40,894 research outputs found
The Eighteenth Century
This chapter has three sections: 1. General and Prose; 2. The Novel; 3. Poetry. Section 1 is by Eliza O’Brien; section 2 is by Elles Smallegoor and Sandro Jung; section 3 is by David E. Shuttleton
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
Hawking radiation in a -dimensional static spherically-symmetric black Hole surrounded by quintessence
We present a solution of Einstein equations with quintessential matter
surrounding a -dimensional black hole, whose asymptotic structures are
determined by the state of the quintessential matter. We examine the
thermodynamics of this black hole and find that the mass of the black hole
depends on the equation of state of the quintessence, while the first law is
universal. Investigating the Hawking radiation in this black hole background,
we observe that the Hawking radiation dominates on the brane in the low-energy
regime. For different asymptotic structures caused by the equation of state of
the quintessential matter surrounding the black hole, we learn that the
influences by the state parameter of the quintessence on Hawking radiation are
different.Comment: 16 pages, Accepted for publication in Phys. Rev.
Ionic and Electronic Conductivity of Nanostructured, Samaria-Doped Ceria
The ionic and electronic conductivities of samaria doped ceria electrolytes, Ce_(0.85)Sm_(0.15)O_(1.925−δ), with nanometric grain size have been evaluated. Nanostructured bulk specimens were obtained using a combination of high specific-surface-area starting materials and suitable sintering profiles under conventional, pressureless conditions. Bulk specimens with relatively high density (≥92% of theoretical density) and low medium grain size (as small as 33 nm) were achieved. Electrical A.C. impedance spectra were recorded over wide temperature (150 to 650°C) and oxygen partial pressure ranges (0.21 to 10^(−31) atm). Under all measurement conditions the total conductivity decreased monotonically with decreasing grain size. In both the electrolytic and mixed conducting regimes this behavior is attributed to the high number density of high resistance grain boundaries. The results suggest a possible variation in effective grain boundary width with grain size, as well as a possible variation in specific grain boundary resistance with decreasing oxygen partial pressure. No evidence appears for either enhanced reducibility or enhanced electronic conductivity upon nanostructuring
Strong gravitational lensing in a squashed Kaluza-Klein black hole spacetime
We investigate the strong gravitational lensing in a Kaluza-Klein black hole
with squashed horizons. We find the size of the extra dimension imprints in the
radius of the photon sphere, the deflection angle, the angular position and
magnification of the relativistic images. Supposing that the gravitational
field of the supermassive central object of the Galaxy can be described by this
metric, we estimated the numerical values of the coefficients and observables
for gravitational lensing in the strong field limit.Comment: 13pages, 5 figures, Final version appeared in PR
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
Transport in Graphene Tunnel Junctions
We present a technique to fabricate tunnel junctions between graphene and Al
and Cu, with a Si back gate, as well as a simple theory of tunneling between a
metal and graphene. We map the differential conductance of our junctions versus
probe and back gate voltage, and observe fluctuations in the conductance that
are directly related to the graphene density of states. The conventional
strong-suppression of the conductance at the graphene Dirac point can not be
clearly demonstrated, but a more robust signature of the Dirac point is found:
the inflection in the conductance map caused by the electrostatic gating of
graphene by the tunnel probe. We present numerical simulations of our
conductance maps, confirming the measurement results. In addition, Al causes
strong n-doping of graphene, Cu causes a moderate p-doping, and in high
resistance junctions, phonon resonances are observed, as in STM studies.Comment: 22 pages, 5 figure
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