Stellar physics and evolution calculations enable a broad range of research in astrophysics. Modules for Experiments in Stellar Astrophysics (MESA) is a suite of open source, robust, efficient, thread-safe libraries for a wide range of applications in computational stellar astrophysics. A one-dimensional stellar evolution module, MESAstar, combines many of the numerical and physics modules for simulations of a wide range of stellar evolution scenarios ranging from very low mass to massive stars, including advanced evolutionary phases. MESAstar solves the fully coupled structure and composition equations simultaneously. It uses adaptive mesh refinement and sophisticated timestep controls, and supports shared memory parallelism based on OpenMP. State-of-the-art modules provide equation of state, opacity, nuclear reaction rates, element diffusion data, and atmosphere boundary conditions. Each module is constructed as a separate Fortran 95 library with its own explicitly defined public interface to facilitate independent development. Several detailed examples indicate the extensive verification and testing that is continuously performed and demonstrate the wide range of capabilities that MESA possesses. These examples include evolutionary tracks of very low mass stars, brown dwarfs, and gas giant planets to very old ages; the complete evolutionary track of a 1 M ☉ star from the pre-main sequence (PMS) to a cooling white dwarf; the solar sound speed profile; the evolution of intermediate-mass stars through the He-core burning phase and thermal pulses on the He-shell burning asymptotic giant branch phase; the interior structure of slowly pulsating B Stars and Beta Cepheids; the complete evolutionary tracks of massive stars from the PMS to the onset of core collapse; mass transfer from stars undergoing Roche lobe overflow; and the evolution of helium accretion onto a neutron star. MESA can be downloaded from the project Web site (http://mesa.sourceforge.net/).

https://iopscience.iop.org/article/10.1088/0067-0049/192/1/3/pdf

Modules for Experiments in Stellar Astrophysics (MESA)

QUANTUM gravitational effects are usually ignored in calculations of the formation and evolution of black holes. The justification for this is that the radius of curvature of space-time outside the event horizon is very large compared to the Planck length (Għ/c3)1/2 ≈ 10−33 cm, the length scale on which quantum fluctuations of the metric are expected to be of order unity. This means that the energy density of particles created by the gravitational field is small compared to the space-time curvature. Even though quantum effects may be small locally, they may still, however, add up to produce a significant effect over the lifetime of the Universe ≈ 1017 s which is very long compared to the Planck time ≈ 10−43 s. The purpose of this letter is to show that this indeed may be the case: it seems that any black hole will create and emit particles such as neutrinos or photons at just the rate that one would expect if the black hole was a body with a temperature of (κ/2π) (ħ/2k) ≈ 10−6 (M/M)K where κ is the surface gravity of the black hole1. As a black hole emits this thermal radiation one would expect it to lose mass. This in turn would increase the surface gravity and so increase the rate of emission. The black hole would therefore have a finite life of the order of 1071 (M/M)−3 s. For a black hole of solar mass this is much longer than the age of the Universe. There might, however, be much smaller black holes which were formed by fluctuations in the early Universe2. Any such black hole of mass less than 1015 g would have evaporated by now. Near the end of its life the rate of emission would be very high and about 1030 erg would be released in the last 0.1 s. This is a fairly small explosion by astronomical standards but it is equivalent to about 1 million 1 Mton hydrogen bombs. It is often said that nothing can escape from a black hole. But in 1974, Stephen Hawking realized that, owing to quantum effects, black holes should emit particles with a thermal distribution of energies — as if the black hole had a temperature inversely proportional to its mass. In addition to putting black-hole thermodynamics on a firmer footing, this discovery led Hawking to postulate 'black hole explosions', as primordial black holes end their lives in an accelerating release of energy.

Black Hole Explosions

We report on the completion of the first version of a new-generation simulation code, FLASH. The FLASH code solves the fully compressible, reactive hydrodynamic equations and allows for the use of adaptive mesh refinement. It also contains state-of-the-art modules for the equations of state and thermonuclear reaction networks. The FLASH code was developed to study the problems of nuclear flashes on the surfaces of neutron stars and white dwarfs, as well as in the interior of white dwarfs. We expect, however, that the FLASH code will be useful for solving a wide variety of other problems. This first version of the code has been subjected to a large variety of test cases and is currently being used for production simulations of X-ray bursts, Rayleigh-Taylor and Richtmyer-Meshkov instabilities, and thermonuclear flame fronts. The FLASH code is portable and already runs on a wide variety of massively parallel machines, including some of the largest machines now extant.

https://iopscience.iop.org/article/10.1086/317361/pdf

Flash: An adaptive mesh hydrodynamics code for modeling astrophysical thermonuclear flashes

This is a review of the physics and cosmology of the cosmological constant. Focusing on recent developments, I present a pedagogical overview of cosmology in the presence of a cosmological constant, observational constraints on its magnitude, and the physics of a small (and potentially nonzero) vacuum energy.

