11,928 research outputs found
A Link Between the Semi-Major Axis of Extrasolar Gas Giant Planets and Stellar Metallicity
The fact that most extrasolar planets found to date are orbiting metal-rich
stars lends credence to the core accretion mechanism of gas giant planet
formation over its competitor, the disc instability mechanism. However, the
core accretion mechanism is not refined to the point of explaining orbital
parameters such as their unexpected semi-major axes and eccentricities. We
propose a model, which correlates the metallicity of the host star with the
original semi-major axis of its most massive planet, prior to migration,
considering that the core accretion scenario governs giant gas planet
formation. The model predicts that the optimum regions for planetary formation
shift inward as stellar metallicity decreases, providing an explanation for the
observed absence of long period planets in metal-poor stars. We compare our
predictions with the available data on extrasolar planets for stars with masses
similar to the mass of the Sun. A fitting procedure produces an estimate of
what we define as the Zero Age Planetary Orbit (ZAPO) curve as a function of
the metallicity of the star. The model also hints that the lack of planets
circling metal-poor stars may be partly caused by an enhanced destruction
probability during the migration process, since the planets lie initially
closer to the central stars.Comment: Nature of the replacement: According to recent simulations, the
temperature profile, T, is more adequately reproduced by beta = 1 rather than
beta = 2. We have introduced a distance scale factor that solves the very
fast drop of T for low metallicity and introduces naturally the inferior
distance limit of our ZAPO. Under this modification all the fitting process
was altere
Configuration mixing calculation for complete low-lying spectra with the mean-field Hamiltonian
We propose a new theoretical approach to ground and low-energy excited states
of nuclei extending the nuclear mean-field theory. It consists of three steps:
stochastic preparation of many Slater determinants, the parity and angular
momentum projection, and diagonalization of the generalized eigenvalue
problems. The Slater determinants are constructed in the three-dimensional
Cartesian coordinate representation capable of describing arbitrary shape of
nuclei. We examine feasibility and usefulness of the method by applying the
method with the BKN interaction to light 4N-nuclei, 12C, 16O, and 20Ne. We
discuss difficulties of keeping linear independence for basis states projected
on good parity and angular momentum and present a possible prescription.Comment: 12 pages, revtex
Escape configuration lattice near the nematic-isotropic transition: Tilt analogue of blue phases
We predict the possible existence of a new phase of liquid crystals near the
nematic-isotropic () transition. This phase is an achiral, tilt-analogue
of the blue phase and is composed of a lattice of {\em double-tilt},
escape-configuration cylinders. We discuss the structure and the stability of
this phase and provide an estimate of the lattice parameter.Comment: 5 pages, 6 figures (major revision, typos corrected, references
added
Early transitions and tertiary enrolment: The cumulative impact of primary and secondary effects on entering university in Germany
Our aim is to assess how the number of working class students entering German universities can effectively be increased. Therefore, we estimate the proportion of students from the working class that would successfully enter university if certain policy interventions were in place to eliminate primary effects (performance differentials between social classes) and/or secondary effects (choice differentials net of performance) at different transition points. We extend previous research by analysing the sequence of transitions between elementary school enrolment and university enrolment and by accounting for the impact that manipulations at earlier transitions have on the performance distribution and size of the student ‘risk-set’ at subsequent transitions. To this end, we develop a novel simulation procedure which also seeks to find viable solutions to the shortcomings in the German data landscape. Our findings show that interventions are most effective if they take place early in the educational career. Neutralizing secondary effects at the transition to upper secondary school proves to be the single most effective means to increase participation rates in tertiary education among working class students. However, this comes at the expense of lower average performance levels. (DIPF/author
Tidal stirring and the origin of dwarf spheroidals in the Local Group
N-Body/SPH simulations are used to study the evolution of dwarf irregular
galaxies (dIrrs) entering the dark matter halo of the Milky Way or M31 on
plunging orbits. We propose a new dynamical mechanism driving the evolution of
gas rich, rotationally supported dIrrs, mostly found at the outskirts of the
Local Group (LG), into gas free, pressure supported dwarf spheroidals (dSphs)
or dwarf ellipticals (dEs), observed to cluster around the two giant spirals.
The initial model galaxies are exponential disks embedded in massive dark
matter halos and reproduce nearby dIrrs. Repeated tidal shocks at the
pericenter of their orbit partially strip their halo and disk and trigger
dynamical instabilities that dramatically reshape their stellar component.
