2,120 research outputs found
On four numerical schemes for a unipolar degenerate drift-diffusion model
International audienceWe consider a unipolar degenerate drift-diffusion system where the relation between the concentration of the charged species c and the chemical potential h is . For four different finite volume schemes based on four different formulations of the fluxes of the problem, we discuss stability and existence results. For two of them, we report a convergence proof. Numerical experiments illustrate the behaviour of the different schemes
Simultaneous Radio to (Sub-) mm-Monitoring of Variability and Spectral Shape Evolution of potential GLAST Blazars
The Large Area Telescope (LAT) instrument onboard GLAST offers a tremendous
opportunity for future blazar studies. In order to fully benefit from its
capabilities and to maximize the scientific return from the LAT, it is of great
importance to conduct dedicated multi-frequency monitoring campaigns that will
result comprehensive observations. Consequently, we initiated an effort to
conduct a GLAST-dedicated, quasi-simultaneous, broad-band flux-density (and
polarization) monitoring of potential GLAST blazars with the Effelsberg and
OVRO radio telescopes (11cm to 7mm wavelength). Here, we present a short
overview of these activities which will complement the multi-wavelengths
activities of the GLAST/LAT collaboration towards the 'low-energy' radio bands.
Further we will give a brief outlook including the extension of this
coordinated campaign towards higher frequencies and future scientific aims.Comment: 3 pages, to appear in the Proceedings of the First GLAST Symposium,
Stanford University, February 200
A Unified Theory for the Atmospheres of the Hot and Very Hot Jupiters: Two Classes of Irradiated Atmospheres
We highlight the importance of gaseous TiO and VO opacity on the highly
irradiated close-in giant planets. The atmospheres of these planets naturally
fall into two classes that are somewhat analogous to the M- and L-type dwarfs.
Those that are warm enough to have appreciable opacity due to TiO and VO gases
we term the ``pM Class'' planets, and those that are cooler we term ``pL
Class'' planets. We calculate model atmospheres for these planets, including
pressure-temperature profiles, spectra, and characteristic radiative time
constants. We show that pM Class planets have hot stratospheres 2000 K
and appear ``anomalously'' bright in the mid infrared secondary eclipse, as was
recently found for planets HD 149026b and HD 209458b. This class of planets
absorbs incident flux and emits thermal flux from high in their atmospheres.
Consequently, they will have large day/night temperature contrasts and
negligible phase shifts between orbital phase and thermal emission light
curves, because radiative timescales are much shorter than possible dynamical
timescales. The pL Class planets absorb incident flux deeper in the atmosphere
where atmospheric dynamics will more readily redistribute absorbed energy. This
will lead to cooler day sides, warmer night sides, and larger phase shifts in
thermal emission light curves. Around a Sun-like primary this boundary occurs
at 0.04-0.05 AU. The eccentric transiting planets HD 147506b and HD
17156b alternate between the classes. Thermal emission in the optical from pM
Class planets is significant red-ward of 400 nm, making these planets
attractive targets for optical detection. The difference in the observed
day/night contrast between ups Andromeda b (pM Class) and HD 189733b (pL Class)
is naturally explained in this scenario. (Abridged.)Comment: Accepted to the Astrophysical Journa
Signal processing in local neuronal circuits based on activity-dependent noise and competition
We study the characteristics of weak signal detection by a recurrent neuronal
network with plastic synaptic coupling. It is shown that in the presence of an
asynchronous component in synaptic transmission, the network acquires
selectivity with respect to the frequency of weak periodic stimuli. For
non-periodic frequency-modulated stimuli, the response is quantified by the
mutual information between input (signal) and output (network's activity), and
is optimized by synaptic depression. Introducing correlations in signal
structure resulted in the decrease of input-output mutual information. Our
results suggest that in neural systems with plastic connectivity, information
is not merely carried passively by the signal; rather, the information content
of the signal itself might determine the mode of its processing by a local
