62,474 research outputs found
Disk heating by more than one spiral density wave
We consider a differentially rotating, 2D stellar disk perturbed by two
steady state spiral density waves moving at different patterns speeds. Our
investigation is based on direct numerical integration of initially circular
test-particle orbits. We examine a range of spiral strengths and spiral speeds
and show that stars in this time dependent gravitational field can be heated
(their random motions increased).This is particularly noticeable in the
simultaneous propagation of a 2-armed spiral density wave near the corotation
resonance (CR), and a weak 4-armed one near the inner and outer 4:1 Lindblad
resonances. In simulations with 2 spiral waves moving at different pattern
speeds we find: (1) the variance of the radial velocity, sigma_R^2, exceeds the
sum of the variances measured from simulations with each individual pattern;
(2) sigma_R^2 can grow with time throughout the entire simulation; (3)
sigma_R^2 is increased over a wider range of radii compared to that seen with
one spiral pattern; (4) particles diffuse radially in real space whereas they
don't when only one spiral density wave is present. Near the CR with the
stronger, 2-armed pattern, test particles are observed to migrate radially.
These effects take place at or near resonances of both spirals so we interpret
them as the result of stochastic motions. This provides a possible new
mechanism for increasing the stellar velocity dispersion in galactic disks. If
multiple spiral patterns are present in the Galaxy we predict that there should
be large variations in the stellar velocity dispersion as a function of radius.Comment: 20 pages, 13 figures. Submitted to MNRA
Travelling waves in a tissue interaction model for skin pattern formation
Tissue interaction plays a major role in many morphogenetic processes, particularly those associated with skin organ primordia. We examine travelling wave solutions in a tissue interaction model for skin pattern formation which is firmly based on the known biology. From a phase space analysis we conjecture the existence of travelling waves with specific wave speeds. Subsequently, analytical approximations to the wave profiles are derived using perturbation methods. We then show numerically that such travelling wave solutions do exist and that they are in good agreement with our analytical results. Finally, the biological implications of our analysis are discussed
Supersonic turbulent boundary-layer flows with mass injection through slots and/or porous walls
An implicit finite-difference method was used to solve the compressible boundary-layer equations, and to study the effects of mass transfer through porous plates, slots, and a combination of the two. The effects of the external pressure field were also included by using a global pressure interaction scheme. Two different eddy viscosity models were used for the slot and slot-porous combination cases: one was a two-layer model with inner and outer laws, and the other was a multi-layer model with as many as five separate layers. Results of the present method were compared with experimental data at a Mach number of 2.8. Comparisons of the skin friction reduction and Mach number profiles gave good to excellent agreement. Pressure interaction had little effect on the slot injection skin friction but increased the skin friction of the porous and slot-porous combination markedly
Study of the application of perceptrons for prediction of solar flares. solar flare forecasting with a recognizing automaton final report, phase ii
Perceptrons for solar flare prediction with pattern recognition automato
The Impact of Early Positive Results on a Mathematics and Science Partnership: The Experience of the Institute for Chemistry Literacy Through Computational Science
After one year of implementation, the Institute for Chemistry Literacy through Computational Science, an NSF Mathematics and Science Partnership Institute Project led by the University of Illinois at Urbana-Champaignâs Department of Chemistry, College of Medicine, and National Center for Supercomputing Applications, experienced statistically signiïŹcant gains in chemistry content knowledge among students of the rural high school teachers participating in its intensive, year-round professional development course, compared to a control group. The project utilizes a two-cohort, delayed-treatment, random control trial, quasi-experimental research design with the second cohort entering treatment one year following the ïŹrst. The three-year treatment includes intensive two-week summer institutes, occasional school year workshops and year-round, on-line collaborative lesson development, resource sharing, and expert support. The means of student pre-test scores for Cohort I (η=963) and Cohort II (η=862) teachers were not signiïŹcantly different. The mean gain (difference between pre-test and post-test scores) after seven months in the classroom for Cohort I was 9.8 percentage points, compared to 6.7 percentage points for Cohort II. This statistically signiïŹcant difference (p\u3c.001) represented an effect size of .25 standard deviation units, and indicated unusually early conïŹrmation of treatment effects. When post-tests were compared, Cohort I students scored signiïŹcantly higher than Cohort II and supported the gain score differences. The impact of these results on treatment and research plans is discussed. concentrating on the effect of lessening rural teachersâ isolation and increasing access to tools to facilitate learning
Electronic transport in films of colloidal CdSe nanocrystals
We present results for electronic transport measurements on large
three-dimensional arrays of CdSe nanocrystals. In response to a step in the
applied voltage, we observe a power-law decay of the current over five orders
of magnitude in time. Furthermore, we observe no steady-state dark current for
fields up to 10^6 V/cm and times as long as 2x10^4 seconds. Although the
power-law form of the decay is quite general, there are quantitative variations
with temperature, applied field, sample history, and the material parameters of
the array. Despite evidence that the charge injected into the film during the
measurement causes the decay of current, we find field-scaling of the current
at all times. The observation of extremely long-lived current transients
suggests the importance of long-range Coulomb interactions between charges on
different nanocrystals.Comment: 11 pages, 10 figure
Nuclear effects in photoproduction of heavy quarks and vector mesons in ultraperipheral PbPb and pPb collisions at the LHC
The comparison of photoproduction cross sections for and b-b(bar)
in PbPb and pPb collisions can give sensitivity to nuclear shadowing effects.
The photoproduction of vector mesons is even more sensitive to the underlying
gluon distributions. In this study we present the cross sections and rapidity
dependence of the photoproduction of heavy quarks and exclusive production of
vector mesons in ultraperipheral pPb and PbPb collisions at the Large Hadron
Collider at sqrt(s_NN)=5 TeV and sqrt(s_NN)=2.76J/\psi\Upsilon$ in
PbPb collisions in particular exhibit very good sensitivity to gluon shadowing.Comment: 4 pages, 4 figure
Role of Schizosaccharomyces pombe RecQ homolog recombination and checkpoint genes in UV Damage tolerance
The cellular responses to DNA damage are complex and include direct DNA repair pathways that remove the damage and indirect damage responses which allow cells to survive DNA damage that has not been, or cannot be, removed. We have identified the gene mutated in the rad12.502 strain as a Schizosaccharomyces pombe recQ homolog. The same gene (designated rqh1) is also mutated in the hus2.22 mutant. We show that Rqh1 is involved in a DNA damage survival mechanism which prevents cell death when UV-induced DNA damage cannot be removed. This pathway also requires the correct functioning of the recombination machinery and the six checkpoint tad gene products plus the Cds1 kinase. Our data suggest that Rqh1 operates during S phase as part of a mechanism which prevents DNA damage causing cell lethality. This process may involve the bypass of DNA damage sites by the replication fork. Finally, in contrast with the reported literature, we do not find that rqh1 (rad12) mutant cells are defective in UV dimer endonuclease activity
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