662 research outputs found
Mixed quark-nucleon phase in neutron stars and nuclear symmetry energy
The influence of the nuclear symmetry energy on the formation of a mixed
quark-nucleon phase in neutron star cores is studied. We use simple
parametrizations of the nuclear matter equation of state, and the bag model for
the quark phase. The behavior of nucleon matter isobars, which is responsible
for the existence of the mixed phase, is investigated. The role of the nuclear
symmetry energy changes with the value of the bag constant B. For lower values
of B the properties of the mixed phase do not depend strongly on the symmetry
energy. For larger B we find that a critical pressure for the first quark
droplets to form is strongly dependent on the nuclear symmetry energy, but the
pressure at which last nucleons disappear is independent of it.Comment: 12 pages, 16 figures, Phys. Rev. C in pres
The effect of growth-promoting methylobacteria on seedling development in Ginkgo biloba L.
Microbes of the genus Methylobacterium are regularly associated with leaves and fruits of plants. In experimental tests, they promote the growth of germ-free liverworts and mosses, which are ancient land plants. In this study we analyzed the effect of M. mesophilicum, a bacterium that has been isolated from the organs of numerous plant species, including gymnosperms and angiosperms, on the development of sterile embryos of Ginkgo biloba L., a unique living fossil among the embryophyta. In addition, germ-free seeds of Pinus sylvestris were inoculated with the same strain of methylobacteria. In G. biloba seedlings that were raised in a 12 h dark/light regime, a promotion of root development was recorded in samples treated with the methylobacteria. A fresh mass increase of + 25 % occurred within 6 weeks of inoculation with bacteria, compared to the aseptic control. In contrast, shoot development of the same plants was not significantly affected by these bacteria. In Pinus seedlings, organ development was unaffected by the presence of methylobacteria. Our results document a differential sensitivity of the root system versus the shoot towards these ubiquitously distributed plant-associated bacteria. The data are discussed with reference to the isolated taxonomic position of Ginkgo biloba, one of the most primitive gymnosperms in the biosphere that is economically important as a medicinal plant
Plant Development, Auxin, and the Subsystem Incompleteness Theorem
Plant morphogenesis (the process whereby form develops) requires signal cross-talking among all levels of organization to coordinate the operation of metabolic and genomic subsystems operating in a larger network of subsystems. Each subsystem can be rendered as a logic circuit supervising the operation of one or more signal-activated system. This approach simplifies complex morphogenetic phenomena and allows for their aggregation into diagrams of progressively larger networks. This technique is illustrated here by rendering two logic circuits and signal-activated subsystems, one for auxin (IAA) polar/lateral intercellular transport and another for IAA-mediated cell wall loosening. For each of these phenomena, a circuit/subsystem diagram highlights missing components (either in the logic circuit or in the subsystem it supervises) that must be identified experimentally if each of these basic plant phenomena is to be fully understood. We also illustrate the âsubsystem incompleteness theorem,â which states that no subsystem is operationally self-sufficient. Indeed, a whole-organism perspective is required to understand even the most simple morphogenetic process, because, when isolated, every biological signal-activated subsystem is morphogenetically ineffective
The influence of the velocity dispersion on the velocity-density relation in the disk model of the galaxy
Taking the velocity dispersion term into account in the Jeans equation describing the disk model, counterintuitively allows to reduce the local mass-to-light ratio at the outskirts of flattened galaxies, and stop the unbound growth of mass function. This is possible thanks to a more intertwined relation between the gravitational potential and the rotation curve than for spheroidal systems. This effect is illustrated on the example of galaxy UGC 6446 by finding iteratively the column mass density from the rotation curve supplemented with an isotropic velocity dispersion profile in the disk plane. Along with galactic magnetic fields, this effect would allow to reduce the local mass-to-light ratio at the galactic outskirts
Microscopic determination of the nuclear incompressibility within the non-relativistic framework
The nuclear incompressibility is deduced from measurements of the
Isoscalar Giant Monopole Resonance (ISGMR) in medium-heavy nuclei, and the
resulting value turns out to be model dependent. Since the considered nuclei
have neutron excess, it has been suggested that the model dependence is due to
the different behaviour of the symmetry energy in different models. To clarify
this issue, we make a systematic and careful analysis based on new Skyrme
forces which span a wide range of values for , for the value of the
symmetry energy at saturation and for its density dependence. By calculating,
in a fully self-consistent fashion, the ISGMR centroid energy in Pb we
reach, for the first time within the non-relativistic framework, three
important conclusions: (i) the monopole energy, and consequently the deduced
value of , depend on a well defined parameter related to the shape of
the symmetry energy curve and called ; (ii) Skyrme forces of the type
of SLy4 predict around 230 MeV, in agreement with the Gogny force
(previous estimates using Skyrme interactions having been plagued by lack of
full self-consistency); (iii) it is possible to build forces which predict
around 250 MeV, although part of this increase is due to our poor
knowledge of the density dependence and effective mass.Comment: 19 pages, 8 figures. Submitted to PR
Macroscopic Parity Violation and Supernova Asymmetries
Core collapse supernovae are dominated by weakly interacting neutrinos. This
provides a unique opportunity for macroscopic parity violation. We speculate
that parity violation in a strong magnetic field can lead to an asymmetry in
the explosion and a recoil of the newly formed neutron star. We estimate the
asymmetry from neutrino-polarized-neutron elastic scattering, polarized
electron capture and neutrino-nucleus elastic scattering in a (partially)
polarized electron gas.Comment: Nine pages Revtex, two postscript figures (included
Shell model study of the pairing correlations
A systematic study of the pairing correlations as a function of temperature
and angular momentum has been performed in the sd-shell region using the
spherical shell model approach. The pairing correlations have been derived for
even-even, even-odd and odd-odd systems near N=Z and also for the asymmetric
case of N=Z+4. The results indicate that the pairing content and the behavior
of pair correlations is similar in even-even and odd-mass nuclei. For odd-odd
N=Z system, angular momentum I=0 state is an isospin, t=1 neutron-proton paired
configuration. Further, these t=1 correlations are shown to be dramatically
reduced for the asymmetric case of N=Z+4. The shell model results obtained are
qualitatively explained within a simplified degenerate model
Instabilities of infinite matter with effective Skyrme-type interactions
The stability of the equation of state predicted by Skyrme-type interactions
is examined. We consider simultaneously symmetric nuclear matter and pure
neutron matter. The stability is defined by the inequalities that the Landau
parameters must satisfy simultaneously. A systematic study is carried out to
define interaction parameter domains where the inequalities are fulfilled. It
is found that there is always a critical density beyond which the
system becomes unstable. The results indicate in which parameter regions one
can find effective forces to describe correctly finite nuclei and give at the
same time a stable equation of state up to densities of 3-4 times the
saturation density of symmetric nuclear matter.Comment: 20 pages, 5 figures, submitted to Phys.Rev.
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