After only 2-3 orbits low surface brightness (LSB) dIrrs are transformed into
dSphs, while high surface brightness (HSB) dIrrs evolve into dEs. This
evolutionary mechanism naturally leads to the morphology-density relation
observed for LG dwarfs. Dwarfs surrounded by very dense dark matter halos, like
the archetypical dIrr GR8, are turned into Draco or Ursa Minor, the faintest
and most dark matter dominated among LG dSphs. If disks include a gaseous
component, this is both tidally stripped and consumed in periodic bursts of
star formation. The resulting star formation histories are in good qualitative
agreement with those derived using HST color-magnitude diagrams for local
dSphs.Comment: 5 pages, 5 figures, to appear on ApJL. Simulation images and movies
can be found at the Local Group web page at
http://pcblu.uni.mi.astro.it/~lucio/LG/LG.htm
Ciclo 2010/2011 de seleção de porta-enxertos potencialmente tolerantes à morte precoce do pessegueiro.
Diffractive Interaction and Scaling Violation in pp->pi^0 Interaction and GeV Excess in Galactic Diffuse Gamma-Ray Spectrum of EGRET
We present here a new calculation of the gamma-ray spectrum from pp->pi^0 in
the Galactic ridge environment. The calculation includes the diffractive pp
interaction and incorporates the Feynman scaling violation for the first time.
Galactic diffuse gamma-rays come, predominantly, from pi^0->gamma gamma in the
sub-GeV to multi-GeV range. Hunter et al. found, however, an excess in the GeV
range ("GeV Excess") in the EGRET Galactic diffuse spectrum above the
prediction based on experimental pp->pi^0 cross-sections and the Feynman
scaling hypothesis. We show, in this work, that the diffractive process makes
the gamma-ray spectrum harder than the incident proton spectrum by ~0.05 in
power-law index, and, that the scaling violation produces 30-80% more pi^0 than
the scaling model for incident proton energies above 100GeV. Combination of the
two can explain about a half of the "GeV Excess" with the local cosmic proton
(power-law index ~2.7). The excess can be fully explained if the proton
spectral index in the Galactic ridge is a little harder (~0.2 in power-law
index) than the local spectrum. Given also in the paper is that the diffractive
process enhances e^+ over e^- and the scaling violation gives 50-100% higher
p-bar yield than without the violation, both in the multi-GeV range.Comment: 35 pages, 11 figures, to appear in Astrophysical Journa
A branch-point approximant for the equation of state of hard spheres
Using the first seven known virial coefficients and forcing it to possess two
branch-point singularities, a new equation of state for the hard-sphere fluid
is proposed. This equation of state predicts accurate values of the higher
virial coefficients, a radius of convergence smaller than the close-packing
value, and it is as accurate as the rescaled virial expansion and better than
the Pad\'e [3/3] equations of state. Consequences regarding the convergence
properties of the virial series and the use of similar equations of state for
hard-core fluids in dimensions are also pointed out.Comment: 6 pages, 4 tables, 3 figures; v2: enlarged version, extension to
other dimensionalities; v3: typos in references correcte
Dispersion of imbibition fronts
We have studied the dispersive behaviour of imbibition fronts in a porous
medium by X-ray tomography. Injection velocities were varied and the porous
medium was initially prewetted or not. At low velocity in the prewetted medium,
the imbibition profiles are found to be distinctly hyperdispersive. The
profiles are anomalously extended when compared to tracer fronts exhibiting
conventional (Gaussian) dispersion. We observe a strong velocity dependence of
the exponent characterizing the divergence of the dispersion coefficient for
low wetting-fluid saturation. Hyperdispersion is absent at high imbibition
velocities or when the medium is not prewetted.Comment: 8 pages, 5 figures; submitted to Europhysics Letter
Embedded disks in Fornax dwarf ellipticals
We present photometric and kinematic evidence for the presence of stellar
disks, seen practically edge-on, in two Fornax dwarf galaxies, FCC204 (dS0(6))
and FCC288 (dS0(7)). This is the first time such structures have been
identified in Fornax dwarfs. FCC2088 has only a small bulge and a bright
flaring and slightly warped disk that can be traced out to 23" from the center
(2.05 kpc for H_0=75 km/s/Mpc). FCC204's disk can be traced out to 20" (1.78
kpc). This galaxy possesses a large bulge. These results can be compared to the
findings of Jerjen et al. (2000) and Barazza et al. (2002) who discovered
nucleated dEs with spiral and bar features in the Virgo Cluster.Comment: 8 pages, 8 figures, accepted for publication in A&
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