439 research outputs found
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
Crucial Physical Dependencies of the Core-Collapse Supernova Mechanism
We explore with self-consistent 2D F{\sc{ornax}} simulations the dependence
of the outcome of collapse on many-body corrections to neutrino-nucleon cross
sections, the nucleon-nucleon bremsstrahlung rate, electron capture on heavy
nuclei, pre-collapse seed perturbations, and inelastic neutrino-electron and
neutrino-nucleon scattering. Importantly, proximity to criticality amplifies
the role of even small changes in the neutrino-matter couplings, and such
changes can together add to produce outsized effects. When close to the
critical condition the cumulative result of a few small effects (including
seeds) that individually have only modest consequence can convert an anemic
into a robust explosion, or even a dud into a blast. Such sensitivity is not
seen in one dimension and may explain the apparent heterogeneity in the
outcomes of detailed simulations performed internationally. A natural
conclusion is that the different groups collectively are closer to a realistic
understanding of the mechanism of core-collapse supernovae than might have
seemed apparent.Comment: 25 pages; 10 figure
The Spin Periods and Rotational Profiles of Neutron Stars at Birth
We present results from an extensive set of one- and two-dimensional
radiation-hydrodynamic simulations of the supernova core collapse, bounce, and
postbounce phases, and focus on the protoneutron star (PNS) spin periods and
rotational profiles as a function of initial iron core angular velocity, degree
of differential rotation, and progenitor mass. For the models considered, we
find a roughly linear mapping between initial iron core rotation rate and PNS
spin. The results indicate that the magnitude of the precollapse iron core
angular velocities is the single most important factor in determining the PNS
spin. Differences in progenitor mass and degree of differential rotation lead
only to small variations in the PNS rotational period and profile. Based on our
calculated PNS spins, at ~ 200-300 milliseconds after bounce, and assuming
angular momentum conservation, we estimate final neutron star rotation periods.
We find periods of one millisecond and shorter for initial central iron core
periods of below ~ 10 s. This is appreciably shorter than what previous studies
have predicted and is in disagreement with current observational data from
pulsar astronomy. After considering possible spindown mechanisms that could
lead to longer periods we conclude that there is no mechanism that can robustly
spin down a neutron star from ~ 1 ms periods to the "injection" periods of tens
to hundreds of milliseconds observed for young pulsars. Our results indicate
that, given current knowledge of the limitations of neutron star spindown
mechanisms, precollapse iron cores must rotate with periods around 50-100
seconds to form neutron stars with periods generically near those inferred for
the radio pulsar population.Comment: 31 pages, including 20 color figures. High-resolution figures
available from the authors upon request. Accepted to Ap
Mu and Tau Neutrino Thermalization and Production in Supernovae: Processes and Timescales
We investigate the rates of production and thermalization of and
neutrinos at temperatures and densities relevant to core-collapse
supernovae and protoneutron stars. Included are contributions from electron
scattering, electron-positron annihilation, nucleon-nucleon bremsstrahlung, and
nucleon scattering. For the scattering processes, in order to incorporate the
full scattering kinematics at arbitrary degeneracy, the structure function
formalism developed by Reddy et al. (1998) and Burrows and Sawyer (1998) is
employed. Furthermore, we derive formulae for the total and differential rates
of nucleon-nucleon bremsstrahlung for arbitrary nucleon degeneracy in
asymmetric matter. We find that electron scattering dominates nucleon
scattering as a thermalization process at low neutrino energies
( MeV), but that nucleon scattering is always faster
than or comparable to electron scattering above MeV. In
addition, for g cm, MeV, and
neutrino energies MeV, nucleon-nucleon bremsstrahlung always
dominates electron-positron annihilation as a production mechanism for
and neutrinos.Comment: 29 pages, LaTeX (RevTeX), 13 figures, submitted to Phys. Rev. C. Also
to be found at anonymous ftp site http://www.astrophysics.arizona.edu; cd to
pub/thompso
A New Algorithm for Supernova Neutrino Transport and Some Applications
We have developed an implicit, multi-group, time-dependent, spherical
neutrino transport code based on the Feautrier variables, the tangent-ray
method, and accelerated iteration. The code achieves high
