavy elements mix within the star at the earliest stages of the blast. We present numerical simulations of the early evolution of Population III pair-instability supernovae with the new adaptive mesh refinement code CASTRO. In stark contrast to 15 - 40 Msun core-collapse primordial supernovae, we find no mixing in most 150 - 250 Msun pair-instability supernovae out to times well after breakout from the surface of the star. This may be the key to determining the mass of the progenitor of a primeval supernova, because vigorous mixing will cause emission lines from heavy metals such as Fe and Ni to appear much sooner in the light curves of core-collapse supernovae than in those of pair-instability explosions. Our results also imply that unlike low-mass Pop III supernovae, whose collective metal yields can be directly compared to the chemical abundances of extremely metal-poor stars, further detailed numerical simulations will be required to determine the nucleosynthetic imprint of very massive Pop III stars on their direct descendants.Comment: submitted to ApJ, comments welcom

Conservative Initial Mapping For Multidimensional Simulations of Stellar Explosions

Mapping one-dimensional stellar profiles onto multidimensional grids as initial conditions for hydrodynamics calculations can lead to numerical artifacts, one of the most severe of which is the violation of conservation laws for physical quantities such as energy and mass. Here we introduce a numerical scheme for mapping one-dimensional spherically-symmetric data onto multidimensional meshes so that these physical quantities are conserved. We validate our scheme by porting a realistic 1D Lagrangian stellar profile to the new multidimensional Eulerian hydro code CASTRO. Our results show that all important features in the profiles are reproduced on the new grid and that conservation laws are enforced at all resolutions after mapping.Comment: 7 pages, 5 figures, Proceeding for Conference on Computational Physics (CCP 2011

A Case Study of Small Scale Structure Formation in 3D Supernova Simulations

It is suggested in observations of supernova remnants that a number of large- and small-scale structures form at various points in the explosion. Multidimensional modeling of core-collapse supernovae has been undertaken since SN1987A, and both simulations and observations suggest/show that Rayleigh-Taylor instabilities during the explosion is a main driver for the formation of structure in the remnants. We present a case study of structure formation in 3D in a \msol{15} supernova for different parameters. We investigate the effect of moderate asymmetries and different resolutions of the formation and morphology of the RT unstable region, and take first steps at determining typical physical quantities (size, composition) of arising clumps. We find that in this progenitor the major RT unstable region develops at the He/OC interface for all cases considered. The RT instabilities result in clumps that are overdense by 1-2 orders of magnitude with respect to the ambient gas, have size scales on the level of a few % of the remnant diameter, and are not diffused after the first $\sim30$ yrs of the remnant evolution, in the absence of a surrounding medium.Comment: 59 pages, 34 figure

Regulated internalization of caveolae

Caveolae are specialized invaginations of the plasma membrane which have been proposed to play a role in diverse cellular processes such as endocytosis and signal transduction. We have developed an assay to determine the fraction of internal versus plasma membrane caveolae. The GPI-anchored protein, alkaline phosphatase, was clustered in caveolae after antibody-induced crosslinking at low temperature and then, after various treatments, the relative amount of alkaline phosphatase on the cell surface was determined. Using this assay we were able to show a time- and temperature-dependent decrease in cell-surface alkaline phosphatase activity which was dependent on antibody-induced clustering. The decrease in cell surface alkaline phosphatase activity was greatly accelerated by the phosphatase inhibitor, okadaic acid, but not by a protein kinase C activator. Internalization of clustered alkaline phosphatase in the presence or absence of okadaic acid was blocked by cytochalasin D and by the kinase inhibitor staurosporine. Electron microscopy confirmed that okadaic acid induced removal of caveolae from the cell surface. In the presence of hypertonic medium this was followed by the redistribution of groups of caveolae to the center of the cell close to the microtubule-organizing center. This process was reversible, blocked by cytochalasin D, and the centralization of the caveolar clusters was shown to be dependent on an intact microtubule network. Although the exact mechanism of internalization remains unknown, the results show that caveolae are dynamic structures which can be internalized into the cell. This process may be regulated by kinase activity and require an intact actin network

