23,772 research outputs found

### A dynamical proximity analysis of interacting galaxy pairs

Using the impulsive approximation to study the velocity changes of stars during disk-sphere collisions and a method due to Bottlinger to study the post collision orbits of stars, the formation of various types of interacting galaxies is studied as a function of the distance of closest approach between the two galaxies

### The nature of the evolution of galaxies by mergers

The merger theory for the formation of elliptical galaxies is examined by conducting a dynamical study of the expected frequency of merging galaxies on the basis of the collisional theory, using galaxy models without halos. The expected merger rates obtained on the basis of the collisional theory fall about a magnitude below the observational value in the present epoch. In the light of current observational evidence and the results obtained, a marked regularity in the formation of ellipticals is indicated, followed by secular evolution by mergers

### Fat tailed distributions for deaths in conflicts and disasters

We study the statistics of human deaths from wars, conflicts, similar
man-made conflicts as well as natural disasters. The probability distribution
of number of people killed in natural disasters as well as man made situations
show power law decay for the largest sizes, with similar exponent values.
Comparisons with natural disasters, when event sizes are measured in terms of
physical quantities (e.g., energy released in earthquake, volume of rainfall,
land area affected in forest fires, etc.) also show striking resemblances. The
universal patterns in their statistics suggest that some subtle similarities in
their mechanisms and dynamics might be responsible.Comment: 6 pages, 3 figs + 2 table

### Minimizing Running Costs in Consumption Systems

A standard approach to optimizing long-run running costs of discrete systems
is based on minimizing the mean-payoff, i.e., the long-run average amount of
resources ("energy") consumed per transition. However, this approach inherently
assumes that the energy source has an unbounded capacity, which is not always
realistic. For example, an autonomous robotic device has a battery of finite
capacity that has to be recharged periodically, and the total amount of energy
consumed between two successive charging cycles is bounded by the capacity.
Hence, a controller minimizing the mean-payoff must obey this restriction. In
this paper we study the controller synthesis problem for consumption systems
with a finite battery capacity, where the task of the controller is to minimize
the mean-payoff while preserving the functionality of the system encoded by a
given linear-time property. We show that an optimal controller always exists,
and it may either need only finite memory or require infinite memory (it is
decidable in polynomial time which of the two cases holds). Further, we show
how to compute an effective description of an optimal controller in polynomial
time. Finally, we consider the limit values achievable by larger and larger
battery capacity, show that these values are computable in polynomial time, and
we also analyze the corresponding rate of convergence. To the best of our
knowledge, these are the first results about optimizing the long-run running
costs in systems with bounded energy stores.Comment: 32 pages, corrections of typos and minor omission

### Competing field pulse induced dynamic transition in Ising models

The dynamic magnetization-reversal phenomena in the Ising model under a
finite-duration external magnetic field competing with the existing order for
$T<T_c^0$ has been discussed. The nature of the phase boundary has been
estimated from the mean-field equation of motion. The susceptibility and
relaxation time diverge at the MF phase boundary. A Monte Carlo study also
shows divergence of relaxation time around the phase boundary. Fluctuation of
order parameter also diverge near the phase boundary. The behavior of the
fourth order cumulant shows two distinct behavior: for low temperature and
pulse duration region of the phase boundary the value of the cumulant at the
crossing point for different system sizes is much less than that corersponding
to the static transition in the same dimension which indicate a new
universality class for the dynamic transition. Also, for higher temperature and
pulse duration, the transition seem to fall in a mean-field like
weak-singularity universality class.Comment: 12 pages, 17 ps & eps figures, to appear in a Special Issue of Phase
Transitions (2004), Ed. S. Pur

### Wave Propagation in 1-D Spiral geometry

In this article, we investigate the wave equation in spiral geometry and
study the modes of vibrations of a one-dimensional (1-D) string in spiral
shape. Here we show that the problem of wave propagation along a spiral can be
reduced to Bessel differential equation and hence, very closely related to the
problem of radial waves of two-dimensional (2-D) vibrating membrane in circular
geometry

### Elliptic flow of thermal photons and formation time of quark gluon plasma at RHIC

We calculate the elliptic flow of thermal photons from Au+Au collisions at
RHIC energies for a range of values for the formation time $\tau_0$ but fixed
entropy (or particle rapidity density). The results are found to be quite
sensitive to $\tau_0$. The $v_2$ for photons decreases as $\tau_0$ decreases
and admits a larger contribution from the QGP phase which has a smaller $v_2$.
The elliptic flow coefficient for hadrons, on the other hand, is only
marginally dependent on $\tau_0$.Comment: 2 extra figures and discussion added. To appear in Physical Review C
(Rapid Communication

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