738 research outputs found
Constraints on the Formation and Evolution of Circumstellar Disks in Rotating Magnetized Cloud Cores
We use magnetic collapse models to place some constraints on the formation
and angular momentum evolution of circumstellar disks which are embedded in
magnetized cloud cores. Previous models have shown that the early evolution of
a magnetized cloud core is governed by ambipolar diffusion and magnetic
braking, and that the core takes the form of a nonequilibrium flattened
envelope which ultimately collapses dynamically to form a protostar. In this
paper, we focus on the inner centrifugally-supported disk, which is formed only
after a central protostar exists, and grows by dynamical accretion from the
flattened envelope. We estimate a centrifugal radius for the collapse of mass
shells within a rotating, magnetized cloud core. The centrifugal radius of the
inner disk is related to its mass through the two important parameters
characterizing the background medium: the background rotation rate \Omb and
the background magnetic field strength \Bref. We also revisit the issue of
how rapidly mass is deposited onto the disk (the mass accretion rate) and use
several recent models to comment upon the likely outcome in magnetized cores.
Our model predicts that a significant centrifugal disk (much larger than a
stellar radius) will be present in the very early (Class 0) stage of
protostellar evolution. Additionally, we derive an upper limit for the disk
radius as it evolves due to internal torques, under the assumption that the
star-disk system conserves its mass and angular momentum even while most of the
mass is transferred to a central star.Comment: 23 pages, 1 figure, aastex, to appear in the Astrophysical Journal
(10 Dec 1998
Evaluation of the LEP Centre-of-Mass Energy Above the W-Pair Production Threshold
Knowledge of the centre-of-mass energy at LEP2 is of primary importance to
set the absolute energy scale for the measurement of the W-boson mass. The beam
energy above 80 GeV is derived from continuous measurements of the magnetic
bending field by 16 NMR probes situated in a number of the LEP dipoles. The
relationship between the fields measured by the probes and the beam energy is
calibrated against precise measurements of the average beam energy between 41
and 55 GeV made using the resonant depolarisation technique. The linearity of
the relationship is tested by comparing the fields measured by the probes with
the total bending field measured by a flux loop. This test results in the
largest contribution to the systematic uncertainty. Several further corrections
are applied to derive the the centre-of-mass energies at each interaction
point. In addition the centre-of-mass energy spread is evaluated. The beam
energy has been determined with a precision of 25 MeV for the data taken in
1997, corresponding to a relative precision of 2.7x10^{-4}. This is small in
comparison to the present uncertainty on the W mass measurement at LEP.
However, the ultimate statistical precision on the W mass with the full LEP2
data sample should be around 25 MeV, and a smaller uncertainty on the beam
energy is desirable. Prospects for improvements are outlined.Comment: 24 pages, 10 figures, Latex, epsfig; replaced by version accepted by
European Physical Journal
Stability of general-relativistic accretion disks
Self-gravitating relativistic disks around black holes can form as transient
structures in a number of astrophysical scenarios such as binary neutron star
and black hole-neutron star coalescences, as well as the core-collapse of
massive stars. We explore the stability of such disks against runaway and
non-axisymmetric instabilities using three-dimensional hydrodynamics
simulations in full general relativity using the THOR code. We model the disk
matter using the ideal fluid approximation with a -law equation of
state with . We explore three disk models around non-rotating black
holes with disk-to-black hole mass ratios of 0.24, 0.17 and 0.11. Due to metric
blending in our initial data, all of our initial models contain an initial
axisymmetric perturbation which induces radial disk oscillations. Despite these
oscillations, our models do not develop the runaway instability during the
first several orbital periods. Instead, all of the models develop unstable
non-axisymmetric modes on a dynamical timescale. We observe two distinct types
of instabilities: the Papaloizou-Pringle and the so-called intermediate type
instabilities. The development of the non-axisymmetric mode with azimuthal
number m = 1 is accompanied by an outspiraling motion of the black hole, which
significantly amplifies the growth rate of the m = 1 mode in some cases.
Overall, our simulations show that the properties of the unstable
non-axisymmetric modes in our disk models are qualitatively similar to those in
Newtonian theory.Comment: 30 pages, 21 figure
Quasi-Periodic Occultation by a Precessing Accretion Disk and Other Variabilities of SMC X-1
We have investigated the variability of the binary X-ray pulsar, SMC X-1, in
data from several X-ray observatories. We confirm the ~60-day cyclic variation
of the X-ray flux in the long-term monitoring data from the RXTE and CGRO
observatories. X-ray light curves and spectra from the ROSAT, Ginga, and ASCA
observatories show that the uneclipsed flux varies by as much as a factor of
twenty between a high-flux state when 0.71 second pulses are present and a
low-flux state when pulses are absent. In contrast, during eclipses when the
X-rays consist of radiation scattered from circumsource matter, the fluxes and
spectra in the high and low states are approximately the same. These
observations prove that the low state of SMC X-1 is not caused by a reduction
in the intrinsic luminosity of the source, or a spectral redistribution
thereof, but rather by a quasi-periodic blockage of the line of sight, most
likely by a precessing tilted accretion disk. In each of two observations in
the midst of low states a brief increase in the X-ray flux and reappearance of
0.71 second pulses occurred near orbital phase 0.2. These brief increases
result from an opening of the line of sight to the pulsar that may be caused by
wobble in the precessing accretion disk. The records of spin up of the neutron
star and decay of the binary orbit are extended during 1991-1996 by
pulse-timing analysis of ROSAT, ASCA, and RXTE PCA data. The pulse profiles in
various energy ranges from 0.1 to >21 keV are well represented as a combination
of a pencil beam and a fan beam. Finally, there is a marked difference between
the power spectra of random fluctuations in the high-state data from the RXTE
PCA below and above 3.4 keV. Deviation from the fitted power law around 0.06 Hz
may be QPO.Comment: Accepted to ApJ. 33 pages including 11 figure
Indication of the ferromagnetic instability in a dilute two-dimensional electron system
The magnetic field B_c, in which the electrons become fully spin-polarized,
is found to be proportional to the deviation of the electron density from the
zero-field metal-insulator transition in a two-dimensional electron system in
silicon. The tendency of B_c to vanish at a finite electron density suggests a
ferromagnetic instability in this strongly correlated electron system.Comment: 4 pages, postscript figures included. Revised versio
Making predictions in the multiverse
I describe reasons to think we are living in an eternally inflating
multiverse where the observable "constants" of nature vary from place to place.
