6,711 research outputs found
Force autocorrelation function in linear response theory and the origin of friction
Vanishing of the equilibrium Green-Kubo fluctuation expression for the
friction coefficient of a massive particle moving in a finite-volume liquid is
usually interpreted as an unphysical consequence of the finite volume. Here I
show that it is a physical consequence of the finite mass of the rest of the
system, which allows it to be dragged by the moving particle. As a consequence,
it is sufficient to have two infinite masses in the liquid for the friction
coefficient to be finite. In addition, I give the physical interpretation of
different friction coefficients for two infinite-mass particles moving in the
liquid.Comment: 24 pages text and figure
Natal Kicks of Stellar-Mass Black Holes by Asymmetric Mass Ejection in Fallback Supernovae
Integrating trajectories of low-mass X-ray binaries containing black holes
within the Galactic potential, Repetto, Davies & Sigurdsson recently showed
that the large distances of some systems above the Galactic plane can only be
explained if black holes receive appreciable natal kicks. Surprisingly, they
found that the distribution of black hole kick velocities (rather than that of
the momenta) should be similar to that of neutron stars. Here I argue that this
result can be understood if neutron star and black hole kicks are a consequence
of large-scale asymmetries created in the supernova ejecta by the explosion
mechanism. The corresponding anisotropic gravitational attraction of the
asymmetrically expelled matter does not only accelerate new-born neutron stars
by the "gravitational tug-boat mechanism". It can also lead to delayed
black-hole formation by asymmetric fallback of the slowest parts of the initial
ejecta onto the transiently existing neutron star, in course of which the
momentum of the black hole can grow with the fallback mass. Black hole kick
velocities will therefore not be reduced by the ratio of neutron star to black
hole mass as would be expected for kicks caused by anisotropic neutrino
emission of the nascent neutron star.Comment: 7 pages, 1 figure (3 eps files); submitted to MNRA
Neutrino Emission from Supernovae
Supernovae are the most powerful cosmic sources of MeV neutrinos. These
elementary particles play a crucial role when the evolution of a massive star
is terminated by the collapse of its core to a neutron star or a black hole and
the star explodes as supernova. The release of electron neutrinos, which are
abundantly produced by electron captures, accelerates the catastrophic infall
and causes a gradual neutronization of the stellar plasma by converting protons
to neutrons as dominant constituents of neutron star matter. The emission of
neutrinos and antineutrinos of all flavors carries away the gravitational
binding energy of the compact remnant and drives its evolution from the hot
initial to the cold final state. The absorption of electron neutrinos and
antineutrinos in the surroundings of the newly formed neutron star can power
the supernova explosion and determines the conditions in the innermost
supernova ejecta, making them an interesting site for the nucleosynthesis of
iron-group elements and trans-iron nuclei. In this Chapter the basic neutrino
physics in supernova cores and nascent neutron stars will be discussed. This
includes the most relevant neutrino production, absorption, and scattering
processes, elementary aspects of neutrino transport in dense environments, the
characteristic neutrino emission phases with their typical signal features, and
the perspectives connected to a measurement of the neutrino signal from a
future galactic supernova.Comment: Author version of chapter for 'Handbook of Supernovae,' edited by A.
Alsabti and P. Murdin, Springer. 30 pages, 9 figure
Global Anisotropy Versus Small-Scale Fluctuations in Neutrino Flux in Core-Collapse Supernova Explosions
Effects of small-scale fluctuations in the neutrino radiation on
core-collapse supernova explosions are examined. Through a parameter study with
a fixed radiation field of neutrinos, we find substantial differences between
the results of globally anisotropic neutrino radiation and those with
fluctuations. As the number of modes of fluctuations increases, the shock
positions, entropy distributions, and explosion energies approach those of
spherical explosion. We conclude that global anisotropy of the neutrino
radiation is the most effective mechanism of increasing the explosion energy
when the total neutrino luminosity is given. This supports the previous
statement on the explosion mechanism by Shimizu and coworkers.Comment: 14 pages, including 12 figures. To be published in the Astrophysical
Journa
Monte Carlo Study of Supernova Neutrino Spectra Formation
The neutrino flux and spectra formation in a supernova core is studied by
using a Monte Carlo code. The dominant opacity contribution for nu_mu and
nu_tau is elastic scattering on nucleons. In addition we switch on or off a
variety of processes which allow for the exchange of energy or the creation and
destruction of neutrino pairs, notably nucleon bremsstrahlung, the e^+ e^- pair
annihilation process and nu_e-bar nu_e -> nu_{mu,tau} nu_{mu,tau}-bar, recoil
and weak magnetism in elastic nucleon scattering, elastic scattering on
electrons and positrons and elastic scattering on electron neutrinos and
anti-neutrinos. The least important processes are neutrino-neutrino scattering
and e^+ e^- annihilation. The formation of the spectra and fluxes of nu_mu is
dominated by the nucleonic processes, i.e. bremsstrahlung and elastic
scattering with recoil, but also nu_e nu_e-bar annihilation and nu_mu e^\pm
scattering contribute significantly. When all processes are included, the
spectral shape of the emitted neutrino flux is always ``pinched,'' i.e. the
width of the spectrum is smaller than that of a thermal spectrum with the same
average energy. In all of our cases we find that the average nu_mu-bar energy
exceeds the average nu_e-bar energy by only a small amount, 10% being a typical
number. Weak magnetism effects cause the opacity of nu_mu to differ slightly
from that of nu_mu-bar, translating into differences of the luminosities and
average energies of a few percent. Depending on the density, temperature, and
composition profile, the flavor-dependent luminosities L_{nu_e}$, L_{nu_e-bar},
and L_{nu_mu} can mutually differ from each other by up to a factor of two in
either direction.Comment: 33 pages, 16 eps-figs, submitted to ApJ. Sections added: weak
magnetism, discussion of different analytic fits to the spectra and detailed
spectral shap
The r-Process in Black Hole Winds
All the current r-process scenarios relevant to core-collapse supernovae are
facing severe difficulties. In particular, recent core-collapse simulations
with neutrino transport show no sign of a neutron-rich wind from the
proto-neutron star. In this paper, we discuss nucleosynthesis of the r-process
in an alternative astrophysical site, "black hole winds", which are the
neutrino-driven outflow from the accretion torus around a black hole. This
condition is assumed to be realized in double neutron star mergers, neutron
star - black hole mergers, or hypernovae.Comment: 6 pages, 4 figures, invited talk at OMEG10, March 2010, to be
published in the proceedings of OMEG10 (AIP
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