22 research outputs found
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
Continuous-wave gravitational radiation from pulsar glitch recovery
Nonaxisymmetric, meridional circulation inside a neutron star, excited by a
glitch and persisting throughout the post-glitch relaxation phase, emits
gravitational radiation. Here, it is shown that the current quadrupole
contributes more strongly to the gravitational wave signal than the mass
quadrupole evaluated in previous work. We calculate the signal-to-noise ratio
for a coherent search and conclude that a large glitch may be detectable by
second-generation interferometers like the Laser Interferometer
Gravitational-Wave Observatory. It is shown that the viscosity and
compressibility of bulk nuclear matter, as well as the stratification
length-scale and inclination angle of the star, can be inferred from a
gravitational wave detection in principle.Comment: 19 pages, 4 figures, accepted for publication in MNRA
Pulsar glitch recovery and the superfluidity coefficients of bulk nuclear matter
A two-component hydrodynamic model is constructed of the global superfluid
flow induced by two-component Ekman pumping during the recovery stage of a
glitch. The model successfully accounts for the quasi-exponential recovery
observed in pulsars like Vela and the "overshoot" observed in pulsars like the
Crab. By fitting the model to high-resolution timing data, three important
constitutive coefficients in bulk nuclear matter can be extracted: the shear
viscosity, the mutual friction parameter, and the charged fluid fraction. The
fitted coefficients for the Crab and Vela are compared with theoretical
predictions for several equations of state, including the color-flavor locked
and two-flavor color superconductor phases of quark matter.Comment: 19 pages, 12 figure
A study of 315 glitches in the rotation of 102 pulsars
The rotation of more than 700 pulsars has been monitored using the 76-m
Lovell Telescope at Jodrell Bank. Here we report on a new search for glitches
in the observations, revealing 128 new glitches in the rotation of 63 pulsars.
Combining these new data with those already published we present a database
containing 315 glitches in 102 pulsars. The database was used to study the
glitch activity among the pulsar population, finding that it peaks for pulsars
with a characteristic age tau_c ~ 10kyr and decreases for longer values of
tau_c, disappearing for objects with tau_c > 20Myr. The glitch activity is also
smaller in the very young pulsars (tau_c <~ 1kyr). The cumulative effect of
glitches, a collection of instantaneous spin up events, acts to reduce the
regular long term spindown rate |nudot| of the star. The percentage of |nudot|
reversed by glitch activity was found to vary between 0.5% and 1.6% for pulsars
with spindown rates |nudot| between 10^(-14) and 3.2*10^(-11) Hz/s, decreasing
to less than 0.01% at both higher and lower spindown rates. These ratios are
interpreted in terms of the amount of superfluid involved in the generation of
glitches. In this context the activity of the youngest pulsar studied, the Crab
pulsar, may be explained by quake-like activity within the crust. Pulsars with
low spindown rates seem to exhibit mostly small glitches, matching well the
decrease of their crustal superfluid.
Through the analysis of glitch sizes it was found that the particular
glitching behaviour of PSR J0537-6910 and the Vela pulsar may be shared by most
Vela-like pulsars. These objects present most of their glitches with
characteristic frequency and frequency derivative jumps, occurring at regular
intervals of time. Their behaviour is different from other glitching pulsars of
similar characteristic age.Comment: 26 pages, 17 figures, 6 tables. Accepted for publication in MNRA
Magnetic Field Generation in Stars
Enormous progress has been made on observing stellar magnetism in stars from
the main sequence through to compact objects. Recent data have thrown into
sharper relief the vexed question of the origin of stellar magnetic fields,
which remains one of the main unanswered questions in astrophysics. In this
chapter we review recent work in this area of research. In particular, we look
at the fossil field hypothesis which links magnetism in compact stars to
magnetism in main sequence and pre-main sequence stars and we consider why its
feasibility has now been questioned particularly in the context of highly
magnetic white dwarfs. We also review the fossil versus dynamo debate in the
context of neutron stars and the roles played by key physical processes such as
buoyancy, helicity, and superfluid turbulence,in the generation and stability
of neutron star fields.
Independent information on the internal magnetic field of neutron stars will
come from future gravitational wave detections. Thus we maybe at the dawn of a
new era of exciting discoveries in compact star magnetism driven by the opening
of a new, non-electromagnetic observational window.
We also review recent advances in the theory and computation of
magnetohydrodynamic turbulence as it applies to stellar magnetism and dynamo
theory. These advances offer insight into the action of stellar dynamos as well
as processes whichcontrol the diffusive magnetic flux transport in stars.Comment: 41 pages, 7 figures. Invited review chapter on on magnetic field
generation in stars to appear in Space Science Reviews, Springe
The STRESS-NL database: A resource for human acute stress studies across the Netherlands
Stress initiates a cascade of (neuro)biological, physiological, and behavioral changes, allowing us to respond to a challenging environment. The human response to acute stress can be studied in detail in controlled settings, usually in a laboratory environment. To this end, many studies employ acute stress paradigms to probe stress-related outcomes in healthy and patient populations. Though valuable, these studies in themselves often have relatively limited sample sizes. We established a data-sharing and collaborative interdisciplinary initiative, the STRESS-NL database, which combines (neuro)biological, physiological, and behavioral data across many acute stress studies in order to accelerate our understanding of the human acute stress response in health and disease (www.stressdatabase.eu). Researchers in the stress field from 12 Dutch research groups of 6 Dutch universities created a database to achieve an accurate inventory of (neuro)biological, physiological, and behavioral data from laboratory-based human studies that used acute stress tests. Currently, the STRESS-NL database consists of information on 5529 individual participants (2281 females and 3348 males, age range 6-99 years, mean age 27.7 ± 16 years) stemming from 57 experiments described in 42 independent studies. Studies often did not use the same stress paradigm; outcomes were different and measured at different time points. All studies currently included in the database assessed cortisol levels before, during and after experimental stress, but cortisol measurement will not be a strict requirement for future study inclusion. Here, we report on the creation of the STRESS-NL database and infrastructure to illustrate the potential of accumulating and combining existing data to allow meta-analytical, proof-of-principle analyses. The STRESS-NL database creates a framework that enables human stress research to take new avenues in explorative and hypothesis-driven data analyses with high statistical power. Future steps could be to incorporate new studies beyond the borders of the Netherlands; or build similar databases for experimental stress studies in rodents. In our view, there are major scientific benefits in initiating and maintaining such international efforts
Gravitational waves from single neutron stars: an advanced detector era survey
With the doors beginning to swing open on the new gravitational wave
astronomy, this review provides an up-to-date survey of the most important
physical mechanisms that could lead to emission of potentially detectable
gravitational radiation from isolated and accreting neutron stars. In
particular we discuss the gravitational wave-driven instability and
asteroseismology formalism of the f- and r-modes, the different ways that a
neutron star could form and sustain a non-axisymmetric quadrupolar "mountain"
deformation, the excitation of oscillations during magnetar flares and the
possible gravitational wave signature of pulsar glitches. We focus on progress
made in the recent years in each topic, make a fresh assessment of the
gravitational wave detectability of each mechanism and, finally, highlight key
problems and desiderata for future work.Comment: 39 pages, 12 figures, 2 tables. Chapter of the book "Physics and
Astrophysics of Neutron Stars", NewCompStar COST Action 1304. Minor
corrections to match published versio