10,171 research outputs found
On Rank Driven Dynamical Systems
We investigate a class of models related to the Bak-Sneppen model, initially
proposed to study evolution. The BS model is extremely simple and yet captures
some forms of "complex behavior" such as self-organized criticality that is
often observed in physical and biological systems.
In this model, random fitnesses in are associated to agents located
at the vertices of a graph . Their fitnesses are ranked from worst (0) to
best (1). At every time-step the agent with the worst fitness and some others
\emph{with a priori given rank probabilities} are replaced by new agents with
random fitnesses. We consider two cases: The \emph{exogenous case} where the
new fitnesses are taken from an a priori fixed distribution, and the
\emph{endogenous case} where the new fitnesses are taken from the current
distribution as it evolves.
We approximate the dynamics by making a simplifying independence assumption.
We use Order Statistics and Dynamical Systems to define a \emph{rank-driven
dynamical system} that approximates the evolution of the \emph{distribution} of
the fitnesses in these rank-driven models, as well as in the Bak-Sneppen model.
For this simplified model we can find the limiting marginal distribution as a
function of the initial conditions. Agreement with experimental results of the
BS model is excellent.Comment: 12 gigures, 20 page
Line formation in solar granulation: I. Fe line shapes, shifts and asymmetries
Realistic ab-initio 3D, radiative-hydrodynamical convection simulations of
the solar granulation have been applied to FeI and FeII line formation. In
contrast to classical analyses based on hydrostatic 1D model atmospheres the
procedure contains no adjustable free parameters but the treatment of the
numerical viscosity in the construction of the 3D, time-dependent,
inhomogeneous model atmosphere and the elemental abundance in the 3D spectral
synthesis. However, the numerical viscosity is introduced purely for numerical
stability purposes and is determined from standard hydrodynamical test cases
with no adjustments allowed to improve the agreement with the observational
constraints from the solar granulation. The non-thermal line broadening is
mainly provided by the Doppler shifts arising from the convective flows in the
solar photosphere and the solar oscillations. The almost perfect agreement
between the predicted temporally and spatially averaged line profiles for weak
Fe lines with the observed profiles and the absence of trends in derived
abundances with line strengths, seem to imply that the micro- and
macroturbulence concepts are obsolete in these 3D analyses. Furthermore, the
theoretical line asymmetries and shifts show a very satisfactory agreement with
observations with an accuracy of typically 50-100 m/s on an absolute velocity
scale. The remaining minor discrepancies point to how the convection
simulations can be refined further.Comment: Accepted for A&
The Luminous and Carbon-Rich Supernova 2006gz: A Double Degenerate Merger?
Spectra and light curves of SN 2006gz show the strongest signature of
unburned carbon and one of the slowest fading light curves ever seen in a type
Ia event (Delta m_15 = 0.69 +/- 0.04). The early-time Si II velocity is low,
implying it was slowed by an envelope of unburned material. Our best estimate
of the luminosity implies M_V = -19.74 and the production of ~ 1.2 M_sun of
56Ni. This suggests a super-Chandrasekhar mass progenitor. A double degenerate
merger is consistent with these observations.Comment: Accepted for publication in ApJL (5 pages, 4 figures). UBVr'i' light
curves, UVOIR light curves, and spectra available at
http://www.cfa.harvard.edu/supernova/SN2006g
Disk stars in the Milky Way detected beyond 25 kpc from its center
CONTEXT. The maximum size of the Galactic stellar disk is not yet known. Some
studies have suggested an abrupt drop-off of the stellar density of the disk at
Galactocentric distances kpc, which means that in practice no
disk stars or only very few of them should be found beyond this limit. However,
stars in the Milky Way plane are detected at larger distances. In addition to
the halo component, star counts have placed the end of the disk beyond 20 kpc,
although this has not been spectroscopically confirmed so far.
AIMS. Here, we aim to spectroscopically confirm the presence of the disk
stars up to much larger distances.
METHODS. With data from the LAMOST and SDSS-APOGEE spectroscopic surveys, we
statistically derived the maximum distance at which the metallicity
distribution of stars in the Galactic plane is distinct from that of the halo
populations.
RESULTS. Our analysis reveals the presence of disk stars at R>26 kpc (99.7%
C.L.) and even at R>31 kpc (95.4% C.L.).Comment: 4 pages, accepted to be published in A&A-Letter
Near IR diffraction-limited integral-field SINFONI spectroscopy of the Circinus galaxy
Using the adaptive optics assisted near infrared integral field spectrometer
SINFONI on the VLT, we have obtained observations of the Circinus galaxy on
parsec scales. The morphologies of the H_2(1-0)S(1) 2.12um and Br_gamma 2.17um
emission lines are only slightly different, but their velocity maps are similar
and show a gradient along the major axis of the galaxy, consistent with
rotation.Since V_rot/sigma is approximately 1 suggests that random motions are
also important, it is likely that the lines arise in a rotating spheroid or
thickened disk around the AGN. Comparing the Br_gamma flux to the stellar
continuum indicates that the star formation in this region began almost 10^8 yr
ago. We also detect the [SiVI] 1.96um,[AlIX] 2.04um and [CaVIII] 2.32um coronal
lines. In all cases we observe a broad blue wing, indicating the presence of
two or more components in the coronal line region. A correlation between the
ionisation potential and the asymmetry of the profiles was found for these high
excitation species.Comment: 6 pages, 5 figures, Submitted to the Proceedings of the IFS Workshop,
Jul 4-8 2005, Durham, Englan
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