F. X. Timmes

Galacti chemical evolution: Hygrogen through zinc

Using the output from a grid of 60 Type II supernova models (Woosley & Weaver 1995) of varying mass (11 approx. less than (M/solar mass) approx. less than 40) and metallicity (0, 10(exp -4), 0.01, and 1 solar metallicity), the chemical evolution of 76 stable isotopes, from hydrogen to zinc, is calculated. The chemical evolution calculation employs a simple dynamical model for the Galaxy (infall with a 4 Gyr e-folding timescale onto a exponential dsk and 1/r(exp 2) bulge), and standard evolution parameters, such as a Salpeter initial mass function and a quadratic Schmidt star formation rate. The theoretical results are compared in detail with observed stellar abundances in stars with metallicities in the range -3.0 approx. less than (Fe/H) approx. less than 0.0 dex. While our discussion focuses on the solar neighborhood where there are the most observations, the supernova rates, an intrinsically Galactic quality, are also discussed.

A comparative study of the turbulent Rayleigh–Taylor instability using high-resolution three-dimensional numerical simulations: The Alpha-Group collaboration

The turbulent Rayleigh–Taylor instability is investigated in the limit of strong mode-coupling using a variety of high-resolution, multimode, three dimensional numerical simulations (NS). The perturbations are initialized with only short wavelength modes so that the self-similar evolution (i.e., bubble diameter Db∝amplitude hb) occurs solely by the nonlinear coupling (merger) of saturated modes. After an initial transient, it is found that hb∼αbAgt2, where A=Atwood number, g=acceleration, and t=time. The NS yield Db∼hb/3 in agreement with experiment but the simulation value αb∼0.025±0.003 is smaller than the experimental value αb∼0.057±0.008. By analyzing the dominant bubbles, it is found that the small value of αb can be attributed to a density dilution due to fine-scale mixing in our NS without interface reconstruction (IR) or an equivalent entrainment in our NS with IR. This may be characteristic of the mode coupling limit studied here and the associated αb may represent a lower bound that is insensiti...

The conductive propagation of nuclear flames. I - Degenerate C + O and O + Ne + Mg white dwarfs

The paper determines the physical properties - speed, width, and density structure - of conductive burning fronts in degenerate carbon-oxygen (C + O) and oxygen-neon-magnesium (O + Ne + Mg) compositions for a grid of initial densities and compositions. The dependence of the physical properties of the flame on the assumed values of nuclear reaction rates, the nuclear reaction network employed, the thermal conductivity, and the choice of coordinate system are investigated. The occurrence of accretion-induced collapse of a white dwarf is found to be critically dependent on the velocity of the nuclear conductive burning front and the growth rate of hydrodynamic instabilities. Treating the expanding area of the turbulent burning region as a fractal whose tile size is identical to the minimum unstable Rayleigh-Taylor wavelength, it is found, for all reasonable values of the fractal dimension, that for initial C + O or O + Ne + Mg densities above about 9 x 10 exp 9 g/cu cm the white dwarf should collapse to a neutron star. 97 refs.

On Variations in the Peak Luminosity of Type Ia Supernovae

We explore the idea that the observed variations in the peak luminosities of Type Ia supernovae (SNe Ia) originate in part from a scatter in metallicity of the main-sequence stars that become white dwarfs. Previous numerical studies have not self-consistently explored metallicities greater than solar. One-dimensional Chandrasekhar mass models of SNe Ia produce most of their 56Ni in a burn to nuclear statistical equilibrium between the mass shells 0.2 and 0.8 M?, for which the electron-to-nucleon ratio Ye is constant during the burn. We show analytically that under these conditions, charge and mass conservation constrain the mass of 56Ni produced to depend linearly on the original metallicity of the white dwarf progenitor. Detailed postprocessing of W7-like models confirms this linear dependence. The effect that we have identified is most evident at metallicities larger than solar and is in agreement with previous self-consistent calculations over the metallicity range common to both calculations. The observed scatter in the metallicity (-3 Z?) of the solar neighborhood is enough to induce a 25% variation in the mass of 56Ni ejected by SNe Ia. This is sufficient to vary the peak V-band brightness by |?MV| ? 0.2. This scatter in metallicity is present out to the limiting redshifts of current observations (z 1). Sedimentation of 22Ne can possibly amplify the variation in 56Ni mass to 50%. Further numerical studies can determine if other metallicity-induced effects, such as a change in the mass of the 56Ni-producing region, offset or enhance the variation that we identify.

On validating an astrophysical simulation code

We present a case study of validating an astrophysical simulation code. Our study focuses on validating FLASH, a parallel, adaptive-mesh hydrodynamics code for studying the compressible, reactive flows found in many astrophysical environments. We describe the astrophysics problems of interest and the challenges associated with simulating these problems. We describe methodology and discuss solutions to difficulties encountered in verification and validation. We describe verification tests regularly administered to the code, present the results of new verification tests, and outline a method for testing general equations of state. We present the results of two validation tests in which we compared simulations to experimental data. The first is of a laser-driven shock propagating through a multilayer target, a configuration subject to both Rayleigh-Taylor and Richtmyer-Meshkov instabilities. The second test is a classic Rayleigh-Taylor instability, where a heavy fluid is supported against the force of gravity by a light fluid. Our simulations of the multilayer target experiments showed good agreement with the experimental results, but our simulations of the Rayleigh-Taylor instability did not agree well with the experimental results. We discuss our findings and present results of additional simulations undertaken to further investigate the Rayleigh-Taylor instability.

