15,675 research outputs found
Gas Dynamics of the Nickel-56 Decay Heating in Pair-Instability Supernovae
Very massive 140-260 Msun stars can die as highly-energetic pair-instability
supernovae (PI SNe) with energies of up to 100 times those of core-collapse SNe
that can completely destroy the star, leaving no compact remnant behind. These
explosions can synthesize Msun of radioactive Ni56, which can cause
them to rebrighten at later times when photons due to Ni56 decay diffuse out of
the ejecta. However, heat from the decay of such large masses of Ni56 could
also drive important dynamical effects deep in the ejecta that are capable of
mixing elements and affecting the observational signatures of these events. We
have now investigated the dynamical effect of Ni56 heating on PI SN ejecta with
high-resolution two-dimensional hydrodynamic simulations performed with the
CASTRO code. We find that expansion of the hot Ni56 bubble forms a shell at the
base of the silicon layer of the ejecta about 200 days after the explosion but
that no hydrodynamical instabilities develop that would mix Ni56 with the
Si/O-rich ejecta. However, while the dynamical effects of Ni56 heating may be
weak they could affect the observational signatures of some PI SNe by diverting
decay energy into internal expansion of the ejecta at the expense of
rebrightening at later times.Comment: Accepted to ApJ, 14 page
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
Overexpression of the type 1 adenylyl cyclase in the forebrain leads to deficits of behavioral inhibition
The type 1 adenylyl cyclase (AC1) is an activity-dependent, calcium-stimulated adenylyl cyclase expressed in the nervous system that is implicated in memory formation. We examined the locomotor activity, and impulsive and social behaviors of AC1+ mice, a transgenic mouse strain overexpressing AC1 in the forebrain. Here we report that AC1+ mice exhibit hyperactive behaviors and demonstrate increased impulsivity and reduced sociability. In contrast, AC1 and AC8 double knock-out mice are hypoactive, and exhibit increased sociability and reduced impulsivity. Interestingly, the hyperactivity of AC1+ mice can be corrected by valproate, a mood-stabilizing drug. These data indicate that increased expression of AC1 in the forebrain leads to deficits in behavioral inhibition
Time variability of accretion flows: effects of the adiabatic index and gas temperature
We report on next phase of our study of rotating accretion flows onto black
holes. We consider hydrodynamical (HD) accretion flows with a spherically
symmetric density distribution at the outer boundary but with spherical
symmetry broken by the introduction of a small, latitude-dependent angular
momentum. We study accretion flows by means of numerical two-dimensional,
axisymmetric, HD simulations for variety of the adiabatic index, and
the gas temperature at infinity, . Our work is an extension of work
done by Proga & Begelman who consider models for only . Our main
result is that the flow properties such as the topology of the sonic surface
and time behavior strongly depend on but little on . In
particular, for , the mass accretion rate shows large
amplitude, slow time-variability which is a result of mixing between slow and
fast rotating gas. This temporal behavior differs significantly from that in
models with \gamma\simless 5/3 where the accretion rate is relatively
constant and from that in models with \gamma\simgreat 1 where the accretion
exhibits small amplitude quasi-periodic oscillations. The key parameter
responsible for the differences is the sound speed of the accretion flow which
in turn determines whether the flow is dominated by gas pressure, radiation
pressure or rotation. Despite these differences the time-averaged mass
accretion rate in units of the corresponding Bondi rate is a weak function of
and .Comment: 31 pages, 14 figures, accepted for publication in ApJ, for full
resolution version goto http://users.camk.edu.pl/mmosc/ms.pd
PAMELA Positron Excess as a Signal from the Hidden Sector
The recent positron excess observed in the PAMELA satellite experiment
strengthens previous experimental findings. We give here an analysis of this
excess in the framework of the Stueckelberg extension of the standard model
which includes an extra gauge field and matter in the hidden sector.
Such matter can produce the right amount of dark matter consistent with the
WMAP constraints. Assuming the hidden sector matter to be Dirac fermions it is
shown that their annihilation can produce the positron excess with the right
positron energy dependence seen in the HEAT, AMS and the PAMELA experiments.
Further test of the proposed model can come at the Large Hadron Collider. The
predictions of the flux ratio also fit the data.Comment: 9 pages,3 figures; Breit-Wigner enhancement emphasized; published in
PR
How the First Stars Regulated Star Formation. II. Enrichment by Nearby Supernovae
Metals from Population III (Pop III) supernovae led to the formation of less
massive Pop II stars in the early universe, altering the course of evolution of
primeval galaxies and cosmological reionization. There are a variety of
scenarios in which heavy elements from the first supernovae were taken up into
second-generation stars, but cosmological simulations only model them on the
largest scales. We present small-scale, high-resolution simulations of the
chemical enrichment of a primordial halo by a nearby supernova after partial
evaporation by the progenitor star. We find that ejecta from the explosion
crash into and mix violently with ablative flows driven off the halo by the
star, creating dense, enriched clumps capable of collapsing into Pop II stars.
Metals may mix less efficiently with the partially exposed core of the halo, so
it might form either Pop III or Pop II stars. Both Pop II and III stars may
thus form after the collision if the ejecta do not strip all the gas from the
halo. The partial evaporation of the halo prior to the explosion is crucial to
its later enrichment by the supernova.Comment: Accepted to Ap
Towards Fast Computation of Certified Robustness for ReLU Networks
Verifying the robustness property of a general Rectified Linear Unit (ReLU)
network is an NP-complete problem [Katz, Barrett, Dill, Julian and Kochenderfer
CAV17]. Although finding the exact minimum adversarial distortion is hard,
giving a certified lower bound of the minimum distortion is possible. Current
available methods of computing such a bound are either time-consuming or
delivering low quality bounds that are too loose to be useful. In this paper,
we exploit the special structure of ReLU networks and provide two
computationally efficient algorithms Fast-Lin and Fast-Lip that are able to
certify non-trivial lower bounds of minimum distortions, by bounding the ReLU
units with appropriate linear functions Fast-Lin, or by bounding the local
Lipschitz constant Fast-Lip. Experiments show that (1) our proposed methods
deliver bounds close to (the gap is 2-3X) exact minimum distortion found by
Reluplex in small MNIST networks while our algorithms are more than 10,000
times faster; (2) our methods deliver similar quality of bounds (the gap is
within 35% and usually around 10%; sometimes our bounds are even better) for
larger networks compared to the methods based on solving linear programming
problems but our algorithms are 33-14,000 times faster; (3) our method is
capable of solving large MNIST and CIFAR networks up to 7 layers with more than
10,000 neurons within tens of seconds on a single CPU core.
In addition, we show that, in fact, there is no polynomial time algorithm
that can approximately find the minimum adversarial distortion of a
ReLU network with a approximation ratio unless
=, where is the number of neurons in the network.Comment: Tsui-Wei Weng and Huan Zhang contributed equall
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