258 research outputs found
Fallback and Black Hole Production in Massive Stars
The compact remnants of core collapse supernovae - neutron stars and black
holes - have properties that reflect both the structure of their stellar
progenitors and the physics of the explosion. In particular, the masses of
these remnants are sensitive to the density structure of the presupernova star
and to the explosion energy. To a considerable extent, the final mass is
determined by the ``fallback'', during the explosion, of matter that initially
moves outwards, yet ultimately fails to escape. We consider here the simulated
explosion of a large number of massive stars (10 to 100 \Msun) of Population I
(solar metallicity) and III (zero metallicity), and find systematic differences
in the remnant mass distributions. As pointed out by Chevalier(1989),
supernovae in more compact progenitor stars have stronger reverse shocks and
experience more fallback. For Population III stars above about 25 \Msun and
explosion energies less than erg, black holes are a common
outcome, with masses that increase monotonically with increasing main sequence
mass up to a maximum hole mass of about 35 \Msun. If such stars produce primary
nitrogen, however, their black holes are systematically smaller. For modern
supernovae with nearly solar metallicity, black hole production is much less
frequent and the typical masses, which depend sensitively on explosion energy,
are smaller. We explore the neutron star initial mass function for both
populations and, for reasonable assumptions about the initial mass cut of the
explosion, find good agreement with the average of observed masses of neutron
stars in binaries. We also find evidence for a bimodal distribution of neutron
star masses with a spike around 1.2 \Msun (gravitational mass) and a broader
distribution peaked around 1.4 \Msun.Comment: Accepted for publication in Ap
Two-Dimensional Core-Collapse Supernova Models with Multi-Dimensional Transport
We present new two-dimensional (2D) axisymmetric neutrino
radiation/hydrodynamic models of core-collapse supernova (CCSN) cores. We use
the CASTRO code, which incorporates truly multi-dimensional, multi-group,
flux-limited diffusion (MGFLD) neutrino transport, including all relevant
terms. Our main motivation for carrying out this study is to
compare with recent 2D models produced by other groups who have obtained
explosions for some progenitor stars and with recent 2D VULCAN results that did
not incorporate terms. We follow the evolution of 12, 15,
20, and 25 solar-mass progenitors to approximately 600 milliseconds after
bounce and do not obtain an explosion in any of these models. Though the reason
for the qualitative disagreement among the groups engaged in CCSN modeling
remains unclear, we speculate that the simplifying ``ray-by-ray' approach
employed by all other groups may be compromising their results. We show that
``ray-by-ray' calculations greatly exaggerate the angular and temporal
variations of the neutrino fluxes, which we argue are better captured by our
multi-dimensional MGFLD approach. On the other hand, our 2D models also make
approximations, making it difficult to draw definitive conclusions concerning
the root of the differences between groups. We discuss some of the diagnostics
often employed in the analyses of CCSN simulations and highlight the intimate
relationship between the various explosion conditions that have been proposed.
Finally, we explore the ingredients that may be missing in current calculations
that may be important in reproducing the properties of the average CCSNe,
should the delayed neutrino-heating mechanism be the correct mechanism of
explosion.Comment: ApJ accepted version. Minor changes from origina
Very Low Energy Supernovae: Light Curves and Spectra of Shock Breakout
The brief transient emitted as a shock wave erupts through the surface of a
presupernova star carries information about the stellar radius and explosion
energy. Here the CASTRO code, which treats radiation transport using multigroup
flux-limited diffusion, is used to simulate the light curves and spectra of
shock breakout in very low-energy supernovae (VLE SNe), explosions in giant
stars with final kinetic energy much less than 10 erg. VLE SNe light
curves, computed here with the KEPLER code, are distinctively faint, red, and
long-lived, making them challenging to find with transient surveys. The
accompanying shock breakouts are brighter, though briefer, and potentially
easier to detect. Previous analytic work provides general guidance, but
numerical simulations are challenging due to the range of conditions and lack
of equilibration between color and effective temperatures. We consider previous
analytic work and extend discussions of color temperature and opacity to the
lower energy range explored by these events. Since this is the first
application of the CASTRO code to shock breakout, test simulations of normal
energy shock breakout of SN1987A are carried out and compared with the
literature. A set of breakout light curves and spectra are then calculated for
VLE SNe with final kinetic energies in the range ergs for
red supergiants with main sequence masses 15 Msun and 25 Msun. The importance
of uncertainties in stellar atmosphere model, opacity, and ambient medium is
discussed, as are observational prospects with current and forthcoming
missions.Comment: 19 pages; submitted to Astrophysical Journa
Radiation Transport Simulations of Pulsational Pair-Instability Supernovae
Massive stars of helium cores of 35-65 Msun eventually encounter the
electron/positron creation instability, and it triggers explosive carbon or
oxygen burning that produces several thermonuclear eruptions. The resulting
catastrophe collisions of eruptive shells sometimes produce luminous transients
with peak luminosity of erg/sec, known as pulsational
pair-instability supernovae (PPISNe). Previous 2D simulations of colliding
shells show the development of Rayleigh-Taylor (RT) instabilities and mixing.
