6,683 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
Spectral variability in Swift and Chandra observations of the Ultraluminous source NGC 55 ULX1
NGC 55 ULX1 is a bright Ultraluminous X-ray source located 1.78 Mpc away. We
analysed a sample of 20 Swift observations, taken between 2013 April and
August, and two Chandra observations taken in 2001 September and 2004 June. We
found only marginal hints of a limited number of dips in the light curve,
previously reported to occur in this source, although the uncertainties due to
the low counting statistics of the data are large. The Chandra and Swift
spectra showed clearly spectral variability which resembles those observed in
other ULXs. We can account for this spectral variability in terms of changes in
both the normalization and intrinsic column density of a two-components model
consisting of a blackbody (for the soft component) and a multicolour accretion
disc (for the hard component). We discuss the possibility that strong outflows
ejected by the disc are in part responsible for such spectral changes.Comment: 9 pages, 6 figure; accepted to be published on MNRA
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&
Time-resolved UVES observations of a stellar flare on the planet host HD 189733 during primary transit
HD 189733 is an exoplanetary system consisting of a transiting hot Jupiter
and an active K2V-type main sequence star. We aim to use VLT/UVES high
resolution echelle spectra to study a stellar flare. We have performed
simultaneous analyses of the temporal evolution in several chromospheric
stellar lines, namely, the Ca II H and K lines, Halpha, Hbeta, Hgamma, Hdelta,
Hepsilon, the Ca II infrared triplet line, and He I D3. Observations were
carried out with a time resolution of approximately 1 min for a duration of
four hours, including a complete planetary transit. We determine the energy
released during the flare in all studied chromospheric lines combined to be
about 8.7e31 erg, which puts this event at the upper end of flare energies
observed on the Sun. Our analysis does not reveal any significant delay of the
flare peak observed in the Balmer and Ca II H and K lines, although we find a
clear difference in the temporal evolution of these lines. The He I D3 shows
additional absorption possibly related to the flare event. Based on the flux
released in Ca II H and K lines during the flare, we estimate the soft X-ray
flux emission to be 7e30 erg. The observed flare can be ranked as a moderate
flare on a K-type star and confirms a rather high activity level of HD 189733
host star. The cores of the studied chromospheric lines demonstrate the same
behavior and let us study the flare evolution. We demonstrate that the activity
of an exoplanet host star can play an important role in the detection of
exoplanet atmospheres, since these are frequently discovered as an additional
absorption in the line cores. A possible star-planet interaction responsible
for a flare occurrence during a transit can neither be confirmed nor ruled out.Comment: 12 pages, 9 figures, accepted for publication in A&
Mechanisms for Direct Breakup Reactions
We review some simple mechanisms of breakup in nuclear reactions. We mention
the spectator breakup, which is described in the post-form DWBA. The relation
to other formulations is also indicated. An especially important mechanism is
Coulomb dissociation. It is a distinct advantage that the perturbation due to
the electric field of the nucleus is exactly known. Therefore firm conclusions
can be drawn from such measurements. Some new applications of Coulomb
dissociation for nuclear astrophysics are discussed.Comment: 17 pages, 5 figures, to appear in the proceedings of the RCNP-TMU
Symposium on Spins in Nuclear and Hadronic Reactions, October 16-18 199
Composition and thermodynamics of nuclear matter with light clusters
We investigate nuclear matter at finite temperature and density, including
the formation of light clusters up to the alpha particle The novel feature of
this work is to include the formation of clusters as well as their dissolution
due to medium effects in a systematic way using two many-body theories: a
microscopic quantum statistical (QS) approach and a generalized relativistic
mean field (RMF) model. Nucleons and clusters are modified by medium effects.
Both approaches reproduce the limiting cases of nuclear statistical equilibrium
(NSE) at low densities and cluster-free nuclear matter at high densities. The
treatment of the cluster dissociation is based on the Mott effect due to Pauli
blocking, implemented in slightly different ways in the QS and the generalized
RMF approaches. We compare the numerical results of these models for cluster
abundances and thermodynamics in the region of medium excitation energies with
temperatures T <= 20 MeV and baryon number densities from zero to a few times
saturation density. The effect of cluster formation on the liquid-gas phase
transition and on the density dependence of the symmetry energy is studied.
Comparison is made with other theoretical approaches, in particular those,
which are commonly used in astrophysical calculations. The results are relevant
for heavy-ion collisions and astrophysical applications.Comment: 32 pages, 15 figures, minor corrections, accepted for publication in
Physical Review
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
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