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The Cosmological Constant

Using a two-dimensional hydrodynamics code (PROMETHEUS), we explore the continued evolution of rotating helium stars, Mα 10 M☉, in which iron-core collapse does not produce a successful outgoing shock but instead forms a black hole of 2-3 M☉. The model explored in greatest detail is the 14 M☉ helium core of a 35 M☉ main-sequence star. The outcome is sensitive to the angular momentum. For j16 ≡ j/(1016 cm2 s-1) 3, material falls into the black hole almost uninhibited. No outflows are expected. For j16 20, the infalling matter is halted by centrifugal force outside 1000 km where neutrino losses are negligible. The equatorial accretion rate is very low, and explosive oxygen burning may power a weak equatorial explosion. For 3 j16 20, however, a reasonable value for such stars, a compact disk forms at a radius at which the gravitational binding energy can be efficiently radiated as neutrinos or converted to beamed outflow by magnetohydrodynamical (MHD) processes. These are the best candidates for producing gamma-ray bursts (GRBs). Here we study the formation of such a disk, the associated flow patterns, and the accretion rate for disk viscosity parameter α ≈ 0.001 and 0.1. Infall along the rotational axis is initially uninhibited, and an evacuated channel opens during the first few seconds. Meanwhile the black hole is spun up by the accretion (to a ≈ 0.9), and energy is dissipated in the disk by MHD processes and radiated by neutrinos. For the α = 0.1 model, appreciable energetic outflows develop between polar angles of 30° and 45°. These outflows, powered by viscous dissipation in the disk, have an energy of up to a few times 1051 ergs and a mass ~1 M☉ and are rich in 56Ni. They constitute a supernova-like explosion by themselves. Meanwhile accretion through the disk is maintained for approximately 10-20 s but is time variable (±30%) because of hydrodynamical instabilities at the outer edge in a region where nuclei are experiencing photodisintegration. Because the efficiency of neutrino energy deposition is sensitive to the accretion rate, this instability leads to highly variable energy deposition in the polar regions. Some of this variability, which has significant power at 50 ms and overtones, may persist in the time structure of the burst. During the time followed, the average accretion rate for the standard α = 0.1 and j16 = 10 model is 0.07 M☉ s-1. The total energy deposited along the rotational axes by neutrino annihilation is (1-14) × 1051 ergs, depending upon the evolution of the Kerr parameter and uncertain neutrino efficiencies. Simulated deposition of energy in the polar regions, at a constant rate of 5 × 1050 ergs s-1 per pole, results in strong relativistic outflow jets beamed to about 1% of the sky. These jets may be additionally modulated by instabilities in the sides of the "nozzle" through which they flow. The jets blow aside the accreting material, remain highly focused, and are capable of penetrating the star in ~10 s. After the jet breaks through the surface of the star, highly relativistic flow can emerge. Because of the sensitivity of the mass ejection and jets to accretion rate, angular momentum, and disk viscosity, and the variation of observational consequences with viewing angle, a large range of outcomes is possible, ranging from bright GRBs like GRB 971214 to faint GRB-supernovae like SN 1998bw. X-ray precursors are also possible as the jet first breaks out of the star. While only a small fraction of supernovae make GRBs, we predict that collapsars will always make supernovae similar to SN 1998bw. However, hard, energetic GRBs shorter than a few seconds will be difficult to produce in this model and may require merging neutron stars and black holes for their explanation.

https://iopscience.iop.org/article/10.1086/307790/pdf

Collapsars: Gamma-ray bursts and explosions in 'failed supernovae'

