1,482 research outputs found
Fitting the Means to the Ends: One School’s Experience with Quantitative and Qualitative Methods in Curriculum Evaluation During Curriculum Change
Curriculum evaluation plays an important role in substantive curriculum change. The experience of the University of Texas Medical Branch (UTMB) with evaluation processes developed for the new Integrated Medical Curriculum (IMC) illustrates how evaluation methods may be chosen to match the goals of the curriculum evaluation process. Quantitative data such as ratings of courses or scores on external exams are useful for comparing courses or assessing whether standards have been met. Qualitative data such as students’ comments about aspects of courses are useful for eliciting explanations of observed phenomena and describing relationships between curriculum features and outcomes. The curriculum evaluation process designed for the IMC used both types of evaluation methods in a complementary fashion. Quantitative and qualitative methods have been used for formative evaluation of the new IMC courses. They are now being incorporated into processes to judge the IMC against its goals and objectives
Power Law Distribution of Wealth in a Money-Based Model
A money-based model for the power law distribution (PLD) of wealth in an
economically interacting population is introduced. The basic feature of our
model is concentrating on the capital movements and avoiding the complexity of
micro behaviors of individuals. It is proposed as an extension of the Equiluz
and Zimmermann's (EZ) model for crowding and information transmission in
financial markets. Still, we must emphasize that in EZ model the PLD without
exponential correction is obtained only for a particular parameter, while our
pattern will give it within a wide range. The Zipf exponent depends on the
parameters in a nontrivial way and is exactly calculated in this paper.Comment: 5 pages and 4 figure
A Toomre-like stability criterion for the clumpy and turbulent interstellar medium
We explore the gravitational instability of clumpy and turbulent gas discs,
taking into account the Larson-type scaling laws observed in giant molecular
clouds (GMCs) and HI, as well as more general scaling relations. This degree of
freedom is of special interest in view of the coming high-z ISM surveys, and is
thus potentially important for understanding the dynamical effects of
turbulence at all epochs of galaxy evolution. Our analysis shows that
turbulence has a deep impact on the gravitational instability of the disc. It
excites a rich variety of stability regimes, several of which have no classical
counterpart. Among other diagnostics, we provide two useful tools for observers
and simulators: (1) the stability map of turbulence, which illustrates our
stability scenario and relates it to the phenomenology of interstellar
turbulence: GMC/HI observations, simulations and models; (2) a Toomre-like
stability criterion, , which applies to a large class of
clumpy/turbulent discs. We make specific predictions about GMC and cold-HI
turbulence, and point out the implications of our analysis for high-z galaxy
surveys.Comment: MNRAS, in press. Moderate revision to match the accepted versio
The spiral structure of our Milky Way Galaxy
The spiral structure of our Milky Way Galaxy is not yet known. HII regions
and giant molecular clouds are the most prominent spiral tracers. We collected
the spiral tracer data of our Milky Way from the literature, namely, HII
regions and giant molecular clouds (GMCs). With weighting factors based on the
excitation parameters of HII regions or the masses of GMCs, we fitted the
distribution of these tracers with models of two, three, four spiral-arms or
polynomial spiral arms. The distances of tracers, if not available from stellar
or direct measurements, were estimated kinetically from the standard rotation
curve of Brand & Blitz (1993) with =8.5 kpc, and =220 km
s or the newly fitted rotation curves with =8.0 kpc and
=220 km s or =8.4 kpc and =254 km s. We
found that the two-arm logarithmic model cannot fit the data in many regions.
The three- and the four-arm logarithmic models are able to connect most
tracers. However, at least two observed tangential directions cannot be matched
by the three- or four-arm model. We composed a polynomial spiral arm model,
which can not only fit the tracer distribution but also match observed
tangential directions. Using new rotation curves with =8.0 kpc and
=220 km s and =8.4 kpc and =254 km s for
the estimation of kinematic distances, we found that the distribution of HII
regions and GMCs can fit the models well, although the results do not change
significantly compared to the parameters with the standard and
.Comment: 34 Pages, 10 Figures, 5 Tables. Accepted for publication in A&A.
