3,837 research outputs found
Finite size effects in Neutron Star and Nuclear matter simulations
In this work we study molecular dynamics simulations of symmetric nuclear
matter using a semi-classical nucleon interaction model. We show that, at
sub-saturation densities and low temperatures, the solutions are
non-homogeneous structures reminiscent of the ``nuclear pasta'' phases expected
in Neutron Star Matter simulations, but shaped by artificial aspects of the
simulations. We explore different geometries for the periodic boundary
conditions imposed on the simulation cell: cube, hexagonal prism and truncated
octahedron. We find that different cells may yield different solutions for the
same physical conditions (i.e. density and temperature). The particular shape
of the solution at a given density can be predicted analytically by energy
minimization. We also show that even if this behavior is due to finite size
effects, it does not mean that it vanishes for very large systems and it
actually is independent of the system size: The system size sets the only
characteristic length scale for the inhomogeneities.
We then include a screened Coulomb interaction, as a model of Neutron Star
Matter, and perform simulations in the three cell geometries. In this case, the
competition between competing interactions of different range produces the well
known nuclear pasta, with (in most cases) several structures per cell. However,
we find that the results are affected by finite size in different ways
depending on the geometry of the cell. In particular, at the same physical
conditions and system size, the hexagonal prism yields a single structure per
cell while the cubic and truncated octahedron show consistent results with more
than one structure per cell. In this case, the results in every cell are
expected to converge for systems much larger than the characteristic length
scale that arises from the competing interactions.Comment: 17 pages, 10 figure
Isoscaling and the nuclear EOS
Experiments with rare isotopes are shedding light on the role isospin plays
in the equation of state (EoS) of nuclear matter, and isoscaling -an
straight-forward comparison of reactions with different isospin- could deliver
valuable information about it. In this work we test this assertion
pragmatically by comparing molecular dynamics simulations of isoscaling
reactions using different equations of state and looking for changes in the
isoscaling parameters; to explore the possibility of isoscaling carrying
information from the hot-and-dense stage of the reaction, we perform our study
in confined and expanding systems. Our results indicate that indeed isoscaling
can help us learn about the nuclear EoS, but only in some range of excitation
energies
The first INTEGRAL-OMC catalogue of optically variable sources
The Optical Monitoring Camera (OMC) onboard INTEGRAL provides photometry in
the Johnson V-band. With an aperture of 50 mm and a field of view of 5deg x
5deg, OMC is able to detect optical sources brighter than V~18, from a
previously selected list of potential targets of interest. After more than nine
years of observations, the OMC database contains light curves for more than
70000 sources (with more than 50 photometric points each). The objectives of
this work have been to characterize the potential variability of the objects
monitored by OMC, to identify periodic sources and to compute their periods,
taking advantage of the stability and long monitoring time of the OMC. To
detect potential variability, we have performed a chi-squared test, finding
5263 variable sources out of an initial sample of 6071 objects with good
photometric quality and more than 300 data points each. We have studied the
periodicity of these sources using a method based on the phase dispersion
minimization technique, optimized to handle light curves with very different
shapes.In this first catalogue of variable sources observed by OMC, we provide
for each object the median of the visual magnitude, the magnitude at maximum
and minimum brightness in the light curve during the window of observations,
the period, when found, as well as the complete intrinsic and period-folded
light curves, together with some additional ancillary data.Comment: Accepted by Astronomy & Astrophysics; 13 pages, 16 figures. Figures'
resolution has been degraded to fit astro-ph constraint
Beyond Nuclear Pasta: Phase Transitions and Neutrino Opacity of Non-Traditional Pasta
In this work, we focus on different length scales within the dynamics of
nucleons in conditions according to the neutron star crust, with a
semiclassical molecular dynamics model, studying isospin symmetric matter at
subsaturation densities. While varying the temperature, we find that a
solid-liquid phase transition exists, that can be also characterized with a
morphology transition. For higher temperatures, above this phase transition, we
study the neutrino opacity, and find that in the liquid phase, the scattering
of low momenta neutrinos remain high, even though the morphology of the
structures differ significatively from those of the traditional nuclear pasta.Comment: 12 pages, 10 figure
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