28 research outputs found
The structure of flame filaments in chaotic flows
The structure of flame filaments resulting from chaotic mixing within a
combustion reaction is considered. The transverse profile of the filaments is
investigated numerically and analytically based on a one-dimensional model that
represents the effect of stirring as a convergent flow. The dependence of the
steady solutions on the Damkohler number and Lewis number is treated in detail.
It is found that, below a critical Damkohler number Da(crit), the flame is
quenched by the flow. The quenching transition appears as a result of a
saddle-node bifurcation where the stable steady filament solution collides with
an unstable one. The shape of the steady solutions for the concentration and
temperature profiles changes with the Lewis number and the value of Da(crit)
increases monotonically with the Lewis number. Properties of the solutions are
studied analytically in the limit of large Damkohler number and for small and
large Lewis number.Comment: 17 pages, 13 figures, to be published in Physica
Smooth-filamental transition of active tracer fields stirred by chaotic advection
The spatial distribution of interacting chemical fields is investigated in
the non-diffusive limit. The evolution of fluid parcels is described by
independent dynamical systems driven by chaotic advection. The distribution can
be filamental or smooth depending on the relative strength of the dispersion
due to chaotic advection and the stability of the chemical dynamics. We give
the condition for the smooth-filamental transition and relate the H\"older
exponent of the filamental structure to the Lyapunov exponents. Theoretical
findings are illustrated by numerical experiments.Comment: 4 pages, 3 figure
Excitable media in open and closed chaotic flows
We investigate the response of an excitable medium to a localized
perturbation in the presence of a two-dimensional smooth chaotic flow. Two
distinct types of flows are numerically considered: open and closed. For both
of them three distinct regimes are found, depending on the relative strengths
of the stirring and the rate of the excitable reaction. In order to clarify and
understand the role of the many competing mechanisms present, simplified models
of the process are introduced. They are one-dimensional baker-map models for
the flow and a one-dimensional approximation for the transverse profile of the
filaments.Comment: 14 pages, 16 figure
Ergodic Properties of Classical SU(2) Lattice Gauge Theory
We investigate the relationship between the Lyapunov exponents of periodic
trajectories, the average and fluctuations of Lyapunov exponents of ergodic
trajectories, and the ergodic autocorrelation time for the two-dimensional
hyperbola billiard. We then study the fluctuation properties of the ergodic
Lyapunov spectrum of classical SU(2) gauge theory on a lattice. Our results are
consistent with the notion that this system is globally hyperbolic. Among the
many powerful theorems applicable to such systems, we discuss one relating to
the fluctuations in the entropy growth rate.Comment: 21 pages, 7 figure
Dust attenuation in disk-dominated galaxies: evidence for the 2175A dust feature
The attenuation of starlight by interstellar dust is investigated in a sample
of low redshift, disk-dominated star-forming galaxies using photometry from
GALEX and SDSS. By considering broadband colors as a function of galaxy
inclination we are able to confidently separate trends arising from increasing
dust opacity from possible differences in stellar populations, since stellar
populations do not correlate with inclination. All commonly employed dust
attenuation curves (such as the Calzetti curve for starbursts, or a power-law
curve) provide poor fits to the ultraviolet colors for moderately and highly
inclined galaxies. This conclusion rests on the fact that the average FUV-NUV
color varies little from face-on to edge-on galaxies, while other colors such
as NUV-u and u-r vary strongly with inclination. After considering a number of
model variations, we are led to speculate that the presence of the strong dust
extinction feature at 2175A seen in the Milky Way (MW) extinction curve is
responsible for the observed trends. Independent of our interpretation, these
results imply that the modeling of dust attenuation in the ultraviolet is
significantly more complicated than traditionally assumed. These results also
imply a very weak dependence of the FUV-NUV color on total FUV attenuation, and
we conclude from this that it is extremely difficult to use only the observed
UV spectral slope to infer the total UV dust attenuation, as is commonly done.
