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End-point of the rp process and periodic gravitational wave emission
The general end-point of the rp process in rapidly accreting neutron stars is
believed to be a surface distribution of matter whose nuclear composition may
depend on position. Its evolution during compression beyond the neutron-drip
threshold density is determined by the presence of nuclear formation enthalpy
minima at the proton closed shells. At threshold, a sequence of weak
interactions with capture or emission of neutron pairs rapidly transform nuclei
to the most accessible proton closed shell. Therefore, angular asymmetries in
nuclear composition present in accreted matter at neutron drip are preserved
during further compression to higher densities provided transition rates
between closed shells are negligible. Although it has been confirmed that this
condition is satisfied for predicted internal temperatures and for the
formation enthalpy distribution used in this work, it would not be so if the
true enthalpy differences between maxima and minima in the distribution were a
factor of two smaller. For this reason, it does not appear possible to assert
with any confidence that position-dependent surface composition can lead to
significant angle-dependence of the equation of state and to potentially
observable gravitational radiation. The effect of non-radial internal
temperature gradients on angle-dependency of the equation of state is also not
quantifiable.Comment: This version corrects a major error in estimating the effect of
composition asymmetry on the equation of state. Its conclusions are less
definite than those of the previous version. 9 pages RevTex; 1 figure. To be
published in Phys. Rev.
Pulsar emission: Langmuir modes in a relativistic multi-component plasma
Ions, protons and possibly a small flux of electrons and positrons are
accelerated outward from the polar cap of a normal or millisecond pulsar whose
rotational spin is antiparallel with its magnetic moment. The Langmuir modes of
this relativistic plasma have several properties of significance for the origin
of coherent radio emission. The characteristics of the mode are determined by
the sequence of singularities in the dielectric tensor at real angular
frequencies, which in turn is fixed by the electron-positron momentum
distribution. We find that under a certain condition on its momentum
distribution, an electron-positron flux two orders of magnitude smaller than
the Goldreich-Julian flux stabilizes the plasma and extinguishes the mode. But
more generally, both the growth rate and wavenumber of the multi-component
Langmuir mode can be as much as an order of magnitude larger than those of the
two-component ion-proton mode. It appears to be a further effective source for
the plasma turbulence whose decay is probably responsible for the observed
emission.Comment: 6 pages, 1 figur
An incomplete model of RRATs and of nulls mode-changes and subpulses
A model for pulsars with polar-cap magnetic flux density B antiparallel with
rotational spin is described. It recognizes the significance of two elementary
processes, proton production in electromagnetic showers and photoelectric
transitions in ions accelerated through the blackbody radiation field, which
must be present at the polar cap in the antiparallel case, but not for pulsars
of the opposite spin direction. The two populations are likely to be
indistinguishable observationally until curvature radiation pair creation
ceases to be possible. The model generates, and provides a physically realistic
framework for, the polar-cap potential fluctuations and their time-scales that
can produce mode-changes and nulls. The RRATs are then no more than an extreme
case of the more commonly observed nulls. The model is also able to support the
basic features of subpulse drift and to some extent the null-memory phenomenon
that is associated with it. Unfortunately, it appears that the most important
neutron-star parameter for quantitative predictive purposes is the
whole-surface temperature, a quantity which is not readily observable at the
neutron-star ages concerned.Comment: 12 page
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