142 research outputs found
Numerical Simulations of Turbulent Molecular Clouds Regulated by Reprocessed Radiation Feedback from Nascent Super Star Clusters
Radiation feedback from young star clusters embedded in giant molecular
clouds (GMCs) is believed to be important to the control of star formation. For
the most massive and dense clouds, including those in which super star clusters
(SSCs) are born, pressure from reprocessed radiation exerted on dust grains may
disperse a significant portion of the cloud mass back into the interstellar
medium (ISM). Using our radiaton hydrodynamics (RHD) code, Hyperion, we conduct
a series of numerical simulations to test this idea. Our models follow the
evolution of self-gravitating, strongly turbulent clouds in which collapsing
regions are replaced by radiating sink particles representing stellar clusters.
We evaluate the dependence of the star formation efficiency (SFE) on the size
and mass of the cloud and , the opacity of the gas to infrared (IR)
radiation. We find that the single most important parameter determining the
evolutionary outcome is , with needed to disrupt clouds. For , the resulting SFE=50-70% is similar to empirical estimates for some
SSC-forming clouds. The opacities required for GMC disruption likely apply only
in dust-enriched environments. We find that the subgrid model approach of
boosting the direct radiation force by a "trapping factor" equal to a
cloud's mean IR optical depth can overestimate the true radiation force by
factors of . We conclude that feedback from reprocessed IR radiation
alone is unlikely to significantly reduce star formation within GMCs unless
their dust abundances or cluster light-to-mass ratios are enhanced.Comment: 19 pages, 18 figures, accepted for publication in Ap
A Two-moment Radiation Hydrodynamics Module in Athena Using a Time-explicit Godunov Method
We describe a module for the Athena code that solves the gray equations of
radiation hydrodynamics (RHD), based on the first two moments of the radiative
transfer equation. We use a combination of explicit Godunov methods to advance
the gas and radiation variables including the non-stiff source terms, and a
local implicit method to integrate the stiff source terms. We adopt the M1
closure relation and include all leading source terms. We employ the reduced
speed of light approximation (RSLA) with subcycling of the radiation variables
in order to reduce computational costs. Our code is dimensionally unsplit in
one, two, and three space dimensions and is parallelized using MPI. The
streaming and diffusion limits are well-described by the M1 closure model, and
our implementation shows excellent behavior for a problem with a concentrated
radiation source containing both regimes simultaneously. Our operator-split
method is ideally suited for problems with a slowly varying radiation field and
dynamical gas flows, in which the effect of the RSLA is minimal. We present an
analysis of the dispersion relation of RHD linear waves highlighting the
conditions of applicability for the RSLA. To demonstrate the accuracy of our
method, we utilize a suite of radiation and RHD tests covering a broad range of
regimes, including RHD waves, shocks, and equilibria, which show second-order
convergence in most cases. As an application, we investigate radiation-driven
ejection of a dusty, optically thick shell in the interstellar medium (ISM).
Finally, we compare the timing of our method with other well-known iterative
schemes for the RHD equations. Our code implementation, Hyperion, is suitable
for a wide variety of astrophysical applications and will be made freely
available on the Web.Comment: 30 pages, 29 figures, accepted for publication in ApJ
Should One Use the Ray-by-Ray Approximation in Core-Collapse Supernova Simulations?
We perform the first self-consistent, time-dependent, multi-group
calculations in two dimensions (2D) to address the consequences of using the
ray-by-ray+ transport simplification in core-collapse supernova simulations.
Such a dimensional reduction is employed by many researchers to facilitate
their resource-intensive calculations. Our new code (F{\sc{ornax}}) implements
multi-D transport, and can, by zeroing out transverse flux terms, emulate the
ray-by-ray+ scheme. Using the same microphysics, initial models, resolution,
and code, we compare the results of simulating 12-, 15-, 20-, and
25-M progenitor models using these two transport methods. Our
findings call into question the wisdom of the pervasive use of the ray-by-ray+
approach. Employing it leads to maximum post-bounce/pre-explosion shock radii
that are almost universally larger by tens of kilometers than those derived
using the more accurate scheme, typically leaving the post-bounce matter less
bound and artificially more "explodable." In fact, for our 25-M
progenitor, the ray-by-ray+ model explodes, while the corresponding multi-D
transport model does not. Therefore, in two dimensions the combination of
ray-by-ray+ with the axial sloshing hydrodynamics that is a feature of 2D
supernova dynamics can result in quantitatively, and perhaps qualitatively,
incorrect results.Comment: Updated and revised text; 13 pages; 13 figures; Accepted to Ap.
