693 research outputs found
Dust properties and distribution in dwarf galaxies
We present a study of the extinction, traced by the Balmer decrement, in HII
regions in the dwarf galaxies NGC 1569 and NGC 4214. We find that the
large-scale extinction around the most prominent HII regions in both galaxies
forms a shell in which locally the intrinsic extinction can adopt relatively
high values ( mag) despite the low metallicity and thus the
low overall dust content. The small-scale extinction (spatial resolution
0.3'') shows fluctuations that are most likely due to variations in the
dust distribution. We compare the distribution of the extinction to that of the
dust emission, traced by Spitzer emission at 8 and 24 \mi, and to the emission
of cold dust at 850 \mi. We find in general a good agreement between all
tracers, expect for the 850 \mi emission in NGC 4214 which is more extended
than the extinction and the other emissions. Whereas in NGC 1569 the dust
emission at all wavelengths is very similar, NGC 4214 shows spatial variations
in the 24-to-850 \mi ratio. We furthermore compared the 24 \mi and the
extinction-corrected Halpha emission from HII regions in a sample of galaxies
with a wide range of metallicities and found a good correlation between both
emissions, independent of metallicity. We suggest that this lack of dependence
on metallicity might be due to the formation of dust shells with a relatively
constant opacity, like the ones observed here, around ionizing stars.Comment: 5 pages, to appear in proceeding of IAU Symposition 255
"Low-metallicity star formation", ed. L. Hunt, S. Madden, & R. Schneider
(Cambridge: CUP
Irreversible processes without energy dissipation in an isolated Lipkin-Meshkov-Glick model
For a certain class of isolated quantum systems, we report the existence of
irreversible processes in which the energy is not dissipated. After a closed
cycle in which the initial energy distribution is fully recovered, the
expectation value of a symmetry-breaking observable changes from a value
different from zero in the initial state, to zero in the final state. This
entails the unavoidable loss of a certain amount of information, and
constitutes a source of irreversibility. We show that the von Neumann entropy
of time-averaged equilibrium states increases in the same magnitude as a
consequence of the process. We support this result by means of numerical
calculations in an experimentally feasible system, the Lipkin-Meshkov-Glick
model.Comment: 10 pages, 7 figure
Non-thermal quantum phase transitions
We report a kind of quantum phase transition which takes place in isolated
quantum systems with non-thermal equilibrium states and an extra symmetry that
commutes with the Hamiltonian for any values of the system parameters. A
critical energy separates two different phases, one in which the symmetry is
broken. This critical behavior is ruled out as soon as the system is put in
contact with a thermal bath. The critical point is crossed when a sufficent
amount of work is performed on the system, keeping it isolated from the
environment. Different phases are identified by means of an order parameter,
which is only different from zero in the symmetry-breaking phase. The behavior
of the system near the critical point is determined by a set of critical
exponents. We illustrate this phenomenon by means of numerical calculations in
three different two-level systems
From thermal to excited-state quantum phase transitions ---the Dicke model
We study the thermodynamics of the full version of the Dicke model, including
all the possible values of the total angular momentum , with both
microcanonical and canonical ensembles. We focus on how the excited-state
quantum phase transition, which only appears in the microcanonical description
of the maximum angular momentum sector, , change to a standard thermal
phase transition when all the sectors are taken into account. We show that both
the thermal and the excited-state quantum phase transitions have the same
origin; in other words, that both are two faces of the same phenomenon. Despite
all the logarithmic singularities which characterize the excited-state quantum
phase transition are ruled out when all the -sectors are considered, the
critical energy (or temperature) still divides the spectrum in two regions: one
in which the parity symmetry can be broken, and another in which this symmetry
is always well defined.Comment: Submitted to PRE. Comments are welcome. V2: Updated to match
published versio
Entropy, chaos and excited-state quantum phase transitions in the Dicke model
We study non-equilibrium processes in an isolated quantum system ---the Dicke
model--- focusing on the role played by the transition from integrability to
chaos and the presence of excited-state quantum phase transitions. We show that
both diagonal and entanglement entropies are abruptly increased by the onset of
chaos. Also, this increase ends in both cases just after the system crosses the
critical energy of the excited-state quantum phase transition. The link between
entropy production, the development of chaos and the excited-state quantum
phase transition is more clear for the entanglement entropy. On the contrary,
the heat dissipated by the process is affected neither by the onset of chaos,
nor by the excited-state quantum phase transition.Comment: 12 pages, 9 figures, RevTex 4.
A one-parameter family of interpolating kernels for Smoothed Particle Hydrodynamics studies
A set of interpolating functions of the type f(v)={(sin[v pi/2])/(v pi/2)}^n
is analyzed in the context of the smoothed-particle hydrodynamics (SPH)
technique. The behaviour of these kernels for several values of the parameter n
has been studied either analytically as well as numerically in connection with
several tests carried out in two dimensions. The main advantage of this kernel
relies in its flexibility because for n=3 it is similar to the standard widely
used cubic-spline, whereas for n>3 the interpolating function becomes more
centrally condensed, being well suited to track discontinuities such as shock
fronts and thermal waves.Comment: 36 pages, 12 figures (low-resolution), published in J.C.
Los grandes mitos geográficos de la cartografía africana en el siglo XVI
En el siglo XVI, todo intento de reconstrucción geográfica referente a África debía enfrentarse a un conocimiento desigual entre una franja costera familiar y un interior continental ignoto. Sin embargo, lejos de reconocer esta realidad, la cartografía del Cinquecento se caracteriza por la utilización de todo tipo de artimañas con las que poder «inventar» el territorio en aquellos casos en que no era suficientemente conocido. Entre ellas, destaca la utilización de los mitos geográficos, de cuyo análisis y evolución a lo largo del siglo XVI trata nuestro estudio
- …