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