Numerical approaches for multidimensional simulations of stellar explosions

We introduce numerical algorithms for initializing multidimensional simulations of stellar explosions with 1D stellar evolution models. The initial mapping from 1D profiles onto multidimensional grids can generate severe numerical artifacts, one of the most severe of which is the violation of conservation laws for physical quantities. We introduce a numerical scheme for mapping 1D spherically-symmetric data onto multidimensional meshes so that these physical quantities are conserved. We verify our scheme by porting a realistic 1D Lagrangian stellar profile to the new multidimensional Eulerian hydro code CASTRO. Our results show that all important features in the profiles are reproduced on the new grid and that conservation laws are enforced at all resolutions after mapping. We also introduce a numerical scheme for initializing multidimensional supernova simulations with realistic perturbations predicted by 1D stellar evolution models. Instead of seeding 3D stellar profiles with random perturbations, we imprint them with velocity perturbations that reproduce the Kolmogorov energy spectrum expected for highly turbulent convective regions in stars. Our models return Kolmogorov energy spectra and vortex structures like those in turbulent flows before the modes become nonlinear. Finally, we describe approaches to determining the resolution for simulations required to capture fluid instabilities and nuclear burning. Our algorithms are applicable to multidimensional simulations besides stellar explosions that range from astrophysics to cosmology.Comment: 17 pages, 12 figure

The Nucleosynthetic Imprint of 15-40 Solar Mass Primordial Supernovae on Metal-Poor Stars

The inclusion of rotationally-induced mixing in stellar evolution can alter the structure and composition of presupernova stars. We survey the effects of progenitor rotation on nucleosynthetic yields in Population III and II supernovae using the new adaptive mesh refinement (AMR) code CASTRO. We examine spherical explosions in 15, 25 and 40 solar mass stars at Z = 0 and 10^-4 solar metallicity with three explosion energies and two rotation rates. Rotation in the Z = 0 models resulted in primary nitrogen production and a stronger hydrogen burning shell which led all models to die as red supergiants. On the other hand, the Z=10^-4 solar metallicity models that included rotation ended their lives as compact blue stars. Because of their extended structure, the hydrodynamics favors more mixing and less fallback in the metal free stars than the Z = 10^-4 models. As expected, higher energy explosions produce more enrichment and less fallback than do lower energy explosions, and less massive stars produce more enrichment and leave behind smaller remnants than do more massive stars. We compare our nucleosynthetic yields to the chemical abundances in the three most iron-poor stars yet found and reproduce the abundance pattern of one, HE 0557-4840, with a zero metallicity 15 solar mass, 2.4 x 10^51 erg supernova. A Salpeter IMF averaged integration of our yields for Z=0 models with explosion energies of 2.4x10^51 ergs or less is in good agreement with the abundances observed in larger samples of extremely metal-poor stars, provided 15 solar mass stars are included. Since the abundance patterns of extremely metal-poor stars likely arise from a representative sample of progenitors, our yields suggest that low-mass supernovae contributed the bulk of the metals to the early universe.Comment: 16 pages, 11 figures; submitted to Ap

Type II Supernovae: Model Light Curves and Standard Candle Relationships

A survey of Type II supernovae explosion models has been carried out to determine how their light curves and spectra vary with their mass, metallicity, and explosion energy. The presupernova models are taken from a recent survey of massive stellar evolution at solar metallicity supplemented by new calculations at subsolar metallicity. Explosions are simulated by the motion of a piston near the edge of the iron core and the resulting light curves and spectra are calculated using full multi-wavelength radiation transport. Formulae are developed that describe approximately how the model observables (light curve luminosity and duration) scale with the progenitor mass, explosion energy, and radioactive nucleosynthesis. Comparison with observational data shows that the explosion energy of typical supernovae (as measured by kinetic energy at infinity) varies by nearly an order of magnitude -- from 0.5 to 4.0 x 10^51 ergs, with a typical value of ~0.9 x 10^51 ergs. Despite the large variation, the models exhibit a tight relationship between luminosity and expansion velocity, similar to that previously employed empirically to make SNe IIP standardized candles. This relation is explained by the simple behavior of hydrogen recombination in the supernova envelope, but we find a sensitivity to progenitor metallicity and mass that could lead to systematic errors. Additional correlations between light curve luminosity, duration, and color might enable the use of SNe IIP to obtain distances accurate to ~20% using only photometric data.Comment: 12 pages, ApJ in pres