The nucleosynthetic yield of isotopes lighter than A = 66 (zinc) is determined for a grid of stellar masses and metallicities including stars of 11, 12, 13, 15, 18, 19, 20, 22, 25, 30, 35, and 40 M{sub {circle_dot}} and metallicities Z = 0, 10{sup {minus}4}, 0.01, 0.1, and 1 times solar (a slightly reduced mass grid is employed for non-solar metallicities). Altogether 78 different model supernova explosions are calculated. In each case nucleosynthesis has already been determined for 200 isotopes in each of 600 to 1200 zones of the presupernova star, including the effects of time dependent convection. Here each star is exploded using a piston to give a specified final kinetic energy at infinity (typically 1.2 {times} 10{sup 51} erg), and the explosive modifications to the nucleosynthesis, including the effects of neutrino irradiation, determined. A single value of the critical {sup 12}C({sub {alpha},{gamma}}){sup 16}O reaction rate corresponding to S(300 keV) = 170 keV barns is used in all calculations. The synthesis of each isotope is discussed along with its sensitivity to model parameters. In each case, the final mass of the collapsed remnant is also determined and often found not to correspond to the location of the pistonmore » (typically the edge of the iron core), but to a ``mass cut`` farther out. This mass cut is sensitive not only to the explosion energy, but also to the presupernova structure, stellar mass, and the metallicity. Unless the explosion mechanism, for unknown reasons, provides a much larger characteristic energy in more massive stars, it appears likely that stars larger than about 30 M{sub {center_dot}} will experience considerable reimplosion of heavy elements following the initial launch of a successful shock. While such explosions will produce a viable, bright Type II supernova light curve, lacking the radioactive tail, they will have dramatically reduced yields of heavy elements and may leave black hole remnants of up to 10 and more solar masses.« less

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The Evolution and Explosion of Massive Stars. II. Explosive Hydrodynamics and Nucleosynthesis

amount of energy, a tiny fraction of which is sufficient to explode the star as a supernova. The authors examine our current understanding of the lives and deaths of massive stars, with special attention to the relevant nuclear and stellar physics. Emphasis is placed upon their post-helium-burning evolution. Current views regarding the supernova explosion mechanism are reviewed, and the hydrodynamics of supernova shock propagation and ‘‘fallback’’ is discussed. The calculated neutron star masses, supernova light curves, and spectra from these model stars are shown to be consistent with observations. During all phases, particular attention is paid to the nucleosynthesis of heavy elements. Such stars are capable of producing, with few exceptions, the isotopes between mass 16 and 88 as well as a large fraction of still heavier elements made by the r and p processes.

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The evolution and explosion of massive stars

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.

Galacti chemical evolution: Hygrogen through zinc

Using the output from a grid of 60 Type II supernova models (Woosley \& Weaver 1994) of varying mass (11 $\ltaprx$ M/M\sun $\ltaprx$ 40) and metallicity (0, 10$^{-4}$, 001, 01, and 1 Z\sol), 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 billion year $e$-folding time scale onto a exponential disk and 1/r$^2$ 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 -30 $\ltaprx$ [Fe/H] $\ltaprx$ 00 dex While our discussion focuses on the solar neighborhood where there are the most observations, the supernovae rates, an intrinsically Galactic quantity, are also discussed

Galactic Chemical Evolution: Hydrogen Through Zinc

Embodiments include a surgical device and a method. An embodiment of the surgical instrument includes at least one grasping jaw, the at least one grasping jaw being adapted to deliver surgical staples by a force generated from a force generator mechanism that is contained within the at least one grasping jaw or is in a proximity to the at least one grasping jaw. Another embodiment includes at least one grasping jaw, at least one delivery mechanism adapted to deliver surgical fasteners, the delivery mechanism being located in a proximity to or contained within the at least one grasping jaw, the surgical fasteners containing at least one shape-transforming material, at least one sensor, at least one chemical tissue sealant and at least one cutter. A method includes: grasping a body organ/tissue with at least one grasping jaw, adjusting a configuration of the grasping in response to a signal or a datum or an image, and releasing a surgical staple/fastener in response to the signal, datum or image.

Thomas A. Weaver

Papers

The Evolution and Explosion of Massive Stars. II. Explosive Hydrodynamics and Nucleosynthesis

The evolution and explosion of massive stars

Galacti chemical evolution: Hygrogen through zinc

Galactic Chemical Evolution: Hydrogen Through Zinc

Steerable surgical stapler