angular resolution, is good to O(), is equivalent to a Boltzmann solver
(without gravitational redshifts), and solves the transport equation at all
optical depths with precision. In this paper, we present our formulation of the
relevant numerics and microphysics and explore protoneutron star atmospheres
for snapshot post-bounce models. Our major focus is on spectra, neutrino-matter
heating rates, Eddington factors, angular distributions, and phase-space
occupancies. In addition, we investigate the influence on neutrino spectra and
heating of final-state electron blocking, stimulated absorption, velocity terms
in the transport equation, neutrino-nucleon scattering asymmetry, and weak
magnetism and recoil effects. Furthermore, we compare the emergent spectra and
heating rates obtained using full transport with those obtained using
representative flux-limited transport formulations to gauge their accuracy and
viability. Finally, we derive useful formulae for the neutrino source strength
due to nucleon-nucleon bremsstrahlung and determine bremsstrahlung's influence
on the emergent and neutrino spectra.Comment: 58 pages, single-spaced LaTeX, 23 figures, revised title, also
available at http://jupiter.as.arizona.edu/~burrows/papers, accepted for
publication in the Ap.
Equation-of-State Dependent Features in Shock-Oscillation Modulated Neutrino and Gravitational-Wave Signals from Supernovae
We present 2D hydrodynamic simulations of the long-time accretion phase of a
15 solar mass star after core bounce and before the launch of a supernova
explosion. Our simulations are performed with the Prometheus-Vertex code,
employing multi-flavor, energy-dependent neutrino transport and an effective
relativistic gravitational potential. Testing the influence of a stiff and a
soft equation of state for hot neutron star matter, we find that the non-radial
mass motions in the supernova core due to the standing accretion shock
instability (SASI) and convection impose a time variability on the neutrino and
gravitational-wave signals. These variations have larger amplitudes as well as
higher frequencies in the case of a more compact nascent neutron star. After
the prompt shock-breakout burst of electron neutrinos, a more compact accreting
remnant radiates neutrinos with higher luminosities and larger mean energies.
The observable neutrino emission in the direction of SASI shock oscillations
exhibits a modulation of several 10% in the luminosities and ~1 MeV in the mean
energies with most power at typical SASI frequencies of 20-100 Hz. At times
later than 50-100 ms after bounce the gravitational-wave amplitude is dominated
by the growing low-frequency (<200 Hz) signal associated with anisotropic
neutrino emission. A high-frequency wave signal is caused by nonradial gas
flows in the outer neutron star layers, which are stirred by anisotropic
accretion from the SASI and convective regions. The gravitational-wave power
then peaks at about 300-800 Hz with distinctively higher spectral frequencies
originating from the more compact and more rapidly contracting neutron star.
The detectability of the SASI effects in the neutrino and gravitational-wave
signals is briefly discussed. (abridged)Comment: 21 pages, 11 figures, 45 eps files; revised version including
discussion of signal detectability; accepted by Astronomy & Astrophysics;
high-resolution images can be obtained upon reques
Theortetical Models of Extrasolar Giant Planets
The recent discoveries of giant planets around nearby stars have galvanized
the planetary science community, astronomers, and the public at large. Since
{\it direct} detection is now feasible, and is suggested by the recent
acquisition of Gl229 B, it is crucial for the future of extrasolar planet
searches that the fluxes, evolution, and physical structure of objects from
Saturn's mass to 15 Juipter masses be theoretically investigated. We discuss
our first attempts to explore the characteristics of extrasolar giant planets
(EGPs), in aid of both NASA's and ESA's recent plans to search for such planets
around nearby stars.Comment: LaTeX, using espcrc2.sty style files from Elsevier, 10 pages, 4
figures, to be published in the Proceedings of the International Conference
on "Sources and Detection of Dark Matter in the Universe," ed. by D. Sanders
et al. (Nuclear Physics B Supplement), 199
Physics of Neutron Star Kicks
It is no longer necessary to `sell' the idea of pulsar kicks, the notion that
neutron stars receive a large velocity (a few hundred to a thousand km
s) at birth. However, the origin of the kicks remains mysterious. We
review the physics of different kick mechanisms, including hydrodynamically
driven, neutrino and magnetically driven kicks.Comment: 8 pages including 1 figure. To be published in "Stellar Astrophysics"
(Pacific Rim Conference Proceedings), (Kluwer Pub.
