17 research outputs found
Dynamic Screening and Thermonuclear Reaction Rates
We show that there are no dynamic screening corrections to the Salpeter's
enhancement factor in the weak-screening limit.Comment: 7 pages LaTex, no figures, Submitted to Ap
Screening in Thermonuclear Reaction Rates in the Sun
We evaluate the effect of electrostatic screening by ions and electrons on
low-Z thermonuclear reactions in the sun. We use a mean field formalism and
calculate the electron density of the screening cloud using the appropriate
density matrix equation of quantum statistical mechanics. Because of well
understood physical effects that are included for the first time in our
treatment, the calculated enhancement of reaction rates does not agree with the
frequently used interpolation formulae. Our result does agree, within small
uncertainties, with Salpeter's weak screening formula. If weak screening is
used instead of the commonly employed screening prescription of Graboske et
al., the predicted B neutrino flux is increased by 7% and the predicted
chlorine rate is increased by 0.4 SNU.Comment: 15 pages, 1 figure, submitted to ApJ. Acknowledgments, a footnote,
and an explanation added. Additional information at www.sns.ias.edu/~jn
Convection-Dominated Accretion Flows
Non-radiating, advection-dominated, accretion flows are convectively
unstable. We calculate the two-dimensional (r-theta) structure of such flows
assuming that (1) convection transports angular momentum inwards, opposite to
normal viscosity and (2) viscous transport by other mechanisms (e.g., magnetic
fields) is weak (alpha << 1). Under such conditions convection dominates the
dynamics of the accretion flow and leads to a steady state structure that is
marginally stable to convection. We show that the marginally stable flow has a
constant temperature and rotational velocity on spherical shells, a net flux of
energy from small to large radii, zero net accretion rate, and a radial density
profile proportional to r^{-1/2}, flatter than the r^{-3/2} profile
characteristic of spherical accretion flows. This solution accurately describes
the full two-dimensional structure of recent axisymmetric numerical simulations
of advection-dominated accretion flows.Comment: final version accepted by ApJ; discussion expanded, references adde
Photon acceleration in variable ultra-relativistic outflows and high-energy spectra of Gamma-Ray Bursts
MeV seed photons produced in shocks in a variable ultra-relativistic outflow
gain energy by the Fermi mechanism, because the photons Compton scatter off
relativistically colliding shells. The Fermi-modified high-energy photon
spectrum has a non-universal slope and a universal cutoff. A significant
increase in the total radiative efficiency is possible. In some gamma ray
bursts, most of the power might be emitted at the high-energy cutoff for this
mechanism, which would be close to 100 MeV for outflows with a mean bulk
Lorentz factor of 100.Comment: 8 pages, submitted to ApJ
Turbulence and Particle Heating in Advection-Dominated Accretion Flows
We extend and reconcile recent work on turbulence and particle heating in
advection-dominated accretion flows. For approximately equipartition magnetic
fields, the turbulence primarily heats the electrons. For weaker magnetic
fields, the protons are primarily heated. The division between electron and
proton heating occurs between and (where
is the ratio of the gas to the magnetic pressure), depending on unknown
details of how Alfv\'en waves are converted into whistlers on scales of the
proton Larmor radius. We also discuss the possibility that magnetic
reconnection could be a significant source of electron heating.Comment: 17 pages (Latex), incl. 2 Figures; submitted to Ap
Graviton Mass or Cosmological Constant?
To describe a massive graviton in 4D Minkowski space-time one introduces a
quadratic term in the Lagrangian. This term, however, can lead to a
readjustment or instability of the background instead of describing a massive
graviton on flat space. We show that for all local Lorentz-invariant mass terms
Minkowski space is unstable. We start with the Pauli-Fierz (PF) term that is
the only local mass term with no ghosts in the linearized approximation. We
show that nonlinear completions of the PF Lagrangian give rise to instability
of Minkowski space. We continue with the mass terms that are not of a PF type.
Although these models are known to have ghosts in the linearized
approximations, nonlinear interactions can lead to background change due to
which the ghosts are eliminated. In the latter case, however, the graviton
perturbations on the new background are not massive. We argue that a consistent
theory of a massive graviton on flat space can be formulated in theories with
extra dimensions. They require an infinite number of fields or non-local
description from a 4D point of view.Comment: 16 pages; references and comments adde
Radiative Efficiency of Collisionless Accretion
Radiative efficiency of a slowly accreting black hole is estimated using a
two-temperature model of accretion. The radiative efficiency depends on the
magnetic field strength near the Schwarzschild radius. For weak magnetic fields
(magnetic energy=equipartition/1000), the low efficiency 0.0001 assumed in some
theoretical models might be achieved. For stronger fields, a significant
fraction of viscous heat is dissipated by electrons and radiated away resulting
in a larger efficiency. At equipartition magnetic fields, we estimate
efficiency = of order 10%.Comment: 12 pages, Latex, Submitted to Ap
THE 7 Be ELECTRON CAPTURE RATE IN THE SUN
For solar conditions, we numerically integrate the density matrix equation for a thermal electron in the field of a 7Be ion and other plasma ions and smeared-out electrons. Our results are in agreement with previous calculations that are based on a different physical picture, a picture which postulates the existence of distinct continuum and bound state orbits for electrons. The density matrix calculation of the electron capture rate is independent of the nature of electron states in the solar plasma. To within a 1 % accuracy, the effects of screening can be described at high temperatures by a Salpeter-like factor of exp(−Ze2 /kTRD), which can be derived from the density matrix equation. The total theoretical uncertainty in the electron capture rate is about ±2%. Subject headings: nuclear reactions – 3