Edited
Legal determinants of external finance revisited : the inverse relationship between investor protection and societal well-being
This paper investigates relationships between corporate governance traditions and quality of life as measured by a number of widely reported indicators. It provides an empirical analysis of indicators of societal health in developed economies using a classification based on legal traditions. Arguably the most widely cited work in the corporate governance literature has been the collection of papers by La Porta et al. which has shown, inter alia, statistically significant relationships between legal traditions and various proxies for investor protection. We show statistically significant relationships between legal traditions and various proxies for societal health. Our comparative evidence suggests that the interests of investors may not be congruent with the interests of wider society, and that the criteria for judging the effectiveness of approaches to corporate governance should not be restricted to financial metrics
The Quantum Hall Effect in Drag: Inter-layer Friction in Strong Magnetic Fields
We study the Coulomb drag between two spatially separated electron systems in
a strong magnetic field, one of which exhibits the quantum Hall effect. At a
fixed temperature, the drag mimics the behavior of in the quantum
Hall system, in that it is sharply peaked near the transitions between
neighboring plateaux. We assess the impact of critical fluctuations near the
transitions, and find that the low temperature behavior of the drag measures an
exponent that characterizes anomalous low frequency dissipation; the
latter is believed to be present following the work of Chalker.Comment: 13 pages, Revtex 2.0, 1 figure upon request, P-93-11-09
An Analytic Model of Angular Momentum Transport by Gravitational Torques: From Galaxies to Massive Black Holes
We present analytic calculations of angular momentum transport and gas inflow
in galaxies, from scales of ~kpc to deep in the potential of a central black
hole (BH). We compare these analytic calculations to numerical simulations and
use them to develop a sub-grid model of BH growth that can be incorporated into
semi-analytic models or cosmological simulations. Both analytic calculations
and simulations argue that the strongest torque on gas arises when
non-axisymmetric perturbations to the stellar gravitational potential produces
orbit crossings and shocks in the gas. This is true both at large radii ~0.01-1
kpc, where bar-like modes dominate the non-axisymmetric potential, and at
smaller radii <10 pc, where a lopsided/eccentric disk dominates. The
traditional orbit crossing criterion is not always adequate to predict the
locations of, and inflow due to, shocks in gas+stellar disks with finite sound
speeds. We derive a modified criterion that predicts the presence of shocks in
stellar dominated systems even absent formal orbit crossing. We then derive
analytic expressions for the loss of angular momentum and the resulting gas
inflow rates in the presence of such shocks. We test our analytic predictions
using hydrodynamic simulations at a range of galactic scales, and show that
they successfully predict the mass inflow rates and quasi-steady gas surface
densities with small scatter (0.3 dex). We use our analytic results to
construct a new estimate of the BH accretion rate given galaxy properties at
larger radii. This captures the key scalings in the numerical simulations.
Alternate estimates such as the local viscous accretion rate or the spherical
Bondi rate fail systematically to reproduce the simulations.Comment: 23 pages, 10 figures, minor revisions to match version accepted to
MNRA
The simulation of molecular clouds formation in the Milky Way
Using 3D hydrodynamic calculations we simulate formation of molecular clouds
in the Galaxy. The simulations take into account molecular hydrogen chemical
kinetics, cooling and heating processes. Comprehensive gravitational potential
accounts for contributions from the stellar bulge, two and four armed spiral
structure, stellar disk, dark halo and takes into account self-gravitation of
the gaseous component. Gas clouds in our model form in the spiral arms due to
shear and wiggle instabilities and turn into molecular clouds after t\simgt
100 Myr. At the times Myr the clouds form hierarchical
structures and agglomerations with the sizes of 100 pc and greater. We analyze
physical properties of the simulated clouds and find that synthetic statistical
distributions like mass spectrum, "mass-size" relation and velocity dispersion
are close to those observed in the Galaxy. The synthetic (galactic
longitude - radial velocity) diagram of the simulated molecular gas
distribution resembles observed one and displays a structure with appearance
similar to Molecular Ring of the Galaxy. Existence of this structure in our
modelling can be explained by superposition of emission from the galactic bar
and the spiral arms at 3-4 kpc.Comment: 10 pages, 8 figure
The Milky Way in X-rays for an outside observer: Log(N)-Log(S) and Luminosity Function of X-ray binaries from RXTE/ASM data
We study the Log(N)-Log(S) and X-ray luminosity function in the 2-10 keV
energy band, and the spatial (3-D) distribution of bright, log(L_X) > 34-35
erg/s, X-ray binaries in the Milky Way. In agreement with theoretical
expectations and earlier results we found significant differences between the
spatial distributions of low (LMXB) and high (HMXB) mass X-ray binaries. The
volume density of LMXB sources peaks strongly at the Galactic Bulge. HMXBs tend
to avoid the inner 3-4 kpc of the Galaxy, HMXBs are more concentrated towards
the Galactic Plane and show clear signatures of the spiral structure in their
spatial distribution. LMXB sources have a flatter Log(N)-Log(S) distribution
and luminosity function than HMXBs. The integrated 2-10 keV luminosities of
X-ray binaries, averaged over 1996--2000, are 2-3 * 10^39 (LMXB) and 2-3 *
10^38 (HMXB) erg/s. Normalised to the stellar mass and the star formation rate,
respectively, these correspond to 5 * 10^28 erg/s/M_sol for LMXBs and 5 * 10^37
erg/s/(M_sol/yr) for HMXBs. Due to the shallow slopes of the luminosity
functions the integrated emission of X-ray binaries is dominated by the 5-10
most luminous sources which determine the appearance of the Milky Way in the
standard X-ray band for an outside observer. In particular variability of
individual sources or an outburst of a bright transient source can increase the
integrated luminosity of the Milky Way by as much as a factor of ~2. Although
the average LMXB luminosity function shows a break near the Eddington
luminosity for a 1.4 M_sol neutron star, at least 11 sources showed episodes of
super-Eddington luminosity during ASM observations. We provide the maps of
distribution of X-ray binaries in the Milky Way in various projections, which
can be compared to images of nearby galaxies taken by CHANDRA and XMM-Newton.Comment: 23 pages, 19 figures, accepted by A&
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