640 research outputs found
Delayed Detonation at a Single Point in Exploding White Dwarfs
Delayed detonation in an exploding white dwarf, which propagates from an
off-center transition point, rather than from a spherical transition shell, is
described and simulated. The differences between the results of 2D simulations
and the 1D case are presented and discussed. The two dimensional effects become
significant in transition density below 3.e7 g/cm^3, where the energetics, the
production of Fe group elements and the symmetry of the explosion are all
affected. In the 2D case the explosion is less energetic and less Ni is
produced in the detonation phase of the explosion. For low transition density
the reduction in Ni mass can reach 20-30 percent. The asymmetry in abundances
between regions close to the transition point and regions far from that point
is large, and could be a source to polarization patterns in the emitted light.
We conclude that the spatial and temporal distribution of transition locations,
is an important parameter which must be included in delayed detonation models
for Type Ia supernovae. \Comment: 11 pages, 1 figur
Incentive Arbitration and Time-Related Bargaining Costs
In the context of the Nash Bargaining Problem an arbitrator is supposed to choose a solution point on the basis of âfairnessâ requirements. By introducing time-related costs to the partiesâ utility functions, a new class of arbitration changes the rules of the negotiations, and particularly the partiesâ payoďŹs, and lets the parties negotiate. The changes are such that one or both parties are reimbursed by the arbitrator for their bargaining costs. A procedure in which only the party who makes the ďŹnal concession is compensated, leads (in the case of complete information) to single dominating equilibrium strategies and to a unique solution. This solution can also serve as an arbitration point in the traditional sense
An Extension of the Nash Bargaining Problem: Introducing Time-Related Bargaining Costs
The Nash Bargaining is reformulated by introducing time-related costs into the von Neumann-Morgenstern utility functions of the parties. A set of mathematical requirements on the solution is satisďŹed uniquely by a function called the Extended RaiďŹa Solution. The properties of this function are investigated. It is shown to coincide with the Nash Cooperative Solution (of a related Bargaining Problem) when the parties have identical discount rates. An interpretation of the solution and of the formal requirements is detailed. In this model the âbargaining powerâ of the parties is determined by the bargaining costs related to the duration of the negotiations
The Proto-neutron Star Phase of the Collapsar Model and the Route to Long-soft Gamma-ray Bursts and Hypernovae
Recent stellar evolutionary calculations of low-metallicity massive
fast-rotating main-sequence stars yield iron cores at collapse endowed with
high angular momentum. It is thought that high angular momentum and black hole
formation are critical ingredients of the collapsar model of long-soft
gamma-ray bursts (GRBs). Here, we present 2D multi-group,
flux-limited-diffusion MHD simulations of the collapse, bounce, and immediate
post-bounce phases of a 35-Msun collapsar-candidate model of Woosley & Heger.
We find that, provided the magneto-rotational instability (MRI) operates in the
differentially-rotating surface layers of the millisecond-period neutron star,
a magnetically-driven explosion ensues during the proto-neutron star phase, in
the form of a baryon-loaded non-relativistic jet, and that a black hole,
central to the collapsar model, does not form. Paradoxically, and although much
uncertainty surrounds stellar mass loss, angular momentum transport, magnetic
fields, and the MRI, current models of chemically homogeneous evolution at low
metallicity yield massive stars with iron cores that may have too much angular
momentum to avoid a magnetically-driven, hypernova-like, explosion in the
immediate post-bounce phase. We surmise that fast rotation in the iron core may
inhibit, rather than enable, collapsar formation, which requires a large
angular momentum not in the core but above it. Variations in the angular
momentum distribution of massive stars at core collapse might explain both the
diversity of Type Ic supernovae/hypernovae and their possible association with
a GRB. A corollary might be that, rather than the progenitor mass, the angular
momentum distribution, through its effect on magnetic field amplification,
distinguishes these outcomes.Comment: 5 pages, 1 table, 2 figures, accepted to ApJ
The Sensitivity of Multidimensional Nova Calculations to the Outer Boundary Conditions
Multidimensional reactive flow models of accreted hydrogen rich envelopes on
top of degenerate cold white dwarfs are very effective tools for the study of
critical, non spherically symmetric, behaviors during the early stages of nova
outbursts. Such models can shed light both on the mechanism responsible for the
heavy element enrichment observed to characterize nova envelope matter and on
the role of perturbations during the early stages of ignition of the runaway.
