8,128 research outputs found
Structural parameters for globular clusters in M31 and generalizations for the fundamental plane
The structures of globular clusters (GCs) reflect their dynamical states and
past histories. High-resolution imaging allows the exploration of morphologies
of clusters in other galaxies. Surface brightness profiles from new Hubble
Space Telescope observations of 34 globular clusters in M31 are presented,
together with fits of several different structural models to each cluster. M31
clusters appear to be adequately fit by standard King models, and do not
obviously require alternate descriptions with relatively stronger halos, such
as are needed to fit many GCs in other nearby galaxies. The derived structural
parameters are combined with corrected versions of those measured in an earlier
survey to construct a comprehensive catalog of structural and dynamical
parameters for M31 GCs with a sample size similar to that for the Milky Way.
Clusters in M31, the Milky Way, Magellanic Clouds, Fornax dwarf spheroidal and
NGC 5128 define a very tight fundamental plane with identical slopes. The
combined evidence for these widely different galaxies strongly reinforces the
view that old globular clusters have near-universal structural properties
regardless of host environment.Comment: AJ in press; 59 pages including 16 figure
Neutrino-Lepton Masses, Zee Scalars and Muon g-2
Evidence for neutrino oscillations is pointing to the existence of tiny but
finite neutrino masses. Such masses may be naturally generated via radiative
corrections in models such as the Zee model where a singlet Zee-scalar plays a
key role. We minimally extend the Zee model by including a right-handed singlet
neutrino \nu_R. The radiative Zee-mechanism can be protected by a simple U(1)_X
symmetry involving only the \nu_R and a Zee-scalar. We further construct a
class of models with a single horizontal U(1)_FN (a la Frogatt-Nielsen) such
that the mass patterns of the neutrinos and leptons are naturally explained. We
then analyze the muon anomalous magnetic moment (g-2) and the flavor changing
\mu --> e\gamma decay. The \nu_R interaction in our minimal extension is found
to induce the BNL g-2 anomaly, with a light charged Zee-scalar of mass 100-300
GeV.Comment: Version for Phys. Rev. Lett. (typos corrected, minor refinements
Using a conceptual framework during learning attenuates the loss of expert-type knowledge structure
BACKGROUND: During evolution from novice to expert, knowledge structure develops into an abridged network organized around pathophysiological concepts. The objectives of this study were to examine the change in knowledge structure in medical students in one year and to investigate the association between the use of a conceptual framework (diagnostic scheme) and long-term knowledge structure. METHODS: Medical students' knowledge structure of metabolic alkalosis was studied after instruction and one year later using concept-sorting. Knowledge structure was labeled 'expert-type' if students shared ≥ 2 concepts with experts and 'novice-type' if they shared < 2 concepts. Conditional logistic regression was used to study the association between short-term knowledge structure, the use of a diagnostic scheme and long-term knowledge structure. RESULTS: Thirty-four medical students completed the concept-sorting task on both occasions. Twenty-four used a diagnostic scheme for metabolic alkalosis. Short-term knowledge structure was not a correlate of long-term knowledge structure, whereas use of a diagnostic scheme was associated with increased odds of expert-type long-term knowledge structure (odds ratio 12.6 [1.4, 116.0], p = 0.02). There was an interaction between short-term knowledge structure and the use of a diagnostic scheme. In the group who did not use a diagnostic scheme the number of students changing from expert-type to novice-type was greater than vice versa (p = 0.046). There was no significant change in the group that used the diagnostic scheme (p = 0.6). CONCLUSION: The use of a diagnostic scheme by students may attenuate the loss of expert-type knowledge structure
A universal velocity distribution of relaxed collisionless structures
Several general trends have been identified for equilibrated,
self-gravitating collisionless systems, such as density or anisotropy profiles.
