12,934 research outputs found
Modified newtonian dynamics and non-relativistic ChSAS gravity
In the context of the non-relativistic theories, a generalization of the
Chern--Weil-theorem allows us to show that extended Chern--Simons actions for
gravity in d=4 invariant under some specific non-relativistic groups lead to
modified Poisson equations. In some particular cases, these modified equations
have the form of the so-called MOND approach to gravity. The modifications
could be understood as due to the effects of dark matter. This result could
leads us to think that dark matter can be interpreted as a non-relativistic
limit of dark energy
On Nilcompactifications of Prime Spectra of Commutative Rings
Given a ring R and S one of its ideals, we obtain a compactification of the
prime spectrum of S through a mainly algebraic process. We name it the
R-nilcompactification of SpecS. We study some categorical properties of this
construction.Comment: 12 pages, 8 Tikz figure
Benchmarking Nonequilibrium Green's Functions against Configuration Interaction for time-dependent Auger decay processes
We have recently proposed a Nonequilibrium Green's Function (NEGF) approach
to include Auger decay processes in the ultrafast charge dynamics of
photoionized molecules. Within the so called Generalized Kadanoff-Baym Ansatz
the fundamental unknowns of the NEGF equations are the reduced one-particle
density matrix of bound electrons and the occupations of the continuum states.
Both unknowns are one-time functions like the density in Time-Dependent
Functional Theory (TDDFT). In this work we assess the accuracy of the approach
against Configuration Interaction (CI) calculations in one-dimensional model
systems. Our results show that NEGF correctly captures qualitative and
quantitative features of the relaxation dynamics provided that the energy of
the Auger electron is much larger than the Coulomb repulsion between two holes
in the valence shells. For the accuracy of the results dynamical
electron-electron correlations or, equivalently, memory effects play a pivotal
role. The combination of our NEGF approach with the Sham-Schl\"uter equation
may provide useful insights for the development of TDDFT exchange-correlation
potentials with a history dependence.Comment: 7 pages, 3 figure
Sodium: a charge-transfer insulator at high pressures
By means of first-principles methods we analyze the optical response of
transparent dense sodium as a function of applied pressure. We discover an
unusual kind of charge-transfer exciton that proceeds from the interstitial
distribution of valence electrons repelled away from the ionic cores by the
Coulomb interaction and the Pauli repulsion. The predicted absorption spectrum
shows a strong anisotropy with light polarization that just at pressures above
the metal-insulator transition manifests as sodium being optically transparent
in one direction but reflective in the other. This result provides a key
information about the crystal structure of transparent sodium, a new
unconventional inorganic electride.Comment: revtex4, 5+8 page
The Magellanic Bridge cluster NGC 796: Deep optical AO imaging reveals the stellar content and initial mass function of a massive open cluster
NGC 796 is a massive young cluster located 59 kpc from us in the diffuse
intergalactic medium of the 1/5-1/10 Magellanic Bridge, allowing to
probe variations in star formation and stellar evolution processes as a
function of metallicity in a resolved fashion, providing a link between
resolved studies of nearby solar-metallicity and unresolved distant metal-poor
clusters located in high-redshift galaxies. In this paper, we present adaptive
optics H imaging of NGC 796 (at 0.5", which is ~0.14 pc at the
cluster distance) along with optical spectroscopy of two bright members to
quantify the cluster properties. Our aim is to explore if star formation and
stellar evolution varies as a function of metallicity by comparing the
properties of NGC 796 to higher metallicity clusters. We find from isochronal
fitting of the cluster main sequence in the colour-magnitude diagram an age of
20 Myr. Based on the cluster luminosity function, we derive a
top-heavy stellar initial mass function (IMF) with a slope =
1.990.2, hinting at an metallicity and/or environmental dependence of the
IMF which may lead to a top-heavy IMF in the early Universe. Study of the
H emission line stars reveals that Classical Be stars constitute a
higher fraction of the total B-type stars when compared with similar clusters
at greater metallicity, providing some support to the chemically homogeneous
theory of stellar evolution. Overall, NGC 796 has a total estimated mass of
990 , and a core radius of 1.40.3 pc which classifies
it as a massive young open cluster, unique in the diffuse interstellar medium
of the Magellanic Bridge.Comment: Accepted for publication in the Astrophysical Journal. Contains 14
pages, 11 figures, and 3 table
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