Numerical Simulations of Turbulent Molecular Clouds Regulated by Radiation Feedback Forces II: Radiation-Gas Interactions and Outflows
Momentum deposition by radiation pressure from young, massive stars may help
to destroy molecular clouds and unbind stellar clusters by driving large-scale
outflows. We extend our previous numerical radiation hydrodynamic study of
turbulent, star-forming clouds to analyze the detailed interaction between
non-ionizing UV radiation and the cloud material. Our simulations trace the
evolution of gas and star particles through self-gravitating collapse, star
formation, and cloud destruction via radiation-driven outflows. These models
are idealized in that we include only radiation feedback and adopt an
isothermal equation of state. Turbulence creates a structure of dense filaments
and large holes through which radiation escapes, such that only ~50% of the
radiation is (cumulatively) absorbed by the end of star formation. The surface
density distribution of gas by mass as seen by the central cluster is roughly
lognormal with sigma_ln(Sigma) = 1.3-1.7, similar to the externally-projected
surface density distribution. This allows low surface density regions to be
driven outwards to nearly 10 times their initial escape speed v_esc. Although
the velocity distribution of outflows is broadened by the lognormal surface
density distribution, the overall efficiency of momentum injection to the gas
cloud is reduced because much of the radiation escapes. The mean outflow
velocity is approximately twice the escape speed from the initial cloud radius.
Our results are also informative for understanding galactic-scale wind driving
by radiation, in particular the relationship between velocity and surface
density for individual outflow structures, and the resulting velocity and mass
distributions arising from turbulent sources.Comment: ApJ, in press (28 pages, 14 figures
Neutrino Signals of Core-Collapse Supernovae in Underground Detectors
For a suite of fourteen core-collapse models during the dynamical first
second after bounce, we calculate the detailed neutrino "light" curves expected
in the underground neutrino observatories Super-Kamiokande, DUNE, JUNO, and
IceCube. These results are given as a function of neutrino-oscillation modality
(normal or inverted hierarchy) and progenitor mass (specifically, post-bounce
accretion history), and illuminate the differences between the light curves for
1D (spherical) models that don't explode with the corresponding 2D
(axisymmetric) models that do. We are able to identify clear signatures of
explosion (or non-explosion), the post-bounce accretion phase, and the
accretion of the silicon/oxygen interface. In addition, we are able to estimate
the supernova detection ranges for various physical diagnostics and the
distances out to which various temporal features embedded in the light curves
might be discerned. We find that the progenitor mass density profile and
supernova dynamics during the dynamical explosion stage should be identifiable
for a supernova throughout most of the galaxy in all the facilities studied and
that detection by any one of them, but in particular more than one in concert,
will speak volumes about the internal dynamics of supernovae.Comment: Accepted to Monthly Notices of the Royal Astronomical Societ
Fornax: a Flexible Code for Multiphysics Astrophysical Simulations
This paper describes the design and implementation of our new multi-group,
multi-dimensional radiation hydrodynamics (RHD) code Fornax and provides a
suite of code tests to validate its application in a wide range of physical
regimes. Instead of focusing exclusively on tests of neutrino radiation
hydrodynamics relevant to the core-collapse supernova problem for which Fornax
is primarily intended, we present here classical and rigorous demonstrations of
code performance relevant to a broad range of multi-dimensional hydrodynamic
and multi-group radiation hydrodynamic problems. Our code solves the
comoving-frame radiation moment equations using the M1 closure, utilizes
conservative high-order reconstruction, employs semi-explicit matter and
radiation transport via a high-order time stepping scheme, and is suitable for
application to a wide range of astrophysical problems. To this end, we first
describe the philosophy, algorithms, and methodologies of Fornax and then
perform numerous stringent code tests, that collectively and vigorously
exercise the code, demonstrate the excellent numerical fidelity with which it
captures the many physical effects of radiation hydrodynamics, and show
excellent strong scaling well above 100k MPI tasks.Comment: Accepted to the Astrophysical Journal Supplement Series; A few more
textual and reference updates; As before, one additional code test include
Modeling UV Radiation Feedback from Massive Stars: I. Implementation of Adaptive Ray Tracing Method and Tests
We present an implementation of an adaptive ray tracing (ART) module in the
Athena hydrodynamics code that accurately and efficiently handles the radiative
transfer involving multiple point sources on a three-dimensional Cartesian
grid. We adopt a recently proposed parallel algorithm that uses non-blocking,
asynchronous MPI communications to accelerate transport of rays across the
computational domain. We validate our implementation through several standard
test problems including the propagation of radiation in vacuum and the
expansions of various types of HII regions. Additionally, scaling tests show
that the cost of a full ray trace per source remains comparable to that of the
hydrodynamics update on up to processors. To demonstrate
application of our ART implementation, we perform a simulation of star cluster
formation in a marginally bound, turbulent cloud, finding that its star
formation efficiency is when both radiation pressure forces and
photoionization by UV radiation are treated. We directly compare the radiation
forces computed from the ART scheme with that from the M1 closure relation.