The major obstacle to making predictions in this context is that we must
regulate the infinities of eternal inflation. I review a number of proposed
regulators, or measures. Recent work has ruled out a number of measures by
showing that they conflict with observation, and focused attention on a few
proposals. Further, several different measures have been shown to be
equivalent. I describe some of the many nontrivial tests these measures will
face as we learn more from theory, experiment, and observation.Comment: 20 pages, 3 figures; invited review for Classical and Quantum
Gravity; v2: references improve
Anomalous Dimensions of Non-Chiral Operators from AdS/CFT
Non-chiral operators with positive anomalous dimensions can have interesting
applications to supersymmetric model building. Motivated by this, we develop a
new method for obtaining the anomalous dimensions of non-chiral double-trace
operators in N=1 superconformal field theories (SCFTs) with weakly-coupled AdS
duals. Via the Hamiltonian formulation of AdS/CFT, we show how to directly
compute the anomalous dimension as a bound state energy in the gravity dual.
This simplifies previous approaches based on the four-point function and the
OPE. We apply our method to a class of effective AdS5 supergravity models, and
we find that the binding energy can have either sign. If such models can be UV
completed, they will provide the first calculable examples of SCFTs with
positive anomalous dimensions.Comment: 38 pages, 2 figures, refs adde
Holographic models of de Sitter QFTs
We describe the dynamics of strongly coupled field theories in de Sitter
spacetime using the holographic gauge/gravity duality. The main motivation for
this is to explore the possibility of dynamical phase transitions during
cosmological evolution. Specifically, we study two classes of theories: (i)
conformal field theories on de Sitter in the static patch which are maintained
in equilibrium at temperatures that may differ from the de Sitter temperature
and (ii) confining gauge theories on de Sitter spacetime. In the former case we
show the such states make sense from the holographic viewpoint in that they
have regular bulk gravity solutions. In the latter situation we add to the
evidence for a confinement/deconfinement transition for a large N planar gauge
theory as the cosmological acceleration is increased past a critical value. For
the field theories we study, the critical acceleration corresponds to a de
Sitter temperature which is less than the Minkowski space deconfinement
transition temperature by a factor of the spacetime dimension.Comment: 35 pages, LaTeX, 4 figures, v2: refs adde
Waiting list registration for kidney transplants must improve
Onderzoeken hoe de samenstelling van de wachtlijst voor postmortale-niertransplantaties zich heeft ontwikkeld en of de wachtlijst de werkelijke behoefte weergeeft.OBJECTIVE: To investigate how the composition of the waiting list for postmortem kidney transplant has developed, and whether the waiting list reflects actual demand.DESIGN: Retrospective research and cohort study.METHOD: We used data from the period 2000-2014 from the Dutch Transplant Foundation, 'RENINE' and Eurotransplant. This concerned data on postmortem kidney donation, live donor transplants, the waiting list and kidney transplantation.RESULTS: The postmortem kidney transplant waiting list included transplantable (T) and non-transplantable (NT) patients. The number of T-patients declined from 1271 in 2000 to 650 in 2014, and the median waiting time between the start of dialysis and postmortem kidney transplant decreased from 4.1 years in 2006 to 3.1 years in 2014. The total number of patients on the waiting list, however, increased from 2263 in 2000 to 2560 in 2014 and in the same period the number of new patient registrations increased from 772 to 1212. In about 80% of the NT-patients the reason for their NT status was not registered. A cohort analysis showed that NT-patients have a 2-times lower chance of a postmortem kidney transplant and a 2-times higher chance of leaving the waiting list without transplantation or of live-donor transplantation.CONCLUSION: The demand for donor kidneys remains high. The increased number of transplants resulted in a declining waiting list for T-patients while the total waiting list is getting longer. Waiting list registration and maintenance need to be improved, to give better insight into the real demand.</p
Sharply increasing effective mass: a precursor of the spontaneous spin polarization in a dilute two-dimensional electron system
We have measured the effective mass, m, and Lande g-factor in very dilute
two-dimensional electron systems in silicon. Two independent methods have been
used: (i) measurements of the magnetic field required to fully polarize the
electrons' spins and (ii) analysis of the Shubnikov-de Haas oscillations. We
have observed a sharp increase of the effective mass with decreasing electron
density while the g-factor remains nearly constant and close to its value in
bulk silicon. The corresponding strong rise of the spin susceptibility may be a
precursor of a spontaneous spin polarization; unlike in the Stoner scenario, it
originates from the enhancement of the effective mass rather than the increase
of g-factor. Furthermore, using tilted magnetic fields, we have found that the
enhanced effective mass is independent of the degree of spin polarization and,
therefore, its increase is not related to spin exchange effects, in
contradiction with existing theories. Our results show that the dilute 2D
electron system in silicon behaves well beyond a weakly interacting Fermi
liquid.Comment: This paper summarizes results reported in our recent publications on
the subjec
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