3,799 research outputs found
General spherical anisotropic Jeans models of stellar kinematics: including proper motions and radial velocities
Cappellari (2008) presented a flexible and efficient method to model the
stellar kinematics of anisotropic axisymmetric and spherical stellar systems.
The spherical formalism could be used to model the line-of-sight velocity
second moments allowing for essentially arbitrary radial variation in the
anisotropy and general luminous and total density profiles. Here we generalize
the spherical formalism by providing the expressions for all three components
of the projected second moments, including the two proper motion components. A
reference implementation is now included in the public JAM package available at
http://purl.org/cappellari/softwareComment: 3 pages, 2 figures, LaTeX. Not submitted anywhere but here. Software
implementing the update to the JAM method described in this paper is
available at http://purl.org/cappellari/softwar
Online Neural Path Guiding with Normalized Anisotropic Spherical Gaussians
The variance reduction speed of physically-based rendering is heavily
affected by the adopted importance sampling technique. In this paper we propose
a novel online framework to learn the spatial-varying density model with a
single small neural network using stochastic ray samples. To achieve this task,
we propose a novel closed-form density model called the normalized anisotropic
spherical gaussian mixture, that can express complex irradiance fields with a
small number of parameters. Our framework learns the distribution in a
progressive manner and does not need any warm-up phases. Due to the compact and
expressive representation of our density model, our framework can be
implemented entirely on the GPU, allowing it produce high quality images with
limited computational resources
A nonlinear dynamics approach to Bogoliubov excitations of Bose-Einstein condensates
We assume the macroscopic wave function of a Bose-Einstein condensate as a
superposition of Gaussian wave packets, with time-dependent complex width
parameters, insert it into the mean-field energy functional corresponding to
the Gross-Pitaevskii equation (GPE) and apply the time-dependent variational
principle. In this way the GPE is mapped onto a system of coupled equations of
motion for the complex width parameters, which can be analyzed using the
methods of nonlinear dynamics. We perform a stability analysis of the fixed
points of the nonlinear system, and demonstrate that the eigenvalues of the
Jacobian reproduce the low-lying quantum mechanical Bogoliubov excitation
spectrum of a condensate in an axisymmetric trap.Comment: 7 pages, 3 figures, Proceedings of the "8th International Summer
School/Conference Let's Face Chaos Through Nonlinear Dynamics", CAMTP,
University of Maribor, Slovenia, 26 June - 10 July 201
Mass estimates from stellar proper motions: The mass of Centauri
We lay out and apply methods to use proper motions of individual kinematic
tracers for estimating the dynamical mass of star clusters. We first describe a
simple projected mass estimator and then develop an approach that evaluates
directly the likelihood of the discrete kinematic data given the model
predictions. Those predictions may come from any dynamical modelling approach,
and we implement an analytic King model, a spherical isotropic Jeans equation
model and an axisymmetric, anisotropic Jeans equation model.We apply these
approaches to the enigmatic globular cluster omega Centauri, combining the
proper motion from van Leeuwen et al (2000) with improved photometric cluster
membership probabilities. We show that all mass estimates based on spherical
isotropic models yield , where our modelling allows us to show how the statistical precision of
this estimate improves as more proper motion data of lower signal-to-noise are
included. MLM predictions, based on an anisotropic axisymmetric Jeans model,
indicate for Cen that the inclusion of anisotropies is not important
for the mass estimates, but that accounting for the flattening is: flattened
models imply , 10%
lower than when restricting the analysis to a spherical model. The best current
distance estimates imply an additional uncertainty in the mass estimate of 12%.Comment: Accepted for publication in MNRA
Measuring the inclination and mass-to-light ratio of axisymmetric galaxies via anisotropic Jeans models of stellar kinematics
We present a simple and efficient anisotropic generalization of the
semi-isotropic (two-integral) axisymmetric Jeans formalism which is used to
model the stellar kinematics of galaxies. The following is assumed: (i) a
constant mass-to-light ratio M/L and (ii) a velocity ellipsoid that is aligned
with cylindrical coordinates (R,z) and characterized by the classic anisotropy
parameter beta_z=1-sigma_z^2/sigma_R^2. Our simple models are fit to SAURON
integral-field observations of the stellar kinematics for a set of fast-rotator
early-type galaxies. With only two free parameters (beta_z and the inclination)
the models generally provide remarkably good descriptions of the shape of the
first (V) and second (V_rms=sqrt{V^2+sigma^2}) velocity moments, once a
detailed description of the surface brightness is given. This is consistent
with previous findings on the simple dynamical structure of these objects. With
the observationally-motivated assumption that beta_z>0, the method is able to
recover the inclination. The technique can be used to determine the dynamical
mass-to-light ratios and angular momenta of early-type fast-rotators and spiral
galaxies, especially when the quality of the data does not justify more
sophisticated modeling approaches. This formalism allows for the inclusion of
dark matter, supermassive black holes, spatially varying anisotropy, and
multiple kinematic components.Comment: 16 pages, 7 figures, LaTeX. Published in MNRAS. Software implementing
the JAM method described in this paper is available at
