3,173 research outputs found
The analog data assimilation
In light of growing interest in data-driven methods for oceanic, atmospheric, and climate sciences, this work focuses on the field of data assimilation and presents the analog data assimilation (AnDA). The proposed framework produces a reconstruction of the system dynamics in a fully data-driven manner where no explicit knowledge of the dynamical model is required. Instead, a representative catalog of trajectories of the system is assumed to be available. Based on this catalog, the analog data assimilation combines the nonparametric sampling of the dynamics using analog forecasting methods with ensemble-based assimilation techniques. This study explores different analog forecasting strategies and derives both ensemble Kalman and particle filtering versions of the proposed analog data assimilation approach. Numerical experiments are examined for two chaotic dynamical systems: the Lorenz-63 and Lorenz-96 systems. The performance of the analog data assimilation is discussed with respect to classical model-driven assimilation. A Matlab toolbox and Python library of the AnDA are provided to help further research building upon the present findings.Fil: Lguensat, Redouane. Université Bretagne Loire; FranciaFil: Tandeo, Pierre. Université Bretagne Loire; FranciaFil: Ailliot, Pierre. University of Western Brittany. Laboratoire de Mathématiques de Bretagne Atlantique; FranciaFil: Pulido, Manuel Arturo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Nordeste. Instituto de Modelado e Innovación Tecnológica. Universidad Nacional del Nordeste. Facultad de Ciencias Exactas Naturales y Agrimensura. Instituto de Modelado e Innovación Tecnológica; ArgentinaFil: Fablet, Ronan. Université Bretagne Loire; Franci
Estimation and uncertainty of reversible Markov models
Reversibility is a key concept in Markov models and Master-equation models of
molecular kinetics. The analysis and interpretation of the transition matrix
encoding the kinetic properties of the model relies heavily on the
reversibility property. The estimation of a reversible transition matrix from
simulation data is therefore crucial to the successful application of the
previously developed theory. In this work we discuss methods for the maximum
likelihood estimation of transition matrices from finite simulation data and
present a new algorithm for the estimation if reversibility with respect to a
given stationary vector is desired. We also develop new methods for the
Bayesian posterior inference of reversible transition matrices with and without
given stationary vector taking into account the need for a suitable prior
distribution preserving the meta- stable features of the observed process
during posterior inference. All algorithms here are implemented in the PyEMMA
software - http://pyemma.org - as of version 2.0
The SWELLS survey. IV. Precision measurements of the stellar and dark matter distributions in a spiral lens galaxy
We construct a fully self-consistent mass model for the lens galaxy J2141 at
z=0.14, and use it to improve on previous studies by modelling its
gravitational lensing effect, gas rotation curve and stellar kinematics
simultaneously. We adopt a very flexible axisymmetric mass model constituted by
a generalized NFW dark matter halo and a stellar mass distribution obtained by
deprojecting the MGE fit to the high-resolution K'-band LGSAO imaging data of
the galaxy, with the (spatially constant) M/L ratio as a free parameter. We
model the stellar kinematics by solving the anisotropic Jeans equations. We
find that the inner logarithmic slope of the dark halo is weakly constrained
(gamma = 0.82^{+0.65}_{-0.54}), and consistent with an unmodified NFW profile.
We infer the galaxy to have (i) a dark matter fraction within 2.2 disk radii of
0.28^{+0.15}_{-0.10}, independent of the galaxy stellar population, implying a
maximal disk for J2141; (ii) an apparently uncontracted dark matter halo, with
concentration c_{-2} = 7.7_{-2.5}^{+4.2} and virial velocity v_{vir} =
242_{-39}^{+44} km/s, consistent with LCDM predictions; (iii) a slightly oblate
halo (q_h = 0.75^{+0.27}_{-0.16}), consistent with predictions from
baryon-affected models. Comparing the stellar mass inferred from the combined
analysis (log_{10} Mstar/Msun = 11.12_{-0.09}^{+0.05}) with that inferred from
SPS modelling of the galaxies colours, and accounting for a cold gas fraction
of 20+/-10%, we determine a preference for a Chabrier IMF over Salpeter IMF by
a Bayes factor of 5.7 (substantial evidence). We infer a value beta_{z} = 1 -
sigma^2_{z}/sigma^2_{R} = 0.43_{-0.11}^{+0.08} for the orbital anisotropy
parameter in the meridional plane, in agreement with most studies of local disk
galaxies, and ruling out at 99% CL that the dynamics of this system can be
described by a two-integral distribution function. [Abridged]Comment: Accepted for publication in MNRAS. 17 pages, 9 figure
Turbulent Disks are Never Stable: Fragmentation and Turbulence-Promoted Planet Formation
A fundamental assumption in our understanding of disks is that when the
Toomre Q>>1, the disk is stable against fragmentation into self-gravitating
objects (and so cannot form planets via direct collapse). But if disks are
turbulent, this neglects a spectrum of stochastic density fluctuations that can
produce rare, high-density mass concentrations. Here, we use a
recently-developed analytic framework to predict the statistics of these
fluctuations, i.e. the rate of fragmentation and mass spectrum of fragments
formed in a turbulent Keplerian disk. Turbulent disks are never completely
stable: we calculate the (always finite) probability of forming
self-gravitating structures via stochastic turbulent density fluctuations in
such disks. Modest sub-sonic turbulence above Mach number ~0.1 can produce a
few stochastic fragmentation or 'direct collapse' events over ~Myr timescales,
even if Q>>1 and cooling is slow (t_cool>>t_orbit). In trans-sonic turbulence
this extends to Q~100. We derive the true Q-criterion needed to suppress such
events, which scales exponentially with Mach number. We specify to turbulence
driven by MRI, convection, or spiral waves, and derive equivalent criteria in
terms of Q and the cooling time. Cooling times >~50*t_dyn may be required to
completely suppress fragmentation. These gravoturbulent events produce mass
spectra peaked near ~M_disk*(Q*M_disk/M_star)^2 (rocky-to-giant planet masses,
increasing with distance from the star). We apply this to protoplanetary disk
models and show that even minimum mass solar nebulae could experience
stochastic collapse events, provided a source of turbulence.Comment: 15 pages, 5 figures (+appendix), accepted to ApJ (added
clarifications and discussion to match accepted version
Galaxy Masses
Galaxy masses play a fundamental role in our understanding of structure
formation models. This review addresses the variety and reliability of mass
estimators that pertain to stars, gas, and dark matter. The different sections
on masses from stellar populations, dynamical masses of gas-rich and gas-poor
galaxies, with some attention paid to our Milky Way, and masses from weak and
strong lensing methods, all provide review material on galaxy masses in a
self-consistent manner.Comment: 145 pages, 28 figures, to appear in Reviews of Modern Physics. Figure
22 is missing here, and Figs. 15, 26-28 are at low resolution. This version
has a slightly different title and some typos fixed in Chapter 5. For the
full review with figures, please consult:
http://www.astro.queensu.ca/~courteau/GalaxyMasses_28apr2014.pd
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