394 research outputs found
Structure of pair winds from compact objects with application to emission from bare strange stars
We present the results of numerical simulations of stationary, spherically
outflowing, electron-positron pair winds, with total luminosities in the range
10^{34}- 10^{42} ergs/s. In the concrete example described here, the wind
injection source is a hot, bare, strange star, predicted to be a powerful
source of electron-positron pairs created by the Coulomb barrier at the quark
surface. We find that photons dominate in the emerging emission, and the
emerging photon spectrum is rather hard and differs substantially from the
thermal spectrum expected from a neutron star with the same luminosity. This
might help distinguish the putative bare strange stars from neutron stars.Comment: 4 pages, 6 figures, 1 table, added references, to appear in the
proceedings of the conference "Isolated Neutron Stars: from the Surface to
the Interior", London, UK, 24-28 April 200
Photon emission from bare quark stars
We investigate the photon emission from the electrosphere of a quark star. It
is shown that at temperatures T\sim 0.1-1 MeV the dominating mechanism is the
bremsstrahlung due to bending of electron trajectories in the mean Coulomb
field of the electrosphere. The radiated energy for this mechanism is much
larger than that for the Bethe-Heitler bremsstrahlung. The energy flux from the
mean field bremsstrahlung exceeds the one from the tunnel e^{+}e^{-} pair
creation as well. We demonstrate that the LPM suppression of the photon
emission is negligible.Comment: 35 pages, 5 figure
A hydrodynamic model for asymmetric explosions of rapidly rotating collapsing supernovae with a toroidal atmosphere
We numerically solved the two-dimensional axisymmetric hydrodynamic problem
of the explosion of a low-mass neutron star in a circular orbit. In the initial
conditions, we assumed a nonuniform density distribution in the space
surrounding the collapsed iron core in the form of a stationary toroidal
atmosphere that was previously predicted analytically and computed numerically.
The configuration of the exploded neutron star itself was modeled by a
torus with a circular cross section whose central line almost coincided with
its circular orbit. Using an equation of state for the stellar matter and the
toroidal atmosphere in which the nuclear statistical equilibrium conditions
were satisfied, we performed a series of numerical calculations that showed the
propagation of a strong divergent shock wave with a total energy of 0.2x10^51
erg at initial explosion energy release of 1.0x10^51 erg. In our calculations,
we rigorously took into account the gravitational interaction, including the
attraction from a higher-mass (1.9M_solar) neutron star located at the
coordinate origin, in accordance with the rotational explosion mechanism for
collapsing supernovae.W e compared in detail our results with previous similar
results of asymmetric supernova explosion simulations and concluded that we
found a lower limit for the total explosion energy.Comment: 13 pages, 5 figures, 2 table
A Rotating Collapsar and Possible Interpretation of the LSD Neutrino Signal from SN 1987A
We consider an improved rotational mechanism of the explosion of a collapsing
supernova. We show that this mechanism leads to two-stage collapse with a phase
difference of \sim 5 h. Based on this model, we attempt a new interpretation of
the events in underground neutrino detectors on February 23, 1987, related to
the supernova SN 1987A.Comment: 18 pages, 3 figures, 9 table
Surface structure of Quark stars with magnetic fields
We investigate the impact of magnetic fields on the electron distribution in
the electrosphere of quark stars. For moderately strong magnetic fields G, quantization effects are generally weak due to the large number
density of electrons at surface, but can nevertheless affect the spectral
features of quark stars. We outline the main observational characteristics of
quark stars as determined by their surface emission, and briefly discuss their
formation in explosive events termed Quark-Novae, which may be connected to the
-process.Comment: 9 pages, 3 figures. Contribution to the proceedings of the IXth
Workshop on High Energy Physics Phenomenology (WHEPP-9), Bhubaneswar, India,
3-14 Jan. 200
Synchronization of multi-phase oscillators: An Axelrod-inspired model
Inspired by Axelrod's model of culture dissemination, we introduce and
analyze a model for a population of coupled oscillators where different levels
of synchronization can be assimilated to different degrees of cultural
organization. The state of each oscillator is represented by a set of phases,
and the interaction --which occurs between homologous phases-- is weighted by a
decreasing function of the distance between individual states. Both ordered