http://www-astro.physics.ox.ac.uk/~mxc/idl
Relation between the eigenfrequencies of Bogoliubov excitations of Bose-Einstein condensates and the eigenvalues of the Jacobian in a time-dependent variational approach
We study the relation between the eigenfrequencies of the Bogoliubov
excitations of Bose-Einstein condensates, and the eigenvalues of the Jacobian
stability matrix in a variational approach which maps the Gross-Pitaevskii
equation to a system of equations of motion for the variational parameters. We
do this for Bose-Einstein condensates with attractive contact interaction in an
external trap, and for a simple model of a self-trapped Bose-Einstein
condensate with attractive 1/r interaction. The stationary solutions of the
Gross-Pitaevskii equation and Bogoliubov excitations are calculated using a
finite-difference scheme. The Bogoliubov spectra of the ground and excited
state of the self-trapped monopolar condensate exhibits a Rydberg-like
structure, which can be explained by means of a quantum defect theory. On the
variational side, we treat the problem using an ansatz of time-dependent
coupled Gaussians combined with spherical harmonics. We first apply this ansatz
to a condensate in an external trap without long-range interaction, and
calculate the excitation spectrum with the help of the time-dependent
variational principle. Comparing with the full-numerical results, we find a
good agreement for the eigenfrequencies of the lowest excitation modes with
arbitrary angular momenta. The variational method is then applied to calculate
the excitations of the self-trapped monopolar condensates, and the
eigenfrequencies of the excitation modes are compared.Comment: 15 pages, 12 figure
Real-Time Hand Tracking Using a Sum of Anisotropic Gaussians Model
Real-time marker-less hand tracking is of increasing importance in
human-computer interaction. Robust and accurate tracking of arbitrary hand
motion is a challenging problem due to the many degrees of freedom, frequent
self-occlusions, fast motions, and uniform skin color. In this paper, we
propose a new approach that tracks the full skeleton motion of the hand from
multiple RGB cameras in real-time. The main contributions include a new
generative tracking method which employs an implicit hand shape representation
based on Sum of Anisotropic Gaussians (SAG), and a pose fitting energy that is
smooth and analytically differentiable making fast gradient based pose
optimization possible. This shape representation, together with a full
perspective projection model, enables more accurate hand modeling than a
related baseline method from literature. Our method achieves better accuracy
than previous methods and runs at 25 fps. We show these improvements both
qualitatively and quantitatively on publicly available datasets.Comment: 8 pages, Accepted version of paper published at 3DV 201
N-body realizations of cuspy dark matter haloes
We describe an algorithm for generating equilibrium initial conditions for numerical experiments with dark matter haloes. Our haloes are modelled using a general form for the mass density p{r), making it possible to represent most of the popular density profiles in the literature. The finite mass 7-models and the cuspy density profiles found in recent high-resolution cosmological TV-body simulations having a density power-law fall-off at large distances proportional to are included as special cases. The algorithm calculates the phase-space distribution function of each model assuming spherical symmetry and either an isotropic velocity dispersion tensor or an anisotropic velocity dispersion tensor of the type proposed by Osipkov and Merritt. The particle velocities are assigned according to the exact velocity distribution, making this method ideal for experiments requiring a high degree of stability. Numerical tests confirm that the resulting models are highly stable. This approach is motivated by the instabilities that arise when a local Maxwellian velocity distribution is adopted. For example, after approximating the velocity distribution by a Gaussian we show that a Hernquist halo with an initial r(^-1) density cusp immediately develops a constant density core. Moreover, after a single crossing time the orbital anisotropy has evolved over the entire system. Previous studies that use this approximation to construct halo or galaxy models could be compromised by this behaviour. Using the derived distribution functions we show the exact 1-d velocity distributions and we compare them with the Gaussian velocity distributions with the same second moment for different distances from the halo centre. We show that instabilities arise because a Gaussian velocity distribution is a very poor approximation to the true velocity distribution of particles. We also perform a series of numerical simulations evolving several dark matter halo models in isolation, with the intention of checking the stability of the initialization procedure in both configuration and velocity space. A subset of the models are evolved under the assumption that the velocity distribution at any given point is a Gaussian and the time evolution of the density profiles and velocity structure is monitored. Finally, a number of applications are discussed, including issues of relaxation in dark matter haloes as well as mergers of haloes in scattering experiments
A simple method for evaluating low-energy electron-molecule scattering cross sections using discrete basis functions
We present a simple, approximate method for calculating low-energy electron-molecule scattering cross sections using only the results of a basis set diagonalization of the molecular Hamiltonian. The method is based on the approximate conservation of orbital angular momentum in collisions between slow electrons and molecules lacking a permanent dipole moment (low l spoiling). Results are presented for e--H2, and e--N2, in the static-exchange approximation
- …