3,400 research outputs found
On thermal nucleation of quark matter in compact stars
The possibility of a hadron-quark phase transition in extreme astrophysical
phenomena such as the collapse of a supernova is not discarded by the modern
knowledge of the high-energy nuclear and quark-matter equations of state. Both
the density and the temperature attainable in such extreme processes are
possibly high enough to trigger a chiral phase transition. However, the time
scales involved are an important issue. Even if the physical conditions for the
phase transition are favorable (for a system in equilibrium), there may not be
enough time for the dynamical process of phase conversion to be completed. We
analyze the relevant time scales for the phase conversion via thermal
nucleation of bubbles of quark matter and compare them to the typical
astrophysical time scale, in order to verify the feasibility of the scenario of
hadron-quark phase conversion during, for example, the core-collapse of a
supernova.Comment: 6 pages, 4 figures, talk given at the International Conference
SQM2009, Buzios, Rio de Janeiro, Brazil, Sep.27-Oct.2, 200
Nucleation of quark matter in protoneutron star matter
The phase transition from hadronic to quark matter may take place already
during the early post-bounce stage of core collapse supernovae when matter is
still hot and lepton rich. If the phase transition is of first order and
exhibits a barrier, the formation of the new phase occurs via the nucleation of
droplets. We investigate the thermal nucleation of a quark phase in supernova
matter and calculate its rate for a wide range of physical parameters. We show
that the formation of the first droplet of a quark phase might be very fast and
therefore the phase transition to quark matter could play an important role in
the mechanism and dynamics of supernova explosions.Comment: v3: fits version published in Physical Review
Effects from inhomogeneities in the chiral transition
We consider an approximation procedure to evaluate the finite-temperature
one-loop fermionic density in the presence of a chiral background field which
systematically incorporates effects from inhomogeneities in the chiral field
through a derivative expansion. We apply the method to the case of a simple
low-energy effective chiral model which is commonly used in the study of the
chiral phase transition, the linear sigma-model coupled to quarks. The
modifications in the effective potential and their consequences for the bubble
nucleation process are discussed.Comment: 11 pages, 5 figures. v2: appendix and references added, published
versio
Formation of ions by high energy photons
We calculate the electron energy spectrum of ionization by a high energy
photon, accompanied by creation of electron-positron pair. The total cross
section of the process is also obtained. The asymptotics of the cross section
does not depend on the photon energies. At the photon energies exceeding a
certain value this appeares to to be the dominant mechanism of
formation of the ions. The dependence of on the value of nuclear
charge is obtained. Our results are consistent with experimental data.Comment: 16 pages, 6 figure
The charge ordered state in half-doped Bi-based manganites studied by O and Bi NMR
We present a Bi and O NMR study of the Mn electron spin
correlations developed in the charge ordered state of
BiSrMnO and BiCaMnO. The unusually
large local magnetic field indicates the dominant
character of the lone electron pair of Bi-ions in both compounds. The
mechanism connecting the character of the lone pairs to the high
temperature of charge ordering is still not clarified. The observed
difference in for BiSrMnO to
BiCaMnO is probably due to a decrease in the canting of
the staggered magnetic moments of Mn-ions from. The modification of the
O spectra below demonstrates that the line due to the apical
oxygens is a unique local tool to study the development of the Mn spin
correlations. In the AF state the analysis of the O spectrum of
PrCaMnO and BiSrMnO prompts us to
try two different theoretical descriptions of the charge-ordered state, a
site-centered model for the first manganite and a bond-centered model for the
second one.Comment: 10 pages, 7 figure
Electromagnetic field evolution in relativistic heavy-ion collisions
The hadron string dynamics (HSD) model is generalized to include the creation
and evolution of retarded electromagnetic fields as well as the influence of
the magnetic and electric fields on the quasiparticle propagation. The
time-space structure of the fields is analyzed in detail for non-central Au+Au
collisions at 200 GeV. It is shown that the created magnetic
field is highly inhomogeneous but in the central region of the overlapping
nuclei it changes relatively weakly in the transverse direction. For the impact
parameter 10 fm the maximal magnetic field - perpendicularly to the
reaction plane - is obtained of order 5 for a very short time
0.2 fm/c, which roughly corresponds to the time of a maximal overlap of
the colliding nuclei. We find that at any time the location of the maximum in
the distribution correlates with that of the energy density of the
created particles. In contrast, the electric field distribution, being also
highly inhomogeneous, has a minimum in the center of the overlap region.