These are integrated quantities which naturally depend on the underlying
velocity distribution function (VDF) of the system. We study this VDF through a
set of numerical simulations, which allow us to extract both the radial and the
tangential VDF. We find that the shape of the VDF is universal, in the sense
that it depends only on two things namely the dispersion (radial or tangential)
and the local slope of the density. Both the radial and the tangential VDF's
are universal for a collection of simulations, including controlled collisions
with very different initial conditions, radial infall simulation, and
structures formed in cosmological simulations.Comment: 13 pages, 6 figures; oversimplified analysis corrected; changed
abstract and conclusions; significantly extended discussio
The Evolution of PSR J0737-3039B and a Model for Relativistic Spin Precession
We present the evolution of the radio emission from the 2.8-s pulsar of the
double pulsar system PSR J0737-3039A/B. We provide an update on the Burgay et
al. (2005) analysis by describing the changes in the pulse profile and flux
density over five years of observations, culminating in the B pulsar's radio
disappearance in 2008 March. Over this time, the flux density decreases by
0.177 mJy/yr at the brightest orbital phases and the pulse profile evolves from
a single to a double peak, with a separation rate of 2.6 deg/yr. The pulse
profile changes are most likely caused by relativistic spin precession, but can
not be easily explained with a circular hollow-cone beam as in the model of
Clifton & Weisberg (2008). Relativistic spin precession, coupled with an
elliptical beam, can model the pulse profile evolution well. This particular
beam shape predicts geometrical parameters for the two bright orbital phases
which are consistent and similar to those derived by Breton et al. (2008).
However, the observed decrease in flux over time and B's eventual disappearance
cannot be easily explained by the model and may be due to the changing
influence of A on B.Comment: 20 pages, 18 figures, Accepted by ApJ on 2 August 201
Gravitational wave astronomy with the SKA
On a time scale of years to decades, gravitational wave (GW) astronomy will
become a reality. Low frequency (nanoHz) GWs are detectable through long-term
timing observations of the most stable pulsars. Radio observatories worldwide
are currently carrying out observing programmes to detect GWs, with data sets
being shared through the International Pulsar Timing Array project. One of the
most likely sources of low frequency GWs are supermassive black hole binaries
(SMBHBs), detectable as a background due to a large number of binaries, or as
continuous or burst emission from individual sources. No GW signal has yet been
detected, but stringent constraints are already being placed on galaxy
evolution models. The SKA will bring this research to fruition.
In this chapter, we describe how timing observations using SKA1 will
contribute to detecting GWs, or can confirm a detection if a first signal
already has been identified when SKA1 commences observations. We describe how
SKA observations will identify the source(s) of a GW signal, search for
anisotropies in the background, improve models of galaxy evolution, test
theories of gravity, and characterise the early inspiral phase of a SMBHB
system.
We describe the impact of the large number of millisecond pulsars to be
discovered by the SKA; and the observing cadence, observation durations, and
instrumentation required to reach the necessary sensitivity. We describe the
noise processes that will influence the achievable precision with the SKA. We
assume a long-term timing programme using the SKA1-MID array and consider the
implications of modifications to the current design. We describe the possible
benefits from observations using SKA1-LOW. Finally, we describe GW detection
prospects with SKA1 and SKA2, and end with a description of the expectations of
GW astronomy.Comment: 19 pages, 3 figures, to be published in: "Advancing Astrophysics with
the Square Kilometre Array", Proceedings of Science, PoS(AASKA14)03
Upper Limits On Periodic, Pulsed Radio Emission from the X-Ray Point Source in Cassiopeia A
The Chandra X-ray Observatory recently discovered an X-ray point source near
the center of Cassiopeia A, the youngest known Galactic supernova remnant. We
have conducted a sensitive search for radio pulsations from this source with
the Very Large Array, taking advantage of the high angular resolution of the
array to resolve out the emission from the remnant itself. No convincing
signatures of a dispersed, periodic source or of isolated dispersed pulses were
found, whether for an isolated or a binary source. We derive upper limits of 30
and 1.3 mJy at 327 and 1435 MHz for the phase-averaged pulsed flux density from
this source. The corresponding luminosity limits are lower than those for any
pulsar with age less than 10^4 years. The sensitivities of our search to single
pulses were 25 and 1.0 Jy at 327 and 1435 MHz. For comparison, the Crab pulsar
emits roughly 80 pulses per minute with flux densities greater than 100 Jy at
327 MHz and 8 pulses per minute with flux densities greater than 50 Jy at 1435
MHz. These limits are consistent with the suggestion that the X-ray point
source in Cas A adds to the growing number of neutron stars which are not radio
pulsars.Comment: accepted by ApJ Letter
Explicit Integration of the Full Symmetric Toda Hierarchy and the Sorting Property
We give an explicit formula for the solution to the initial value problem of
the full symmetric Toda hierarchy. The formula is obtained by the
orthogonalization procedure of Szeg\"{o}, and is also interpreted as a
consequence of the QR factorization method of Symes \cite{symes}. The sorting
property of the dynamics is also proved for the case of a generic symmetric
matrix in the sense described in the text, and generalizations of tridiagonal
formulae are given for the case of matrices with nonzero diagonals.Comment: 13 pages, Latex
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