60 research outputs found
Oscillatory superfluid Ekman pumping in Helium II and neutron stars
The linear response of a superfluid, rotating uniformly in a cylindrical
container and threaded with a large number of vortex lines, to an impulsive
increase in the angular velocity of the container is investigated. At zero
temperature and with perfect pinning of vortices to the top and bottom of the
container, we demonstrate that the system oscillates persistently with a
frequency proportional to the vortex line tension parameter to the quarter
power. This low-frequency mode is generated by a secondary flow analogous to
classical Ekman pumping that is periodically reversed by the vortex tension in
the boundary layers. We compare analytic solutions to the two-fluid equations
of Chandler & Baym (1986) with the spin-up experiments of Tsakadze & Tsakadze
(1980) in helium II and find the frequency agrees within a factor of four,
although the experiment is not perfectly suited to the application of the
linear theory. We argue that this oscillatory Ekman pumping mode, and not
Tkachenko modes provide a natural explanation for the observed oscillation. In
neutron stars, the oscillation period depends on the pinning interaction
between neutron vortices and flux tubes in the outer core. Using a simplified
pinning model, we demonstrate that strong pinning can accommodate modes with
periods of days to years, which are only weakly damped by mutual friction over
longer timescales.Comment: 25 pages, 6 figures, submitted to Journal of Fluid Mechanic
Spin down of superfluid-filled vessels: theory versus experiment
The spin up of helium II is studied by calculating the spin-down recovery of
a superfluid-filled container after an impulsive acceleration and comparing
with experiments. The calculation takes advantage of a recently published
analytic solution for the spin up of a Hall-Vinen-Bekharevich-Khalatnikov
superfluid that treats the back-reaction torque exerted by the viscous
component self-consistently in arbitrary geometry for the first time. Excellent
agreement at the 0.5% level is obtained for experiments at ,
after correcting for the non-uniform rotation in the initial state, confirming
that vortex tension and pinning (which are omitted from the theory) play a
minimal role under certain conditions (small Rossby number, smooth walls). The
dependence of the spin-down time on temperature and the mass fraction of the
viscous component are also investigated. Closer to the lambda point, the
predicted onset of turbulence invalidates the linear Ekman theory.Comment: 5 figures, 1 tabl
Gravitational radiation from pulsar glitches
The nonaxisymmetric Ekman flow excited inside a neutron star following a
rotational glitch is calculated analytically including stratification and
compressibility. For the largest glitches, the gravitational wave strain
produced by the hydrodynamic mass quadrupole moment approaches the sensitivity
range of advanced long-baseline interferometers. It is shown that the
viscosity, compressibility, and orientation of the star can be inferred in
principle from the width and amplitude ratios of the Fourier peaks (at the spin
frequency and its first harmonic) observed in the gravitational wave spectrum
in the plus and cross polarizations. These transport coefficients constrain the
equation of state of bulk nuclear matter, because they depend sensitively on
the degree of superfluidity.Comment: 28 page
Gross-Pitaevskii model of pulsar glitches
The first large-scale quantum mechanical simulations of pulsar glitches are
presented, using a Gross-Pitaevskii model of the crust-superfluid system in the
presence of pinning. Power-law distributions of simulated glitch sizes are
obtained, in accord with astronomical observations, with exponents ranging from
-0.55 to -1.26. Examples of large-scale simulations, containing
vortices, reveal that these statistics persist in the many-vortex limit.
Waiting-time distributions are also constructed. These and other statistics
support the hypothesis that catastrophic unpinning mediated by collective
vortex motion produces glitches; indeed, such collective events are seen in
time-lapse movies of superfluid density. Three principal trends are observed.
(1) The glitch rate scales proportional to the electromagnetic spin-down
torque. (2) A strong positive correlation is found between the strength of
vortex pinning and mean glitch size. (3) The spin-down dynamics depend less on
the pinning site abundance once the latter exceeds one site per vortex,
suggesting that unpinned vortices travel a distance comparable to the
inter-vortex spacing before repinning.Comment: 24 pages, 19 figures, accepted for publication in MNRA
The effect of realistic equations of state and general relativity on the "snowplow" model for pulsar glitches
Many pulsars are observed to "glitch", i.e. show sudden jumps in their
rotational frequency , some of which can be as large as in a subset of pulsars known as giant
glitchers. Recently Pizzochero (2011) has shown that an analytic model based on
realistic values for the pinning forces in the crust and for the angular
momentum transfer in the star can describe the average properties of giant
glitches, such as the inter-glitch waiting time, the step in frequency and that
in frequency derivative. In this paper we extend the model (originally
developed in Newtonian gravity and for a polytropic equation of state) to
realistic backgrounds obtained by integrating the relativistic equations of
stellar structure and using physically motivated equations of state to describe
matter in the neutron star. We find that this more detailed treatment still
reproduces the main features of giant glitches in the Vela pulsar and allows us
to set constraints on the equation of state. In particular we find that stiffer
equations of state are favoured and that it is unlikely that the Vela pulsar
has a high mass (larger than ).Comment: 15 pages, 8 figures, submitted to MNRA
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