170 research outputs found
3D AMR hydrosimulations of a compact source scenario for the Galactic Centre cloud G2
The nature of the gaseous and dusty cloud G2 in the Galactic Centre is still
under debate. We present three-dimensional hydrodynamical adaptive mesh
refinement (AMR) simulations of G2, modeled as an outflow from a "compact
source" moving on the observed orbit. The construction of mock
position-velocity (PV) diagrams enables a direct comparison with observations
and allow us to conclude that the observational properties of the gaseous
component of G2 could be matched by a massive () and slow ()
outflow, as observed for T Tauri stars. In order for this to be true, only the
material at larger () distances from the source must be
actually emitting, otherwise G2 would appear too compact compared to the
observed PV diagrams. On the other hand, the presence of a central dusty source
might be able to explain the compactness of G2's dust component. In the present
scenario, 5-10 years after pericentre the compact source should decouple from
the previously ejected material, due to the hydrodynamic interaction of the
latter with the surrounding hot and dense atmosphere. In this case, a new
outflow should form, ahead of the previous one, which would be the smoking gun
evidence for an outflow scenario.Comment: resubmitted to MNRAS after referee report, 16 pages, 11 figure
Molecular geometry optimization with a genetic algorithm
We present a method for reliably determining the lowest energy structure of
an atomic cluster in an arbitrary model potential. The method is based on a
genetic algorithm, which operates on a population of candidate structures to
produce new candidates with lower energies. Our method dramatically outperforms
simulated annealing, which we demonstrate by applying the genetic algorithm to
a tight-binding model potential for carbon. With this potential, the algorithm
efficiently finds fullerene cluster structures up to starting
from random atomic coordinates.Comment: 4 pages REVTeX 3.0 plus 3 postscript figures; to appear in Physical
Review Letters. Additional information available under "genetic algorithms"
at http://www.public.iastate.edu/~deaven
Stellar feedback efficiencies: supernovae versus stellar winds
The final, definitive version of this paper has been published in Monthly Notices of the Royal Astronomical Society, Vol. 456(1): 710-730, February 2016, DOI: 10.1093/mnras/stv2699, published by Oxford University Press on behalf of MNRAS.Stellar winds and supernova (SN) explosions of massive stars (`stellar feedback') create bubbles in the interstellar medium (ISM) and insert newly produced heavy elements and kinetic energy into their surroundings, possibly driving turbulence. Most of this energy is thermalized and immediately removed from the ISM by radiative cooling. The rest is available for driving ISM dynamics. In this work we estimate the amount of feedback energy retained as kinetic energy when the bubble walls have decelerated to the sound speed of the ambient medium. We show that the feedback of the most massive star outweighs the feedback from less massive stars. For a giant molecular cloud (GMC) mass of 105 Mâ (as e.g. found in the Orion GMCs) and a star formation efficiency of 8 per cent the initial mass function predicts a most massive star of approximately 60 Mâ. For this stellar evolution model we test the dependence of the retained kinetic energy of the cold GMC gas on the inclusion of stellar winds. In our model winds insert 2.34 times the energy of an SN and create stellar wind bubbles serving as pressure reservoirs. We find that during the pressure-driven phases of the bubble evolution radiative losses peak near the contact discontinuity (CD), and thus the retained energy depends critically on the scales of the mixing processes across the CD. Taking into account the winds of massive stars increases the amount of kinetic energy deposited in the cold ISM from 0.1 per cent to a few per cent of the feedback energy.Peer reviewe
Zero Temperature Phases of the Electron Gas
The stability of different phases of the three-dimensional non-relativistic
electron gas is analyzed using stochastic methods. With decreasing density, we
observe a {\it continuous} transition from the paramagnetic to the
ferromagnetic fluid, with an intermediate stability range () for the {\it partially} spin-polarized liquid. The freezing
transition into a ferromagnetic Wigner crystal occurs at . We
discuss the relative stability of different magnetic structures in the solid
phase, as well as the possibility of disordered phases.Comment: 4 pages, REVTEX, 3 ps figure
3D AMR simulations of G2 as an outflow
We study the evolution of G2 in a \textit{Compact Source Scenario}, where G2
is the outflow from a low-mass central star moving on the observed orbit. This
is done through 3D AMR simulations of the hydrodynamic interaction of G2 with
the surrounding hot accretion flow. A comparison with observations is done by
means of mock position-velocity (PV) diagrams. We found that a massive
() and slow
() outflow can reproduce G2's
properties. A faster outflow () might
also be able to explain the material that seems to follow G2 on the same orbit.Comment: 2 pages, 1 figure, Proceedings of IAU Symposium 322: The
Multi-Messenger Astrophysics of the Galactic Centr
Sparse random matrices and vibrational spectra of amorphous solids
