50,637 research outputs found
Shape evolution in Yttrium and Niobium neutron-rich isotopes
The isotopic evolution of the ground-state nuclear shapes and the systematics
of one-quasiproton configurations are studied in neutron-rich odd-A Yttrium and
Niobium isotopes. We use a selfconsistent Hartree-Fock-Bogoliubov formalism
based on the Gogny energy density functional with two parametrizations, D1S and
D1M. The equal filling approximation is used to describe odd-A nuclei
preserving both axial and time reversal symmetries. Shape-transition signatures
are identified in the N=60 isotopes in both charge radii and spin-parities of
the ground states. These signatures are a common characteristic for nuclei in
the whole mass region. The nuclear deformation and shape coexistence inherent
to this mass region are shown to play a relevant role in the understanding of
the spectroscopic features of the ground and low-lying one-quasiproton states.
Finally, a global picture of the neutron-rich A=100 mass region from Krypton up
to Molybdenum isotopes is illustrated with the systematics of the nuclear
charge radii isotopic shifts.Comment: 21 pages, 14 figures. To be published in Phys. Rev.
Systematics of one-quasiparticle configurations in neutron-rich Sr, Zr, and Mo odd isotopes with the Gogny energy density functional
The systematics of one-quasiparticle configurations in neutron-rich Sr, Zr,
and Mo odd isotopes is studied within the Hartree-Fock-Bogoliubov plus Equal
Filling Approximation method preserving both axial and time reversal
symmetries. Calculations based on the Gogny energy density functional with both
the standard D1S parametrization and the new D1M incarnation of this functional
are included in our analysis. The nuclear deformation and shape coexistence
inherent to this mass region are shown to play a relevant role in the
understanding of the spectroscopic features of the ground and low-lying
one-quasineutron states.Comment: 11 page
Signatures of shape transition in odd-A neutron-rich Rubidium isotopes
The isotopic evolution of the ground-state nuclear shapes and the systematics
of one-quasiproton configurations are studied in odd-A Rubidium isotopes. We
use a selfconsistent Hartree-Fock-Bogoliubov formalism based on the Gogny
energy density functional with two parametrizations, D1S and D1M, and
implemented with the equal filling approximation. We find clear signatures of a
sharp shape transition at N=60 in both charge radii and spin-parity of the
ground states, which are robust, consistent to each other, and in agreement
with experiment. We point out that the combined analysis of these two
observables could be used to predict unambiguously new regions where shape
transitions might develop.Comment: 6 pages, 7 figures. To appear in Phys. Rev. C (Rapid Communications
Microscopic description of quadrupole-octupole coupling in Sm and Gd isotopes with the Gogny Energy Density Functional
The interplay between the collective dynamics of the quadrupole and octupole
deformation degree of freedom is discussed in a series of Sm and Gd isotopes
both at the mean field level and beyond, including parity symmetry restoration
and configuration mixing. Physical properties like negative parity excitation
energies, E1 and E3 transition probabilities are discussed and compared to
experimental data. Other relevant intrinsic quantities like dipole moments,
ground state quadrupole moments or correlation energies associated to symmetry
restoration and configuration mixing are discussed. For the considered
isotopes, the quadrupole-octupole coupling is found to be weak and most of the
properties of negative parity states can be described in terms of the octupole
degree of freedom alone.Comment: 31 pages, 11 figure
Berezinskii-Kosterlitz-Thouless Transition in Spin-Charge Separated Superconductor
A model for spin-charge separated superconductivity in two dimensions is
introduced where the phases of the spinon and holon order parameters couple
gauge-invariantly to a statistical gauge-field representing chiral
spin-fluctuations. The model is analyzed in the continuum limit and in the
low-temperature limit. In both cases we find that physical electronic phase
correlations show a superconducting-normal phase transition of the
Berezinskii-Kosterlitz-Thouless type, while statistical gauge-field excitations
are found to be strictly gapless. The normal-to-superconductor phase boundary
for this model is also obtained as a function of carrier density, where we find
that its shape compares favorably with that of the experimentally observed
phase diagram for the oxide superconductors.Comment: 35 pages, TeX, CSLA-P-93-
A high Eddington-ratio, true Seyfert 2 galaxy candidate: implications for broad-line-region models
A bright, soft X-ray source was detected on 2010 July 14 during an
XMM--Newton slew at a position consistent with the galaxy GSN 069 (z=0.018).
Previous ROSAT observations failed to detect the source and imply that GSN 069
is now >240 times brighter than it was in 1994 in the soft X-ray band. We
report here results from a ~1 yr monitoring with Swift and XMM-Newton, as well
as from optical spectroscopy. GSN 069 is an unabsorbed, ultra-soft source in
X-rays, with no flux detected above ~1 keV. The soft X-rays exhibit significant
variability down to timescales of hundreds of seconds. The UV-to-X-ray spectrum
of GSN 069 is consistent with a pure accretion disc model which implies an
Eddington ratio of ~0.5 and a black hole mass of ~ 1.2 million solar masses. A
new optical spectrum, obtained ~3.5 months after the XMM-Newton slew detection,
is consistent with earlier spectra and lacks any broad line component,
classifying the source as a Seyfert 2 galaxy. The lack of cold X-ray absorption
and the short timescale variability in the soft X-rays rule out a standard
Seyfert 2 interpretation of the X-ray data. We discuss our results within the
framework of two possible scenarios for the broad-line-region (BLR) in AGN,
namely the two-phase model (cold BLR clouds in pressure equilibrium with a
hotter medium), and models in which the BLR is part of an outflow, or
disc-wind. Finally, we point out that GSN 069 may be a member of a population
of super-soft AGN whose SED is completely dominated by accretion disc emission,
as it is the case in some black hole X-ray binary transients during their
outburst evolution. The disc emission for a typical AGN with larger black hole
mass than GSN 069 does not enters the soft X-ray band, so that GSN 069-like
objects would likely be missed by current X-ray surveys, or mis-classified as
Compton-thick candidates. (ABRIDGED)Comment: Accepted for publication in MNRA
VLA Observations of H I in the Helix Nebula (NGC 7293)
We report the detection of 21-cm line emission from H I in the planetary
nebula NGC 7293 (the Helix). The observations, made with the Very Large Array,
show the presence of a ring of atomic hydrogen that is associated with the
outer portion of the ionized nebula. This ring is most probably gas ejected in
the AGB phase that has been subsequently photodissociated by radiation from the
central star. The H I emission spreads over about 50 km/s in radial velocity.
The mass in H I is approximately 0.07 solar masses, about three times larger
than the mass in molecular hydrogen and comparable with the mass in ionized
hydrogen.Comment: 19 pages, 9 figure
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