3,417 research outputs found
Contextualised problem-based approach for teaching undergraduate database module
In this paper, a new approach has been used in teaching the second year undergraduate database module. The approach is a combination of contextualisation, problem-based approach, group work and continuous formative assessment. The contextualisation ensures the visibility of teaching/learning activities so that students are aware of the values of activities and how they can fit into a big picture. Problem-based approach gives the students tasks/problems to solve before the relevant lecture takes place, hence can better develop effective reasoning processes, independently learning skills and improve motivation and engagement. Group work is regularly used due to the diversity of student backgrounds and level of prior knowledge of certain topics. By having group work, students can learn from each other and easily clarify confusions among themselves before approaching the lecturer. This gives the lecture more time focusing on common issues. Formative assessment has also been used to support teaching/learning activities and to reinforce their understanding. The work in this paper has been evaluated via an end-of-year online module survey. The results show good effectiveness of the new approach, although there are still spaces for improvement
First Order Feynman-Kac Formula
We study the parabolic integral kernel associated with the weighted Laplacian
and the Feynman-Kac kernels. For manifold with a pole we deduce formulas and
estimates for them and for their derivatives, given in terms of a Gaussian term
and the semi-classical bridge. Assumptions are on the Riemannian data.Comment: 31 pages, to appear in `Stochastic Processes and their Applications
Wind-driven Accretion in Protoplanetary Disks. I: Suppression of the Magnetorotational Instability and Launching of the Magnetocentrifugal Wind
We perform local, vertically stratified shearing-box MHD simulations of
protoplanetary disks (PPDs) at a fiducial radius of 1 AU that take into account
the effects of both Ohmic resistivity and ambipolar diffusion (AD). The
magnetic diffusion coefficients are evaluated self-consistently from a look-up
table based on equilibrium chemistry. We first show that the inclusion of AD
dramatically changes the conventional picture of layered accretion. Without net
vertical magnetic field, the system evolves into a toroidal field dominated
configuration with extremely weak turbulence in the far-UV ionization layer
that is far too inefficient to drive rapid accretion. In the presence of a weak
net vertical field (plasma beta~10^5 at midplane), we find that the MRI is
completely suppressed, resulting in a fully laminar flow throughout the
vertical extent of the disk. A strong magnetocentrifugal wind is launched that
efficiently carries away disk angular momentum and easily accounts for the
observed accretion rate in PPDs. Moreover, under a physical disk wind geometry,
all the accretion flow proceeds through a strong current layer with thickness
of ~0.3H that is offset from disk midplane with radial velocity of up to 0.4
times the sound speed. Both Ohmic resistivity and AD are essential for the
suppression of the MRI and wind launching. The efficiency of wind transport
increases with increasing net vertical magnetic flux and the penetration depth
of the FUV ionization. Our laminar wind solution has important implications on
planet formation and global evolution of PPDs.Comment: 23 pages, 13 figures, accepted to Ap
Dynamics of Solids in the Midplane of Protoplanetary Disks: Implications for Planetesimal Formation
(Abridged) We present local 2D and 3D hybrid numerical simulations of
particles and gas in the midplane of protoplanetary disks (PPDs) using the
Athena code. The particles are coupled to gas aerodynamically, with
particle-to-gas feedback included. Magnetorotational turbulence is ignored as
an approximation for the dead zone of PPDs, and we ignore particle self-gravity
to study the precursor of planetesimal formation. Our simulations include a
wide size distribution of particles, ranging from strongly coupled particles
with dimensionless stopping time tau_s=Omega t_stop=1e-4 to marginally coupled
ones with tau_s=1 (where Omega is the orbital frequency, t_stop is the particle
friction time), and a wide range of solid abundances. Our main results are: 1.
Particles with tau_s>=0.01 actively participate in the streaming instability,
generate turbulence and maintain the height of the particle layer before
Kelvin-Helmholtz instability is triggered. 2. Strong particle clumping as a
consequence of the streaming instability occurs when a substantial fraction of
the solids are large (tau_s>=0.01) and when height-integrated solid to gas mass
ratio Z is super-solar. 3. The radial drift velocity is reduced relative to the
conventional Nakagawa-Sekiya-Hayashi (NSH) model, especially at high Z. We
derive a generalized NSH equilibrium solution for multiple particle species
which fits our results very well. 4. Collision velocity between particles with
tau_s>=0.01 is dominated by differential radial drift, and is strongly reduced
at larger Z. 5. There exist two positive feedback loops with respect to the
enrichment of local disk solid abundance and grain growth. All these effects
promote planetesimal formation.Comment: 25 pages (emulate apj), accepted to Ap
Can the ANITA anomalous events be due to new physics?
The ANITA collaboration has observed two ultra-high-energy upgoing air shower
events that cannot originate from Standard Model neutrinos that have traversed
the Earth. Several beyond-the-standard-model physics scenarios have been
proposed as explanations for these events. In this paper we present some
general arguments making it challenging for new physics to explain the events.
One exceptional class of models that could work is pointed out, in which
metastable dark matter decays to a highly boosted lighter dark matter particle,
that can interact in the Earth to produce the observed events.Comment: 12 pages, 5 figure
An optimistic CoGeNT analysis
Inspired by a recently proposed model of millicharged atomic dark matter
(MADM), we analyze several classes of light dark matter models with respect to
CoGeNT modulated and unmodulated data, and constraints from CDMS, XENON10 and
XENON100. After removing the surface contaminated events from the original
CoGeNT data set, we find an acceptable fit to all these data (but with the
modulating part of the signal making a statistically small contribution), using
somewhat relaxed assumptions about the response of the null experiments at low
recoil energies, and postulating an unknown modulating background in the CoGeNT
data at recoil energies above 1.5 keVee. We compare the fits of MADM---an
example of inelastic magnetic dark matter---to those of standard elastically
and inelastically scattering light WIMPs (eDM and iDM). The iDM model gives the
best fit, with MADM close behind. The dark matter interpretation of the DAMA
annual modulation cannot be made compatible with these results however. We find
that the inclusion of a tidal debris component in the dark matter phase space
distribution improves the fits or helps to relieve tension with XENON
constraints.Comment: 16 pages, 11 figures; v2: added discussion of dipole-dipole
scattering and details of matrix elements; also implications of XENON100 2012
limits. Qualitative conclusions unchanged. Published versio
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