A random matrix approach is used to analyze the vibrational properties of
amorphous solids. We investigated a dynamical matrix M=AA^T with non-negative
eigenvalues. The matrix A is an arbitrary real NxN sparse random matrix with n
independent non-zero elements in each row. The average values =0 and
dispersion =V^2 for all non-zero elements. The density of vibrational
states g(w) of the matrix M for N,n >> 1 is given by the Wigner quarter circle
law with radius independent of N. We argue that for n^2 << N this model can be
used to describe the interaction of atoms in amorphous solids. The level
statistics of matrix M is well described by the Wigner surmise and corresponds
to repulsion of eigenfrequencies. The participation ratio for the major part of
vibrational modes in three dimensional system is about 0.2 - 0.3 and
independent of N. Together with term repulsion it indicates clearly to the
delocalization of vibrational excitations. We show that these vibrations spread
in space by means of diffusion. In this respect they are similar to diffusons
introduced by Allen, Feldman, et al., Phil. Mag. B 79, 1715 (1999) in amorphous
silicon. Our results are in a qualitative and sometimes in a quantitative
agreement with molecular dynamic simulations of real and model glasses.Comment: 24 pages, 7 figure
Density-functional-based predictions of Raman and IR spectra for small Si clusters
We have used a density-functional-based approach to study the response of silicon clusters to applied electric fields. For the dynamical response, we have calculated the Raman activities and infrared (IR) intensities for all of the vibrational modes of several clusters (SiN with N=3-8, 10, 13, 20, and 21) using the local density approximation (LDA). For the smaller clusters (N=3-8) our results are in good agreement with previous quantum-chemical calculations and experimental measurements, establishing that LDA-based IR and Raman data can be used in conjunction with measured spectra to determine the structure of clusters observed in experiment. To illustrate the potential of the method for larger clusters, we present calculated IR and Raman data for two low-energy isomers of Si10 and for the lowest-energy structure of Si13 found to date. For the static response, we compare our calculated polarizabilities for N=10, 13, 20, and 21 to recent experimental measurements. The calculated results are in rough agreement with experiment, but show less variation with cluster size than the measurements. Taken together, our results show that LDA calculations can offer a powerful means for establishing the structures of experimentally fabricated clusters and nanoscale systems
Diffusion Monte Carlo study of circular quantum dots
We present ground and excited state energies obtained from Diffusion Monte
Carlo (DMC) calculations, using accurate multiconfiguration wave functions, for
electrons () confined to a circular quantum dot. We analyze the
electron-electron pair correlation functions and compare the density and
correlation energies to the predictions of local spin density approximation
theory (LSDA). The DMC estimated change in electrochemical potential as
function of the number of electrons in the dot is compared to that from LSDA
and Hartree-Fock (HF) calculations.Comment: 7 pages, 4 eps figures. To be published in Phys. Rev. B, September
15th 2000. See erratum cond-mat/030571
Surface reconstruction induced geometries of Si clusters
We discuss a generalization of the surface reconstruction arguments for the
structure of intermediate size Si clusters, which leads to model geometries for
the sizes 33, 39 (two isomers), 45 (two isomers), 49 (two isomers), 57 and 61
(two isomers). The common feature in all these models is a structure that
closely resembles the most stable reconstruction of Si surfaces, surrounding a
core of bulk-like tetrahedrally bonded atoms. We investigate the energetics and
the electronic structure of these models through first-principles density
functional theory calculations. These models may be useful in understanding
experimental results on the reactivity of Si clusters and their shape as
inferred from mobility measurements.Comment: 9 figures (available from the author upon request) Submitted to Phys.
Rev.
Structure and Dynamics of Liquid Iron under Earth's Core Conditions
First-principles molecular dynamics simulations based on density-functional
theory and the projector augmented wave (PAW) technique have been used to study
the structural and dynamical properties of liquid iron under Earth's core
conditions. As evidence for the accuracy of the techniques, we present PAW
results for a range of solid-state properties of low- and high-pressure iron,
and compare them with experimental values and the results of other
first-principles calculations. In the liquid-state simulations, we address
particular effort to the study of finite-size effects, Brillouin-zone sampling
and other sources of technical error. Results for the radial distribution
function, the diffusion coefficient and the shear viscosity are presented for a
wide range of thermodynamic states relevant to the Earth's core. Throughout
this range, liquid iron is a close-packed simple liquid with a diffusion
coefficient and viscosity similar to those of typical simple liquids under
ambient conditions.Comment: 13 pages, 8 figure
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