Furthermore, the field characteristics are presented as a function of the
collision energy and the centrality of the collisions. To explore the effect of
the back reaction of the fields on hadronic observables a comparison of HSD
results with and without fields is exemplified. Our actual calculations show no
noticeable influence of the electromagnetic fields - created in heavy-ion
collisions - on the effect of the electric charge separation with respect to
the reaction plane.Comment: 17 pages, 22 figures, title changed by editor, accepted for PR
Use of ERTS-A, Skylab, and supporting aircraft to enhance resource management
There are no author-identified significant results in this report
Se NMR measurements of the exchange field in the organic conductor (BETS)FeCl
Se-NMR spectrum and frequency shift measurements in the paramagnetic
metal (PM) and antiferromagnetic insulating (AFI) phases are reported for a
small single crystal of the organic conductor (BETS)FeCl
as a function of temperature () and field alignment for an applied magnetic
field = 9 T. The results show that in the low limit, where the
localized Fe spins ( = 5/2) are almost fully polarized, the
conduction electrons (Se -electrons, spin = 1/2) in the BETS
molecules experience an exchange field () from the Fe
spins with a value of 32.7 1.5 T at 5 K and 9 T aligned opposite to
. This large negative value of is consistent
with that predicted by the resistivity measurements and supports the
Jaccarino-Peter internal field-compensation mechanism being responsible for the
origin of field-induced superconductivity.Comment: 4 pages, 5 figures, submitted to Physical Review Letter
Strange quark matter in explosive astrophysical systems
Explosive astrophysical systems, such as supernovae or compact star binary
mergers, provide conditions where strange quark matter can appear. The high
degree of isospin asymmetry and temperatures of several MeV in such systems may
cause a transition to the quark phase already around saturation density.
Observable signals from the appearance of quark matter can be predicted and
studied in astrophysical simulations. As input in such simulations, an equation
of state with an integrated quark matter phase transition for a large
temperature, density and proton fraction range is required. Additionally,
restrictions from heavy ion data and pulsar observation must be considered. In
this work we present such an approach. We implement a quark matter phase
transition in a hadronic equation of state widely used for astrophysical
simulations and discuss its compatibility with heavy ion collisions and pulsar
data. Furthermore, we review the recently studied implications of the QCD phase
transition during the early post-bounce evolution of core-collapse supernovae
and introduce the effects from strong interactions to increase the maximum mass
of hybrid stars. In the MIT bag model, together with the strange quark mass and
the bag constant, the strong coupling constant provides a parameter
to set the beginning and extension of the quark phase and with this the mass
and radius of hybrid stars.Comment: 6 pages, 5 figures, talk given at the International Conference on
Strangeness in Quark Matter (SQM2009), Buzios, Brasil, September 28 - October
2, 2009, to be published in Journal Phys.
Whole Earth Telescope observations of the pulsating subdwarf B star PG 0014+067
PG 0014+067 is one of the most promising pulsating subdwarf B stars for
seismic analysis, as it has a rich pulsation spectrum. The richness of its
pulsations, however, poses a fundamental challenge to understanding the
pulsations of these stars, as the mode density is too complex to be explained
only with radial and nonradial low degree (l < 3) p-modes without rotational
splittings. One proposed solution, for the case of PG 0014+067 in particular,
assigns some modes with high degree (l=3). On the other hand, theoretical
models of sdB stars suggest that they may retain rapidly rotating cores, and so
the high mode density may result from the presence of a few rotationally-split
triplet (l=1), quintuplet (l=2) modes, along with radial (l=0) p-modes. To
examine alternative theoretical models for these stars, we need better
frequency resolution and denser longitude coverage. Therefore, we observed this
star with the Whole Earth Telescope for two weeks in October 2004. In this
paper we report the results of Whole Earth Telescope observations of the
pulsating subdwarf B star PG 0014+067. We find that the frequencies seen in PG
0014+067 do not appear to fit any theoretical model currently available;
however, we find a simple empirical relation that is able to match all of the
well-determined frequencies in this star.Comment: 19 pages, preprint of paper accepted for publication in The
Astrophysical Journa
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