5,030 research outputs found
Effects of the liquid-gas phase transition and cluster formation on the symmetry energy
Various definitions of the symmetry energy are introduced for nuclei, dilute
nuclear matter below saturation density and stellar matter, which is found in
compact stars or core-collapse supernovae. The resulting differences are
exemplified by calculations in a theoretical approach based on a generalized
relativistic density functional for dense matter. It contains nucleonic
clusters as explicit degrees of freedom with medium dependent properties that
are derived for light clusters from a quantum statistical approach. With such a
model the dissolution of clusters at high densities can be described. The
effects of the liquid-gas phase transition in nuclear matter and of cluster
formation in stellar matter on the density dependence of the symmetry energy
are studied for different temperatures. It is observed that correlations and
the formation of inhomogeneous matter at low densities and temperatures causes
an increase of the symmetry energy as compared to calculations assuming a
uniform uncorrelated spatial distribution of constituent baryons and leptons.Comment: 20 pages, 19 figures, version accepted for publication in EPJA
special volume on Nuclear Symmetry Energ
The center-to-limb variation across the Fraunhofer lines of HD 189733; Sampling the stellar spectrum using a transiting planet
The center-to-limb variation (CLV) describes the brightness of the stellar
disk as a function of the limb angle. Across strong absorption lines, the CLV
can vary quite significantly. We obtained a densely sampled time series of
high-resolution transit spectra of the active planet host star HD 189733 with
UVES. Using the passing planetary disk of the hot Jupiter HD 189733 b as a
probe, we study the CLV in the wings of the Ca II H and K and Na I D1 and D2
Fraunhofer lines, which are not strongly affected by activity-induced
variability. In agreement with model predictions, our analysis shows that the
wings of the studied Fraunhofer lines are limb brightened with respect to the
(quasi-)continuum. The strength of the CLV-induced effect can be on the same
order as signals found for hot Jupiter atmospheres. Therefore, a careful
treatment of the wavelength dependence of the stellar CLV in strong absorption
lines is highly relevant in the interpretation of planetary transit
spectroscopy.Comment: Accepted in A&
A planetary eclipse map of CoRoT-2a. Comprehensive lightcurve modeling combining rotational-modulation and transits
We analyze the surface structure of the planet host star CoRoT-2a using a
consistent model for both the `global' (i.e., rotationally modulated)
lightcurve and the transit lightcurves, using data provided by the CoRoT
mission. Selecting a time interval covering two stellar rotations and six
transits of the planetary companion CoRoT-2b, we adopt a `strip' model of the
surface to reproduce the photometric modulation inside and outside the transits
simultaneously. Our reconstructions show that it is possible to achieve
appropriate fits for the entire sub-interval using a low-resolution surface
model with 36 strips. The surface reconstructions indicate that the brightness
on the eclipsed section of the stellar surface is (6 +/- 1) % lower than the
average brightness of the remaining surface. This result suggests a
concentration of stellar activity in a band around the stellar equator similar
to the behavior observed on the Sun.Comment: accepted by A&A on 12/09/200
Planetary eclipse mapping of CoRoT-2a. Evolution, differential rotation, and spot migration
The lightcurve of CoRoT-2 shows substantial rotational modulation and
deformations of the planet's transit profiles caused by starspots. We
consistently model the entire lightcurve, including both rotational modulation
and transits, stretching over approximately 30 stellar rotations and 79
transits. The spot distribution and its evolution on the noneclipsed and
eclipsed surface sections are presented and analyzed, making use of the high
resolution achievable under the transit path.
We measure the average surface brightness on the eclipsed section to be
(5\pm1) % lower than on the noneclipsed section. Adopting a solar spot
contrast, the spot coverage on the entire surface reaches up to 19 % and a
maximum of almost 40 % on the eclipsed section. Features under the transit
path, i.e. close to the equator, rotate with a period close to 4.55 days.
Significantly higher rotation periods are found for features on the noneclipsed
section indicating a differential rotation of . Spotted
and unspotted regions in both surface sections concentrate on preferred
longitudes separated by roughly 180 deg.Comment: Paper accepted by A&A 17/02/2010. For a better resolution paper
please visit my homepage:
http://www.hs.uni-hamburg.de/EN/Ins/Per/Huber/index.htm
Cluster virial expansion for nuclear matter within a quasiparticle statistical approach
Correlations in interacting many-particle systems can lead to the formation
of clusters, in particular bound states and resonances. Systematic quantum
statistical approaches allow to combine the nuclear statistical equilibrium
description (law of mass action) with mean-field concepts. A chemical picture,
which treats the clusters as distinct entities, serves as an intuitive concept
to treat the low-density limit. Within a generalized Beth-Uhlenbeck approach,
the quasiparticle virial expansion is extended to include arbitrary clusters,
where special attention must be paid to avoid inconsistencies such as double
counting. Correlations are suppressed with increasing density due to Pauli
blocking. The contribution of the continuum to the virial coefficients can be
reduced by considering clusters explicitly and introducing quasiparticle
energies. The cluster-virial expansion for nuclear matter joins known
benchmarks at low densities with those near saturation density.Comment: 18 pages, 6 figures, 2 table
Asymmetry Effects on Nuclear Fragmentation
We show the possibility of extracting important information on the symmetry
term of the Equation of State () directly from multifragmentation
reactions using stable isotopes with different charge asymmetries. We study
n-rich and n-poor collisions at using a new stochastic
transport approach with all isospin effects suitably accounted for. For central
collisions a chemical component in the spinodal instabilities is clearly seen.
This effect is reduced in the neck fragmentation observed for semiperipheral
collisions, pointing to a different nature of the instability. In spite of the
low asymmetry tested with stable isotopes the results are showing an
interesting and promising dependence on the stiffness of the symmetry term,
with an indication towards an increase of the repulsion above normal density.Comment: 8 pages (Latex), 7 Postscript figures, CRIS2000 Conference,
Acicastello, Italy, May 22-26, (2000), Nucl. Phys. A (in press
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