Here we present radiation hydrodynamic PPISNe simulations of a 110 Msun
solar-metallicity star that was promising to produce a superluminous transit in
the early work. Our comprehensive study contains a suite of one-, two-, and
three-dimensional models. We discuss the impact of dimensionality and fluid
instabilities on the resulting light curves. The results show the RT mixing
found in previous multidimensional hydro studies transforms into a thin and
distorted shell due to radiative cooling. Radiation from the wiggly shell peaks
at its bolometric light curve of erg/sec, lasting about
150 days and following with a plateau of erg/sec for
another two hundred days before it fades away. The total radiation energy
emitted from colliding shells is erg, which is of the kinetic energy of the major eruption. The dimensional effects also
manifest on the physical properties, such as irregularity and thickness of the
shell. Our study suggests PPISNe is a promising candidate of luminous SNe, the
radiation of which originates from colliding shells with a homogeneous mixing
of ejecta.Comment: Submitted to ApJ, 16 pages, comments are welcom
Models for Gamma-Ray Bursts and Diverse Transients
The observational diversity of ``gamma-ray bursts'' (GRBs) has been
increasing, and the natural inclination is a proliferation of models. We
explore the possibility that at least part of this diversity is a consequence
of a single basic model for the central engine operating in a massive star of
variable mass, differential rotation rate, and mass loss rate. Whatever that
central engine may be - and here the collapsar is used as a reference point -
it must be capable of generating both a narrowly collimated, highly
relativistic jet to make the GRB, and a wide angle, sub-relativistic outflow
responsible for exploding the star and making the supernova bright. To some
extent, the two components may vary independently, so it is possible to produce
a variety of jet energies and supernova luminosities. We explore, in
particular, the production of low energy bursts and find a lower limit,
erg s to the power required for a jet to escape a massive
star before that star either explodes or is accreted. Lower energy bursts and
and ``suffocated'' bursts may be particularly prevalent when the metallicity is
high, i.e., in the modern universe at low redshift.Comment: 12 pages, Royal Society meeting on GRBs, to appear in Philosophical
Transactions
Relativistic Hydrodynamic Flows Using Spatial and Temporal Adaptive Structured Mesh Refinement
Astrophysical relativistic flow problems require high resolution
three-dimensional numerical simulations. In this paper, we describe a new
parallel three-dimensional code for simulations of special relativistic
hydrodynamics (SRHD) using both spatially and temporally structured adaptive
mesh refinement (AMR). We used the method of lines to discretize the SRHD
equations spatially and a total variation diminishing (TVD) Runge-Kutta scheme
for time integration. For spatial reconstruction, we have implemented piecewise
linear method (PLM), piecewise parabolic method (PPM), third order convex
essentially non-oscillatory (CENO) and third and fifth order weighted
essentially non-oscillatory (WENO) schemes. Flux is computed using either
direct flux reconstruction or approximate Riemann solvers including HLL,
modified Marquina flux, local Lax-Friedrichs flux formulas and HLLC. The AMR
part of the code is built on top of the cosmological Eulerian AMR code {\sl
enzo}. We discuss the coupling of the AMR framework with the relativistic
solvers. Via various test problems, we emphasize the importance of resolution
studies in relativistic flow simulations because extremely high resolution is
required especially when shear flows are present in the problem. We also
present the results of two 3d simulations of astrophysical jets: AGN jets and
GRB jets. Resolution study of those two cases further highlights the need of
high resolutions to calculate accurately relativistic flow problems.Comment: 14 pages, 23 figures. A section on 3D GRB jet simulation added.
Accepted by ApJ
The Relationship Between Nitrogen Content in Soybean Leaves and Infestation Severity of Aphis glycines Mutsumura
Changes in nitrogen content of leaves in different soybean species during the infestation by Aphis glycines Mutsumura was determined. A correlation between the nitrogen content of soybean leaves and infestation severity of Aphis glycines Mutsumura was found. Therefore, the nitrogen content of soybean leaves could be regarded as one of the ecological factors used in prediction of infestation severity of Aphis glycines Mutsumura.Originating text in Chinese.Citation: Hu, Qi, Zhang, Weiqun, Yao, Yuxia, Yan, Shuqin. (1992). The Relationship Between Nitrogen Content in Soybean Leaves and Infestation Severity of Aphis glycines Mutsumura. Journal of Jilin Agricultural University, 14(4), 103-104
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