A New Open-Source Code for Spherically-Symmetric Stellar Collapse to Neutron Stars and Black Holes
We present the new open-source spherically-symmetric general-relativistic
(GR) hydrodynamics code GR1D. It is based on the Eulerian formulation of GR
hydrodynamics (GRHD) put forth by Romero-Ibanez-Gourgoulhon and employs
radial-gauge, polar-slicing coordinates in which the 3+1 equations simplify
substantially. We discretize the GRHD equations with a finite-volume scheme,
employing piecewise-parabolic reconstruction and an approximate Riemann solver.
GR1D is intended for the simulation of stellar collapse to neutron stars and
black holes and will also serve as a testbed for modeling technology to be
incorporated in multi-D GR codes. Its GRHD part is coupled to various
finite-temperature microphysical equations of state in tabulated form that we
make available with GR1D. An approximate deleptonization scheme for the
collapse phase and a neutrino-leakage/heating scheme for the postbounce epoch
are included and described. We also derive the equations for effective rotation
in 1D and implement them in GR1D. We present an array of standard test
calculations and also show how simple analytic equations of state in
combination with presupernova models from stellar evolutionary calculations can
be used to study qualitative aspects of black hole formation in failing
rotating core-collapse supernovae. In addition, we present a simulation with
microphysical EOS and neutrino leakage/heating of a failing core-collapse
supernova and black hole formation in a presupernova model of a 40 solar mass
zero-age main-sequence star. We find good agreement on the time of black hole
formation (within 20%) and last stable protoneutron star mass (within 10%) with
predictions from simulations with full Boltzmann neutrino radiation
hydrodynamics.Comment: 25 pages, 6 figures, 2 appendices. Accepted for publication to the
Classical and Quantum Gravity special issue for MICRA2009. Code may be
downloaded from http://www.stellarcollapse.org Update: corrected title, small
modifications suggested by the referees, added source term derivation in
appendix
Magnetar Driven Bubbles and the Origin of Collimated Outflows in Gamma-ray Bursts
We model the interaction between the wind from a newly formed rapidly
rotating magnetar and the surrounding supernova shock and host star. The
dynamics is modeled using the two-dimensional, axisymmetric thin-shell
equations. In the first ~10-100 seconds after core collapse the magnetar
inflates a bubble of plasma and magnetic fields behind the supernova shock. The
bubble expands asymmetrically because of the pinching effect of the toroidal
magnetic field, just as in the analogous problem of the evolution of pulsar
wind nebulae. The degree of asymmetry depends on E_mag/E_tot. The correct value
of E_mag/E_tot is uncertain because of uncertainties in the conversion of
magnetic energy into kinetic energy at large radii in relativistic winds; we
argue, however, that bubbles inflated by newly formed magnetars are likely to
be significantly more magnetized than their pulsar counterparts. We show that
for a ratio of magnetic to total power supplied by the central magnetar
L_mag/L_tot ~ 0.1 the bubble expands relatively spherically. For L_mag/L_tot ~
0.3, however, most of the pressure in the bubble is exerted close to the
rotation axis, driving a collimated outflow out through the host star. This can
account for the collimation inferred from observations of long-duration
gamma-ray bursts (GRBs). Outflows from magnetars become increasingly
magnetically dominated at late times, due to the decrease in neutrino-driven
mass loss as the young neutron star cools. We thus suggest that the
magnetar-driven bubble initially expands relatively spherically, enhancing the
energy of the associated supernova, while at late times it becomes
progressively more collimated, producing the GRB.Comment: 14 pages, 8 figures, accepted for publication in MNRA
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