We propose several simple tests that might finally identify the grain
population responsible for the 2175A feature.Comment: 16 pages, 10 figures. ApJ accepte
Radio galaxies at z~2.5: results from Keck spectropolarimetry
In classifying the ensemble of powerful extragalactic radio sources,
considerable evidence has accumulated that radio galaxies and quasars are
orientation-dependent manifestations of the same parent population: massive
spheroidal galaxies containing correspondingly massive black holes. [...] In
this study, we exploit the opportunity to eliminate the quasar glare by
performing sensitive spectropolarimetry with the Keck II telescope of a sample
of radio galaxies with redshifts around 2.5. [...] We show that dust-reflected
quasar light generally dominates the restframe ultraviolet continuum of these
sources and that a highly clumped scattering medium results in almost grey
scattering of the active galactic nucleus photons. The observations, however,
do not exclude a substantial star formation rate averaged over a Gyr of
evolution. The sub-mm reradiation from the scattering dust is likely to
represent only a small fraction (~10%) of the total far infrared luminosity. An
analysis of the emission lines excited in the interstellar medium of the host
galaxy by the hard quasar radiation field reveals evidence of a dramatic
chemical evolution within the spheroid during this epoch. Secondary nitrogen
production in intermediate mass stars produces a characteristic signature in
the NV/CIV and NV/HeII line ratios which has been seen previously in the broad
line region of quasars at similar redshifts. We find intriguing correlations
between the strengths of the Ly alpha and NV emission lines and the degree of
ultraviolet continuum polarization which may represent the dispersal of dust
associated with the chemical enrichment of the spheroid.Comment: 24 pages, 12 figures, accepted for publication in Astronomy &
Astrophysics. Properly formatted for A4 and Letter paper formats ps files
available at http://www.eso.org/~jvernet/research.htm
The propagation of uncertainties in stellar population synthesis modeling II: The challenge of comparing galaxy evolution models to observations
Models for the formation and evolution of galaxies readily predict physical
properties such as the star formation rates, metal enrichment histories, and,
increasingly, gas and dust content of synthetic galaxies. Such predictions are
frequently compared to the spectral energy distributions of observed galaxies
via the stellar population synthesis (SPS) technique. Substantial uncertainties
in SPS exist, and yet their relevance to the task of comparing galaxy evolution
models to observations has received little attention. In the present work we
begin to address this issue by investigating the importance of uncertainties in
stellar evolution, the initial stellar mass function (IMF), and dust and
interstellar medium (ISM) properties on the translation from models to
observations. We demonstrate that these uncertainties translate into
substantial uncertainties in the ultraviolet, optical, and near-infrared colors
of synthetic galaxies. Aspects that carry significant uncertainties include the
logarithmic slope of the IMF above 1 Msun, dust attenuation law, molecular
cloud disruption timescale, clumpiness of the ISM, fraction of unobscured
starlight, and treatment of advanced stages of stellar evolution including blue
stragglers, the horizontal branch, and the thermally-pulsating asymptotic giant
branch. The interpretation of the resulting uncertainties in the derived colors
is highly non-trivial because many of the uncertainties are likely systematic,
and possibly correlated with the physical properties of galaxies. We therefore
urge caution when comparing models to observations.Comment: 14 pages, 7 figures. ApJ accepte
Exploring the brown dwarf desert : new substellar companions from the SDSS-III MARVELS survey
Planet searches using the radial velocity technique show a paucity of companions to solar-type stars within ∼5 au in the mass range of ∼10–80 MJup. This deficit, known as the brown dwarf desert, currently has no conclusive explanation. New substellar companions in this region help assess the reality of the desert and provide insight to the formation and evolution of these objects. Here, we present 10 new brown dwarf and 2 low-mass stellar companion candidates around solar-type stars from the Multi-object APO Radial Velocity Exoplanet Large-Area Survey (MARVELS) of the Sloan Digital Sky Survey III. These companions were selected from processed MARVELS data using the latest University of Florida Two Dimensional pipeline, which shows significant improvement and reduction of systematic errors over previous pipelines. The 10 brown dwarf companions range in mass from ∼13 to 76 MJup and have orbital radii of less than 1 au. The two stellar companions have minimum masses of ∼98 and 100 MJup. The host stars of the MARVELS brown dwarf sample have a mean metallicity of [Fe/H] = 0.03 ± 0.08 dex. Given our stellar sample we estimate the brown dwarf occurrence rate around solar-type stars with periods less than ∼300 d to be ∼0.56 per cent