Integration of Nuclear Reaction Networks for Stellar Hydrodynamics

Methods for solving the stiff system of ordinary differential equations that constitute nuclear reaction networks are surveyed. Three semi-implicit time integration algorithms are examined; a traditional first-order-accurate Euler method, a fourth-order-accurate Kaps-Rentrop method, and a variable-order Bader-Deuflhard method. These three integration methods are coupled to eight different linear algebra packages. Four of the linear algebra packages operate on dense matrices (LAPACK, LUDCMP, LEQS, GIFT), three of them are designed for the direct solution of sparse matrices (MA28, UMFPACK, Y12M), and one uses an iterative method for sparse matrices (BiCG). The scaling properties and behavior of the 24 combinations (3 time integration methods times 8 linear algebra packages) are analyzed by running each combination on seven different nuclear reaction networks. These reaction networks range from a hardwired 13 isotope α-chain and heavy-ion reaction network, which is suitable for most multidimensional simulations of stellar phenomena, to a 489 isotope reaction network, which is suitable for determining the yields of isotopes lighter than technetium in spherically symmetric models of Type II supernovae. Each of the time integration methods and linear algebra packages are capable of generating accurate results, but the efficiency of the various methods—evaluated across several different machine architectures and compiler options—differ dramatically. If the execution speed of reaction networks that contain less than about 50 isotopes is an overriding concern, then the variable-order Bader-Deuflhard time integration method coupled with routines generated from the GIFT matrix package or LAPACK with vendor-optimized BLAS routines is a good choice. If the amount of storage needed for any reaction network is a concern, then any of the sparse matrix packages will reduce the storage costs by 70%-90%. When a balance between accuracy, overall efficiency, and ease of use is desirable, then the variable-order Bader-Deuflhard time integration method coupled with the MA28 sparse matrix package is a strong choice.

Constraints on the progenitor of cassiopeia A

We compare a suite of three-dimensional explosion calculations and stellar models incorporating advanced physics with observational constraints on the progenitor of Cassiopeia A. We consider binary and single stars from 16 to 40 M☉ with a range of explosion energies and geometries. The parameter space allowed by observations of nitrogen-rich high-velocity ejecta, ejecta mass, compact remnant mass, and 44Ti and 56Ni abundances individually and as an ensemble is considered. A progenitor of 15-25 M☉ that loses its hydrogen envelope to a binary interaction and undergoes an energetic explosion can match all the observational constraints.

Galactic Evolution of Silicon Isotopes: Application to Presolar SiC Grains from Meteorites

We calculate and discuss the chemical evolution of the isotopic silicon abundances in the interstellar medium (ISM) at distances and times appropriate to the birth of the solar system. This has several objectives, some of which are related to anomalous silicon isotope ratios within presolar grains extracted from meteorites; namely: (1) What is the relative importance for silicon isotopic compositions in the bulk ISM of Type II supernovae, Type Ia supernovae, and AGB stars? (2) Are 29Si and 30Si primary or secondary nucleosynthesis products? (3) In what isotopic direction in a three-isotope plot do core-collapse supernovae of different mass move the silicon isotopic composition? (4) Why do present calcu-lations not reproduce the solar ratios for silicon isotopes, and what does that impose upon studies of anomalous Si isotopes in meteoritic silicon carbide grains? (5) Are chemical-evolution features recorded in the anomalous SiC grains? Our answers are formulated on the basis of the Woosley & Weaver super-nova yield survey. Renormalization with the calculated interstellar medium silicon isotopic composition and solar composition is as an important and recurring concept of this paper. Possible interpretations of the silicon isotope anomalies measured in single SiC grains extracted from carbonaceous meteorites are then presented. The calculations suggest that the temporal evolution of the isotopic silicon abundances in the interstellar medium may be recorded in these grains.

The Accuracy, Consistency, and Speed of Five Equations of State for Stellar Hydrodynamics

We compare the thermodynamic properties and execution speed of five independent equations of state. A wide range of temperatures, densities, and compositions are considered—conditions appropriate for modeling the collapse of a cloud of hydrogen gas (or an exploding supernova) to the outer layers of a neutron star. The pressures and specific thermal energies calculated by each equation-of-state routine are reasonably accurate (typically 0.1% error or less) and agree remarkably well with each other, despite the different approaches and approximations used in each routine. The derivatives of the pressure and specific thermal energies with respect to the temperature and density generally show less accuracy (typically 1% error or less) and more disagreement with one another. Thermodynamic consistency, as measured by deviations from the appropriate Maxwell relations, shows that the Timmes equation of state and the Nadyozhin equation of state achieve thermodynamic consistency to a high degree of precision. The execution speed of the five equation-of-state routines—evaluated across several different machine architectures, compiler options, and modes of operation—differ dramatically. The Arnett equation of state is the fastest of the five routines, with the Nadyozhin equation of state close behind.

26Al and 60Fe from Supernova Explosions

Using recently calculated yields for Type II supernovae, along with models for chemical evolution and the distribution of mass in the interstellar medium, the current abundances and spatial distributions of two key gamma-ray radioactivities,