71,433 research outputs found
Hot interstellar gas and ionization of embedded clouds
Researchers present detailed photoionization calculations for the instellar cloud in which the Sun is embedded. They consider the EUV radiation field with contribution from discrete stellar sources and from a thermal bremsstrahlung-radiative recombination spectrum emitted from the surrounding 10 to the 6th power k coronal substrate. They establish lower limits to the fractional ionization of hydrogen and helium of 0.17 and 0.29 respectively. The high He ionization fraction results primarily from very strong line emission below 500 A originating in the surrounding coronal substrate while the H ionization is dominated by the EUV radiation from the discrete stellar sources. The dual effects of thermal conduction and the EUV spectrum of the 10 to the 6th k plasma on ionization in the cloud skin are explored. The EUV radiation field and Auger ionization have insignificant effects on the resulting ionic column densities of Si IV, C IV, N V and O VI through the cloud skin. Calculations show that the abundances of these species are dominated by collisional ionization in the thermal conduction front. Because of a low charge exchange rate with hydrogen, the ionic column density ratios of N (CIII)/N (CII) and N (NII)/N (NI) are dominated by the EUV radiation field in the local interstellar medium. These ratios should be important diagnostics for the EUV radiation field and serve as surrogate indicators of the interstellar He and H ionization fraction respectively. Spacecraft such as Lyman which is designed to obtain high resolution spectral data down to the Lyman limit at 912 A could sample interstellar lines of these ions
Structure of the electrospheres of bare strange stars
We consider a thin ( fm) layer of electrons (the
electrosphere) at the quark surface of a bare strange star, taking into account
the surface effects at the boundary with the vacuum. The quark surface holds
the electron layer by an extremely strong electric field, generated in the
electrosphere to prevent the electrons from escaping to infinity by
counterbalancing the degeneracy and thermal pressure. Because of the surface
tension and depletion of quarks a very thin (a few fm) charged layer of
quarks forms at the surface of the star. The formation of this layer modifies
the structure of the electrosphere, by significantly changing the electric
field and the density of the electrons, in comparison with the case when the
surface effects are ignored. Some consequences of the modification of the
electrosphere structure on the properties of strange stars are briefly
discussed.Comment: 23 pages, 6 figures, to appear in Ap
Nucleation of quark matter in neutron stars cores
We consider the general conditions of quark droplets formation in high
density neutron matter. The growth of the quark bubble (assumed to contain a
sufficiently large number of particles) can be described by means of a
Fokker-Planck equation. The dynamics of the nucleation essentially depends on
the physical properties of the medium it takes place. The conditions for quark
bubble formation are analyzed within the frameworks of both dissipative and
non-dissipative (with zero bulk and shear viscosity coefficients) approaches.
The conversion time of the neutron star to a quark star is obtained as a
function of the equation of state of the neutron matter and of the microscopic
parameters of the quark nuclei. As an application of the obtained formalism we
analyze the first order phase transition from neutron matter to quark matter in
rapidly rotating neutron stars cores, triggered by the gravitational energy
released during the spinning down of the neutron star. The endothermic
conversion process, via gravitational energy absorption, could take place, in a
very short time interval, of the order of few tens seconds, in a class of dense
compact objects, with very high magnetic fields, called magnetars.Comment: 31 pages, 2 figures, to appear in Ap
Bounds on R-parity violating supersymmetric couplings from leptonic and semi-leptonic meson decays
We present a comprehensive update of the bounds on R-Parity violating
supersymmetric couplings from lepton-flavour- and lepton-number-violating decay
processes. We consider tau and mu decays as well as leptonic and semi-leptonic
decays of mesons. We present several new bounds resulting from tau, eta and
Kaon decays and correct some results in the literature concerning B-meson
decays.Comment: 30 pages; changed title, updated some bounds from the literature from
different references, added reference
Cross-section and polarization of neutrino-produced 's made simple
Practical formulae are derived for the cross-section and polarization of the
lepton produced in deep-inelastic neutrino-nucleon scattering in the
frame of the simple quark-parton model.Comment: 10 pages, no figure
Matching Stages of Heavy Ion Collision Models
Heavy ion reactions and other collective dynamical processes are frequently
described by different theoretical approaches for the different stages of the
process, like initial equilibration stage, intermediate locally equilibrated
fluid dynamical stage and final freeze-out stage. For the last stage the best
known is the Cooper-Frye description used to generate the phase space
distribution of emitted, non-interacting, particles from a fluid dynamical
expansion/explosion, assuming a final ideal gas distribution, or (less
frequently) an out of equilibrium distribution. In this work we do not want to
replace the Cooper-Frye description, rather clarify the ways how to use it and
how to choose the parameters of the distribution, eventually how to choose the
form of the phase space distribution used in the Cooper-Frye formula. Moreover,
the Cooper-Frye formula is used in connection with the freeze-out problem,
while the discussion of transition between different stages of the collision is
applicable to other transitions also. More recently hadronization and molecular
dynamics models are matched to the end of a fluid dynamical stage to describe
hadronization and freeze-out. The stages of the model description can be
matched to each other on spacetime hypersurfaces (just like through the
frequently used freeze-out hypersurface). This work presents a generalized
description of how to match the stages of the description of a reaction to each
other, extending the methodology used at freeze-out, in simple covariant form
which is easily applicable in its simplest version for most applications.Comment: 9 pages, 2 figure
The Additional Symmetries for the BTL and CTL Hierarchies
The Toda lattice (TL) hierarchy was first introduced by K.Ueno and K.Takasaki
in \cite{uenotaksasai} to generalize the Toda lattice equations\cite{toda}.
Along the work of E. Date, M. Jimbo, M. Kashiwara and T. Miwa \cite{DJKM} on
the KP hierarchy, K.Ueno and K.Takasaki in \cite{uenotaksasai} develop the
theory for the TL hierarchy: its algebraic structure, the linearization, the
bilinear identity, function and so on. Also the analogues of the B and C
types for the TL hierarchy, i.e. the BTL and CTL hierarchies, are considered in
\cite{uenotaksasai}, which are corresponding to infinite dimensional Lie
algebras and respectively. In this
paper, we will focus on the study of the additional symmetries for the BTL and
CTL hierarchies.Comment: 13 page
A re-visit of the phase-resolved X-ray and \gamma-ray spectra of the Crab pulsar
We use a modified outer gap model to study the multi-frequency phase-resolved
spectra of the Crab pulsar. The emissions from both poles contribute to the
light curve and the phase-resolved spectra. Using the synchrotron self-Compton
mechanism and by considering the incomplete conversion of curvature photons
into secondary pairs, the observed phase-averaged spectrum from 100 eV - 10 GeV
can be explained very well. The predicted phase-resolved spectra can match the
observed data reasonably well, too. We find that the emission from the north
pole mainly contributes to Leading Wing 1. The emissions in the remaining
phases are mainly dominated by the south pole. The widening of the azimuthal
extension of the outer gap explains Trailing Wing 2. The complicated
phase-resolved spectra for the phases between the two peaks, namely Trailing
Wing 1, Bridge and Leading Wing 2, strongly suggest that there are at least two
well-separated emission regions with multiple emission mechanisms, i.e.
synchrotron radiation, inverse Compton scattering and curvature radiation. Our
best fit results indicate that there may exist some asymmetry between the south
and the north poles. Our model predictions can be examined by GLAST.Comment: 35 pages, 13 figures, accepted to publish in Ap
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