15 research outputs found
Time-dependent, compositionally driven convection in the oceans of accreting neutron stars
We discuss the effect of chemical separation as matter freezes at the base of
the ocean of an accreting neutron star, and the subsequent enrichment of the
ocean in light elements and inward transport of heat through convective mixing.
We extend the steady-state results of Medin & Cumming 2011 to transiently
accreting neutron stars, by considering the time-dependent cases of heating
during accretion outbursts and cooling during quiescence. Convective mixing is
extremely efficient, flattening the composition profile in about one convective
turnover time (weeks to months at the base of the ocean). During accretion
outbursts, inward heat transport has only a small effect on the temperature
profile in the outer layers until the ocean is strongly enriched in light
elements, a process that takes hundreds of years to complete. During
quiescence, however, inward heat transport rapidly cools the outer layers of
the ocean while keeping the inner layers hot. We find that this leads to a
sharp drop in surface emission at around a week followed by a gradual recovery
as cooling becomes dominated by the crust. Such a dip should be observable in
the light curves of these neutron star transients, if enough data is taken at a
few days to a month after the end of accretion. If such a dip is definitively
observed, it will provide strong constraints on the chemical composition of the
ocean and outer crust.Comment: 22 pages, 11 figures, submitted to Ap
Model Atmospheres for X-ray Bursting Neutron Stars
The hydrogen and helium accreted by X-ray bursting neutron stars is
periodically consumed in runaway thermonuclear reactions that cause the entire
surface to glow brightly in X-rays for a few seconds. With models of the
emission, the mass and radius of the neutron star can be inferred from the
observations. By simultaneously probing neutron star masses and radii, X-ray
bursts are one of the strongest diagnostics of the nature of matter at
extremely high densities. Accurate determinations of these parameters are
difficult, however, due to the highly non-ideal nature of the atmospheres where
X-ray bursts occur. Observations from X-ray telescopes such as RXTE and NuStar
can potentially place strong constraints on nuclear matter once uncertainties
in atmosphere models have been reduced. Here we discuss current progress on
modeling atmospheres of X-ray bursting neutron stars and some of the challenges
still to be overcome.Comment: 25 pages, 14 figure
Crystallization of classical multi-component plasmas
We develop a method for calculating the equilibrium properties of the
liquid-solid phase transition in a classical, ideal, multi-component plasma.
Our method is a semi-analytic calculation that relies on extending the accurate
fitting formulae available for the one-, two-, and three-component plasmas to
the case of a plasma with an arbitrary number of components. We compare our
results to those of Horowitz, Berry, & Brown (Phys. Rev. E, 75, 066101, 2007),
who use a molecular dynamics simulation to study the chemical properties of a
17-species mixture relevant to the ocean-crust boundary of an accreting neutron
star, at the point where half the mixture has solidified. Given the same
initial composition as Horowitz et al., we are able to reproduce to good
accuracy both the liquid and solid compositions at the half-freezing point; we
find abundances for most species within 10% of the simulation values. Our
method allows the phase diagram of complex mixtures to be explored more
thoroughly than possible with numerical simulations. We briefly discuss the
implications for the nature of the liquid-solid boundary in accreting neutron
stars.Comment: 14 pages, 5 figures, submitted to Phys. Rev.
Pair cascades in the magnetospheres of strongly-magnetized neutron stars
We present numerical simulations of electron-positron pair cascades in the
magnetospheres of magnetic neutron stars for a wide range of surface fields
(B_p = 10^{12}--10^{15} G), rotation periods (0.1--10 s), and field geometries.
This has been motivated by the discovery in recent years of a number of radio
pulsars with inferred magnetic fields comparable to those of magnetars.
Evolving the cascade generated by a primary electron or positron after it has
been accelerated in the inner gap of the magnetosphere, we follow the spatial
development of the cascade until the secondary photons and pairs leave the
magnetosphere, and we obtain the pair multiplicity and the energy spectra of
the cascade pairs and photons under various conditions. Going beyond previous
works, which were restricted to weaker fields (B < a few x 10^{12} G), we have
incorporated in our simulations detailed treatments of physical processes that
are potentially important (especially in the high field regime) but were either
neglected or crudely treated before, including photon splitting with the
correct selection rules for photon polarization modes, one-photon pair
production into low Landau levels for the e^+e^-, and resonant inverse Compton
scattering from polar cap hot spots. We discuss the implications of our results
for the radio pulsar death line and for the hard X-ray emission from magnetized
neutron stars.Comment: 28 pages, 18 figures, submitted to MNRA
Density-Functional-Theory Calculations of Matter in Strong Magnetic Fields: II. Infinite Chains and Condensed Matter
We present new, ab initio calculations of the electronic structure of
one-dimensional infinite chains and three-dimensional condensed matter in
strong magnetic fields ranging from B=10^12 G to 2x10^15 G, appropriate for
observed magnetic neutron stars. At these field strengths, the magnetic forces
on the electrons dominate over the Coulomb forces, and to a good approximation
the electrons are confined to the ground Landau level. Our calculations are
based on the density functional theory, and use a local magnetic
exchange-correlation function appropriate in the strong field regime. The band
structures of electrons in different Landau orbitals are computed
self-consistently. Numerical results of the ground-state energies and electron
work functions are given for one-dimensional chains of H, He, C, and Fe.
Fitting formulae for the B-dependence of the energies are also provided. For
all the field strengths considered in this paper, hydrogen, helium, and carbon
chains are found to be bound relative to individual atoms (although for B less
than a few x 10^12 G, the relative binding between C and C_infinity is small).
Iron chains are significantly bound for B>10^14 G and are weakly bound if at
all at B<10^13 G. We also study the cohesive property of three-dimensional
condensed matter of H, He, C, and Fe at zero pressure, constructed from
interacting chains in a body-centered tetragonal lattice. Such
three-dimensional condensed matter is found to be bound relative to individual
atoms, with the cohesive energy increasing rapidly with increasing B.Comment: 32 pages, 18 figures. Minor changes, figure omissions. Phys. Rev. A
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