arrays and random networks are considered. We find that the transition between
synchronization and incoherent behaviour is mediated by a clustering regime
with rich organizational structure, where some of the phases of a given
oscillator can be synchronized to a certain cluster, while its other phases are
synchronized to different clusters.Comment: 6 pages, 5 figure
Presupernova Evolution of Rotating Massive Stars I: Numerical Method and Evolution of the Internal Stellar Structure
The evolution of rotating stars with zero-age main sequence (ZAMS) masses in
the range 8 to 25 M_sun is followed through all stages of stable evolution. The
initial angular momentum is chosen such that the star's equatorial rotational
velocity on the ZAMS ranges from zero to ~ 70 % of break-up. Redistribution of
angular momentum and chemical species are then followed as a consequence of
rotationally induced circulation and instablities. The effects of the
centrifugal force on the stellar structure are included. Uncertain mixing
efficiencies are gauged by observations. We find, as noted in previous work,
that rotation increases the helium core masses and enriches the stellar
envelopes with products of hydrogen burning. We determine, for the first time,
the angular momentum distribution in typical presupernova stars along with
their detailed chemical structure. Angular momentum loss due to (non-magnetic)
stellar winds and the redistribution of angular momentum during core hydrogen
burning are of crucial importance for the specific angular momentum of the
core. Neglecting magnetic fields, we find angular momentum transport from the
core to the envelope to be unimportant after core helium burning. We obtain
specific angular momenta for the iron core and overlaying material of
1E16...1E17 erg s. These values are insensitive to the initial angular
momentum. They are small enough to avoid triaxial deformations of the iron core
before it collapses, but could lead to neutron stars which rotate close to
break-up. They are also in the range required for the collapsar model of
gamma-ray bursts. The apparent discrepancy with the measured rotation rates of
young pulsars is discussed.Comment: 62 pages, including 7 tables and 19 figures. Accepted by Ap
Ferromagnetic HfO2/Si/GaAs interface for spin-polarimetry applications
In this letter, we present electrical and magnetic characteristics of HfO2-based metal-oxide-semiconductor capacitors (MOSCAPs), along with the effect of pseudomorphic Si as a passivating interlayer on GaAs(001) grown by molecular beam epitaxy. Ultrathin HfO2 high-k gate dielectric films (3–15 nm) have been grown on Si/GaAs(001) structures through evaporation of a Hf/HfO2 target in NO2 gas. The lowest interface states density Dit at Au/HfO2/Si/GaAs(001) MOS-structures were obtained in the range of (6−13)×101
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Freshwater in the Arctic Ocean 2010–2019
The Arctic climate system is rapidly transitioning into a new regime with a reduction in the extent of sea ice, enhanced mixing in the ocean and atmosphere, and thus enhanced coupling within the ocean–ice–atmosphere system; these physical changes are leading to ecosystem changes in the Arctic Ocean. In this review paper, we assess one of the critically important aspects of this new regime, the variability of Arctic freshwater, which plays a fundamental role in the Arctic climate system by impacting ocean stratification and sea ice formation or melt. Liquid and solid freshwater exports also affect the global climate system, notably by impacting the global ocean overturning circulation. We assess how freshwater budgets have changed relative to the 2000–2010 period. We include discussions of processes such as poleward atmospheric moisture transport, runoff from the Greenland Ice Sheet and Arctic glaciers, the role of snow on sea ice, and vertical redistribution. Notably, sea ice cover has become more seasonal and more mobile; the mass loss of the Greenland Ice Sheet increased in the 2010s (particularly in the western, northern, and southern regions) and imported warm, salty Atlantic waters have shoaled. During 2000–2010, the Arctic Oscillation and moisture transport into the Arctic are in-phase and have a positive trend. This cyclonic atmospheric circulation pattern forces reduced freshwater content on the Atlantic–Eurasian side of the Arctic Ocean and freshwater gains in the Beaufort Gyre. We show that the trend in Arctic freshwater content in the 2010s has stabilized relative to the 2000s, potentially due to an increased compensation between a freshening of the Beaufort Gyre and a reduction in freshwater in the rest of the Arctic Ocean. However, large inter-model spread across the ocean reanalyses and uncertainty in the observations used in this study prevent a definitive conclusion about the degree of this compensation
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