The complexity of convective reactive flow in multi-dimensions makes the
computational model itself complex and sensitive to the details of the
numerics. In this study, we demonstrate that the imposed outer boundary
condition can have a dramatic effect on the solution. Several commonly used
choices for the outer boundary conditions are examined. It is shown that the
solutions obtained from Lagrangian simulations, where the envelope is allowed
to expand and mass is being conserved, are consistent with spherically
symmetric solutions. In Eulerian schemes which utilize an outer boundary
condition of free outflow, the outburst can be artificially quenched.Comment: 12 Pages 3 figures; Accepted for publication in the Astrophysical
Journa
Neutrino Signatures and the Neutrino-Driven Wind in Binary Neutron Star Mergers
We present VULCAN/2D multigroup flux-limited-diffusion radiation-hydrodynamics simulations of binary neutron star mergers, using the Shen equation of state, covering âł 100 ms, and starting from azimuthal-averaged two-dimensional slices obtained from three-dimensional smooth-particle-hydrodynamics simulations of Rosswog & Price for 1.4Mâ (baryonic) neutron stars with no initial spins, co-rotating spins, or counter-rotating spins. Snapshots are post-processed at 10 ms intervals with a multiangle neutrino-transport solver. We find polar-enhanced neutrino luminosities, dominated by ¯νe and âνΟâ neutrinos at the peak, although νe emission may be stronger at late times. We obtain typical peak neutrino energies for νe, ¯νe, and âνΟâ of âź12, âź16, and âź22 MeV, respectively. The supermassive neutron star (SMNS) formed from the merger has a cooling timescale of âž 1 s. Charge-current neutrino reactions lead to the formation of a thermally driven bipolar wind with (M¡) âź 10^â3 Mâ s^â1 and baryon-loading in the polar regions, preventing any production of a Îł-ray burst prior to black hole formation. The large budget of rotational free energy suggests that magneto-rotational effects could produce a much-greater polar mass loss. We estimate that âž 10^â4 Mâ of material with an electron fraction in the range 0.1â0.2 becomes unbound during this SMNS phase as a result of neutrino heating. We present a new formalism to compute the νi ¯νi annihilation rate based on moments of the neutrino-specific intensity computed with our multiangle solver. Cumulative annihilation rates, which decay as âźt^â1.8, decrease over our 100 ms window from a few Ă1050 to âź 1049 erg sâ1, equivalent to a few Ă10^54 to âź10^53 eâe+ pairs per second
Asymmetry and the Nucleosynthetic Signature of Nearly Edge-Lit Detonation in White Dwarf Cores
Most of the leading explosion scenarios for Type Ia supernovae involve the
nuclear incineration of a white dwarf star through a detonation wave. Several
scenarios have been proposed as to how this detonation may actually occur, but
the exact mechanism and environment in which it takes place remain unknown. We
explore the effects of an off-center initiated detonation on the spatial
distribution of the nucleosynthetic yield products in a toy model -- a
pre-expanded near Chandrasekhar-mass white dwarf. We find that a single-point
near edge-lit detonation results in asymmetries in the density and thermal
profiles, notably the expansion timescale, throughout the supernova ejecta. We
demonstrate that this asymmetry of the thermodynamic trajectories should be
common to off-center detonations where a small amount of the star is burned
prior to detonation. The sensitivity of the yields on the expansion timescale
results in an asymmetric distribution of the elements synthesized as reaction
products. We tabulate the shift in the center of mass of the various elements
produced in our model supernova and find an odd-even pattern for elements past
silicon. Our calculations show that off-center single-point detonations in
carbon-oxygen white dwarfs are marked by significant composition asymmetries in
their remnants which bear potentially observable signatures in both velocity
and coordinate space, including an elemental nickel mass fraction which varies
by a factor of two to three from one side of the remnant to the other.Comment: 7 pages, 7 figures, accepted for publication in the Astrophysical
Journa
Two-dimensional, Time-dependent, Multi-group, Multi-angle Radiation Hydrodynamics Test Simulation in the Core-Collapse Supernova Context
We have developed a time-dependent, multi-energy-group, and multi-angle
(S) Boltzmann transport scheme for radiation hydrodynamics simulations, in
one and two spatial dimensions. The implicit transport is coupled to both 1D
(spherically-symmetric) and 2D (axially-symmetric) versions of the explicit
Newtonian hydrodynamics code VULCAN. The 2D variant, VULCAN/2D, can be operated
in general structured or unstructured grids and though the code can address
many problems in astrophysics it was constructed specifically to study the
core-collapse supernova problem. Furthermore, VULCAN/2D can simulate the
radiation/hydrodynamic evolution of differentially rotating bodies. We
summarize the equations solved and methods incorporated into the algorithm and
present results of a time-dependent 2D test calculation. A more complete
description of the algorithm is postponed to another paper. We highlight a 2D
test run that follows for 22 milliseconds the immediate post-bounce evolution
of a collapsed core. We present the relationship between the anisotropies of
the overturning matter field and the distribution of the corresponding flux
vectors, as a function of energy group. This is the first 2D multi-group,
multi-angle, time-dependent radiation/hydro calculation ever performed in core
collapse studies. Though the transport module of the code is not gray and does
not use flux limiters (however, there is a flux-limited variant of VULCAN/2D),
it still does not include energy redistribution and most velocity-dependent
terms.Comment: 19 pages, plus 13 figures in JPEG format. Submitted to the
Astrophysical Journa
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