3,078 research outputs found
Bars and Cold Dark Matter Halos
The central part of a dark matter halo reacts to the presence and evolution
of a bar. Not only does the halo absorb angular momentum from the disk, it can
also be compressed and have its shape modified. We study these issues in a
series of cosmologically motivated, highly resolved N-body simulations of
barred galaxies run under different initial conditions. In all models we find
that the inner halo's central density increases. We model this density increase
using the standard adiabatic approximation and the modified formula by Gnedin
et al. and find that halo mass profiles are better reproduced by this latter.
In models with a strong bar, the dark matter in the central region forms a
bar-like structure (``dark matter bar''), which rotates together with the
normal bar formed by the stellar component (``stellar bar''). The
minor-to-major axial ratio of a halo bar changes with radius with a typical
value 0.7 in the central disk region. DM bar amplitude is mostly a function of
the stellar bar strength. Models in which the bar amplitude increases or stays
roughly constant with time, initially large (40%-60%) misalignment between the
halo and disk bars quickly decreases with time as the bar grows. The halo bar
is nearly aligned with the stellar bar (~10 degrees lag for the halo) after ~2
Gyr. The torque, which the halo bar exerts on the stellar bar, can serve as a
mechanism to regulate the angular momentum transfer from the disk to the halo.Comment: Modified version after referee's suggestions. 17 pages, 12 figures,
accepted by Ap
Ocean spectra for the high-frequency waves as determined from airborne radar measurements
The possibilities of using radar to obtain oceanographic information are described. Ocean spectra for the short gravity and gravity-capillary waves have been derived from radar crosssection measurements for vertical polarization at 428 MHz, 1228 MHz, 4455 MHz, and 8910 MHz...
Anisotropy of the dc conductivity due to orbital-selective spin fluctuations in the nematic phase of iron superconductors
We study the dc conductivity of iron-based superconductors within the orbital-selective spin fluctuation scenario. Within this approach, the anisotropy of spin fluctuations below the spin-nematic transition at TS is also responsible for the orbital ordering, induced by nematic self-energy corrections to the quasiparticle dispersion. As a consequence, the anisotropy of the dc conductivity below TS is determined not only by the anisotropy of the scattering rates as expected within a spin-nematic scenario, but also by the modification of the Fermi velocity due to the orbital reconstruction. More interestingly, it turns out that these two effects contribute to the dc-conductivity anisotropy with opposite signs. By using realistic band-structure parameters we compute the conductivity anisotropy for both 122 and FeSe compounds, discussing the possible origin of the different dc-conductivity anisotropy observed experimentally in these two families of iron-based superconductors
Searching for tidal tails around Centauri using RR Lyrae Stars
We present a survey for RR Lyrae stars in an area of 50 deg around the
globular cluster Centauri, aimed to detect debris material from the
alleged progenitor galaxy of the cluster. We detected 48 RR Lyrae stars of
which only 11 have been previously reported. Ten among the eleven previously
known stars were found inside the tidal radius of the cluster. The rest were
located outside the tidal radius up to distances of degrees from the
center of the cluster. Several of those stars are located at distances similar
to that of Centauri. We investigated the probability that those stars
may have been stripped off the cluster by studying their properties (mean
periods), calculating the expected halo/thick disk population of RR Lyrae stars
in this part of the sky, analyzing the radial velocity of a sub-sample of the
RR Lyrae stars, and finally, studying the probable orbits of this sub-sample
around the Galaxy. None of these investigations support the scenario that there
is significant tidal debris around Centauri, confirming previous
studies in the region. It is puzzling that tidal debris have been found
elsewhere but not near the cluster itself.Comment: 11 pages, 11 figures, Accepte
Deformation of the Fermi surface in the extended Hubbard model
The deformation of the Fermi surface induced by Coulomb interactions is
investigated in the t-t'-Hubbard model. The interplay of the local U and
extended V interactions is analyzed. It is found that exchange interactions V
enhance small anisotropies producing deformations of the Fermi surface which
break the point group symmetry of the square lattice at the Van Hove filling.
This Pomeranchuck instability competes with ferromagnetism and is suppressed at
a critical value of U(V). The interaction V renormalizes the t' parameter to
smaller values what favours nesting. It also induces changes on the topology of
the Fermi surface which can go from hole to electron-like what may explain
recent ARPES experiments.Comment: 5 pages, 4 ps figure
Chat Application with A Codified Information Traveling Option
Cryptology is the science that involves the study of the various techniques to keep information secure. It is the science that deals with the theoretical problems related to the safety in the interchange, between a transmitter and a receptor, of codified information traveling in a communication channel [1].
The project consist of building a software to cipher text messages using a variant on the Playfair cipher method, which is an algorithm used in cryptology. The variant consist on extend the allowed characters from Playfair to include every character on the ASCII code. In this case, a 16x16 matrix will be used, to be able to also include the extended characters. The project was developed in Matlab, a programming and design platform that is able to do matrix manipulation and calculation easily.
For the possible implementation, wersquore going to use a virtual application developed in Matlab using the TCP/IP protocol to send and receive encrypted messages
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