Although the ART and M1 schemes yield similar results on large scales, the
latter is unable to resolve the radiation field accurately near individual
point sources.Comment: 20 pages, 14 figures; accepted for publication in Ap
Electron-Capture and Low-Mass Iron-Core-Collapse Supernovae: New Neutrino-Radiation-Hydrodynamics Simulations
We present new 1D (spherical) and 2D (axisymmetric) simulations of
electron-capture (EC) and low-mass iron-core-collapse supernovae (SN). We
consider six progenitor models: the ECSN progenitor from Nomoto (1984, 1987);
two ECSN-like low-mass low-metallicity iron core progenitors from Heger
(private communication); and the 9-, 10-, and 11- (zero-age main
sequence) progenitors from Sukhbold et al. (2016). We confirm that the ECSN and
ESCN-like progenitors explode easily even in 1D with explosion energies of up
to a 0.15 Bethes (), and are a viable
mechanism for the production of very low-mass neutron stars. However, the 9-,
10-, and 11- progenitors do not explode in 1D and are not even
necessarily easier to explode than higher-mass progenitor stars in 2D. We study
the effect of perturbations and of changes to the microphysics and we find that
relatively small changes can result in qualitatively different outcomes, even
in 1D, for models sufficiently close to the explosion threshold. Finally, we
revisit the impact of convection below the protoneutron star (PNS) surface. We
analyze, 1D and 2D evolutions of PNSs subject to the same boundary conditions.
We find that the impact of PNS convection has been underestimated in previous
studies and could result in an increase of the neutrino luminosity by up to
factors of two.Comment: 18 pages, 17 figures, 3 tables. Major revisions following a fix in
the code input physics. Accepted on Ap
The Athena Astrophysical MHD Code in Cylindrical Geometry
A method for implementing cylindrical coordinates in the Athena
magnetohydrodynamics (MHD) code is described. The extension follows the
approach of Athena's original developers and has been designed to alter the
existing Cartesian-coordinates code as minimally and transparently as possible.
The numerical equations in cylindrical coordinates are formulated to maintain
consistency with constrained transport, a central feature of the Athena
algorithm, while making use of previously implemented code modules such as the
Riemann solvers. Angular-momentum transport, which is critical in astrophysical
disk systems dominated by rotation, is treated carefully. We describe
modifications for cylindrical coordinates of the higher-order spatial
reconstruction and characteristic evolution steps as well as the finite-volume
and constrained transport updates. Finally, we present a test suite of standard
and novel problems in one-, two-, and three-dimensions designed to validate our
algorithms and implementation and to be of use to other code developers. The
code is suitable for use in a wide variety of astrophysical applications and is
freely available for download on the web
Revival of the Fittest: Exploding Core-Collapse Supernovae from 12 to 25 M
We present results of 2D axisymmetric core-collapse supernova simulations,
employing the FORNAX code, of nine progenitor models spanning 12 to 25
M and evolved over a 20,000-km grid. We find that four of the nine
models explode with inelastic scattering off electrons and neutrons as well as
the many-body correction to neutrino-nucleon scattering opacities. We show that
these four models feature sharp Si-O interfaces in their density profiles, and
that the corresponding dip in density reduces the accretion rate around the
stalled shock and prompts explosion. The non-exploding models lack such a steep
feature, suggesting that Si-O interface is one key to explosion. Furthermore,
we show that all of the non-exploding models can be nudged to explosion with
modest changes to macrophysical inputs, including moderate rotation and
perturbations to infall velocities, as well as to microphysical inputs,
including changes to neutrino-nucleon interaction rates, suggesting that all
the models are perhaps close to criticality. Exploding models have energies of
few 10 ergs at the end of our simulation, and are rising,
suggesting the need to continue these simulations over larger grids and for
longer times to reproduce the energies seen in Nature. We find that the
morphology of the explosion contributes to the explosion energy, with more
isotropic ejecta producing larger explosion energies. However, we do not find
evidence for the Lepton-number Emission Self-Sustained Asymmetry. Finally, we
look at PNS properties and explore the role of dimension in our simulations. We
find that convection in the proto-neutron star (PNS) produces larger PNS radii
as well as greater "" luminosities in 2D compared to 1D.Comment: accepted by MNRAS March 201
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