neuronal circuit.Comment: 15 pages, 4 pages, in press for "Chaos
Detailed analysis of Balmer lines in cool dwarf stars
An analysis of H alpha and H beta spectra in a sample of 30 cool dwarf and
subgiant stars is presented using MARCS model atmospheres based on the most
recent calculations of the line opacities. A detailed quantitative comparison
of the solar flux spectra with model spectra shows that Balmer line profile
shapes, and therefore the temperature structure in the line formation region,
are best represented under the mixing length theory by any combination of a low
mixing-length parameter alpha and a low convective structure parameter y. A
slightly lower effective temperature is obtained for the sun than the accepted
value, which we attribute to errors in models and line opacities. The programme
stars span temperatures from 4800 to 7100 K and include a small number of
population II stars. Effective temperatures have been derived using a
quantitative fitting method with a detailed error analysis. Our temperatures
find good agreement with those from the Infrared Flux Method (IRFM) near solar
metallicity but show differences at low metallicity where the two available
IRFM determinations themselves are in disagreement. Comparison with recent
temperature determinations using Balmer lines by Fuhrmann (1998, 2000), who
employed a different description of the wing absorption due to self-broadening,
does not show the large differences predicted by Barklem et al. (2000). In
fact, perhaps fortuitously, reasonable agreement is found near solar
metallicity, while we find significantly cooler temperatures for low
metallicity stars of around solar temperature.Comment: 17 pages, 9 figures, to appear in A&
A Consistency Test of Spectroscopic Gravities for Late-Type Stars
Chemical analyses of late-type stars are usually carried out following the
classical recipe: LTE line formation and homogeneous, plane-parallel,
flux-constant, and LTE model atmospheres. We review different results in the
literature that have suggested significant inconsistencies in the spectroscopic
analyses, pointing out the difficulties in deriving independent estimates of
the stellar fundamental parameters and hence,detecting systematic errors.
The trigonometric parallaxes measured by the HIPPARCOS mission provide
accurate appraisals of the stellar surface gravity for nearby stars, which are
used here to check the gravities obtained from the photospheric iron ionization
balance. We find an approximate agreement for stars in the metallicity range -1
<= [Fe/H] <= 0, but the comparison shows that the differences between the
spectroscopic and trigonometric gravities decrease towards lower metallicities
for more metal-deficient dwarfs (-2.5 <= [Fe/H] <= -1.0), which casts a shadow
upon the abundance analyses for extreme metal-poor stars that make use of the
ionization equilibrium to constrain the gravity. The comparison with the
strong-line gravities derived by Edvardsson (1988) and Fuhrmann (1998a)
confirms that this method provides systematically larger gravities than the
ionization balance. The strong-line gravities get closer to the physical ones
for the stars analyzed by Fuhrmann, but they are even further away than the
iron ionization gravities for the stars of lower gravities in Edvardsson's
sample. The confrontation of the deviations of the iron ionization gravities in
metal-poor stars reported here with departures from the excitation balance
found in the literature, show that they are likely to be induced by the same
physical mechanism(s).Comment: AAS LaTeX v4.0, 35 pages, 10 PostScript files; to appear in The
Astrophysical Journa
Different activation energies in glucose uptake in Saccharomyces cerevisiae DFY1 suggest two transport systems
AbstractThe analysis of initial glucose uptake in Saccharomyces cerevisiae at 25°, 20°, 15° and 10°C by computer-assisted nonlinear regression analysis predicts two transport systems. The first demonstrates MichaelisâMenten kinetics and the second shows first order behaviour. The activation energies of these two systems were calculated by the Arrhenius equation at four different growth phases, namely early exponential (EE), middle exponential (ME2), late exponential (LE) and early stationary (ES) with 2% glucose in the batch medium. The activation energies calculated from the Vm values in EE, ME, LE and ES growth phases were 15.8±1.7, 13.5±1.0, 15.1±0.8 and 13.5±0.7 kcal/mol. These values are in agreement with activation energies calculated for the first mechanism, facilitated diffusion, which is the mechanism deduced from countertransport experiments. The activation energies derived for the second transport system from the first order rate constants in cells grown to EE, ME2, LE and ES were 8.0±2.1, 8.1±1.3, 9.6±3.0 and 7.5±2.6 kcal/mol. These values are still significantly higher than for free diffusion of glucose in water and lower as predicted for passage of glucose through the lipid phase. Therefore, we assume in addition to carrier-mediated facilitated diffusion the entrance of glucose into the cell through a pore
Correlation entropy of synaptic input-output dynamics
The responses of synapses in the neocortex show highly stochastic and
nonlinear behavior. The microscopic dynamics underlying this behavior, and its
computational consequences during natural patterns of synaptic input, are not
explained by conventional macroscopic models of deterministic ensemble mean
dynamics. Here, we introduce the correlation entropy of the synaptic
input-output map as a measure of synaptic reliability which explicitly includes
the microscopic dynamics. Applying this to experimental data, we find that
cortical synapses show a low-dimensional chaos driven by the natural input
pattern.Comment: 7 pages, 6 Figures (7 figure files
Detailed Analysis of Nearby Bulgelike Dwarf Stars III. Alpha and Heavy-element abundances
The present sample of nearby bulgelike dwarf stars has kinematics and
metallicities characteristic of a probable inner disk or bulge origin. Ages
derived by using isochrones give 10-11 Gyr for these stars and metallicities
are in the range -0.80< [Fe/H]< +0.40. We calculate stellar parameters from
spectroscopic data, and chemical abundances of Mg, Si, Ca, Ti, La, Ba, Y, Zr
and Eu are derived by using spectrum synthesis.
We found that [alpha-elements/Fe] show different patterns depending on the
element. Si, Ca and Ti-to-iron ratios decline smoothly for increasing
metallicities, and follow essentially the disk pattern. O and Mg, products of
massive supernovae, and also the r-process element Eu, are overabundant
relative to disk stars, showing a steeper decline for metallicities [Fe/H] >
-0.3 dex. [s-elements/Fe] roughly track the solar values with no apparent trend
with metallicity for [Fe/H] < 0, showing subsolar values for the metal rich
stars. Both kinematical and chemical properties of the bulgelike stars indicate
a distinct identity of this population when compared to disk stars.Comment: 21 pages, 9 figures, to appear in Ap
A Spectroscopic Study of the Ancient Milky Way: F- and G-Type Stars in the Third Data Release of the Sloan Digital Sky Survey
(Abridged) We perform an analysis of spectra and photometry for 22,770 stars
included in the third data release (DR3) of the SDSS. We measure radial
velocities and, based on a model-atmosphere analysis, derive estimates ofthe
atmospheric parameters (effective temperature, surface gravity, and [Fe/H]) for
each star. Stellar evolution models are then used to estimate distances. The
SDSS sample covers a range in stellar brightness of 14 < V < 22, and comprises
large numbers of F- and G-type stars from the thick-disk and halo populations
(up to 100 kpc from the galactic plane), therefore including some of the oldest
stars in the Milky Way. In agreement with previous results from the literature,
we find that halo stars exhibit a broad range of iron abundances, with a peak
at [Fe/H] ~ -1.4. This population exhibits essentially no galactic rotation.
Thick-disk G-dwarf stars at distances from the galactic plane in the range
1<|z|<3 kpc show a much more compact metallicity distribution, with a maximum
at [Fe/H] ~ -0.7, and a median galactic rotation velocity at that metallicity
of 157 +/- 4 km/s (a lag relative to the thin disk of 63 km/s). A comparison of
color indices and metal abundances with isochrones indicates that no
significant star formation has taken place in the halo in the last ~ 11 Gyr,
but there are thick-disk stars which are at least 2 Gyr younger. We find the
metallicities of thick-disk stars to be nearly independent of galactocentric
distance between 5 and 14 kpc, in contrast with the marked gradients found in
the literature for the thin disk. No vertical metallicity gradient is apparent
for the thick disk, but we detect a gradient inits rotational velocity of -16
+/- 4 km/s/kpc between 1 and 3 kpc from the plane.Comment: 18 pages, 16 figures; accepted for publication in the ApJ; also
available from http://hebe.as.utexas.edu
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