396 research outputs found
Dust around Mira variables: An analysis of IRAS LRS spectra
The spatial extent and spectral appearance of the thin dust shell around Mira variables is determined largely by the dust absorptivity, Q(sub abs)(lambda), and the dust condensation temperature T(sub cond). Both Q(sub abs)(lambda) and T(sub cond) are extracted from IRAS low-resolution spectra (LRS) spectra. In order to do this, the assumption that the ratio of total power in the 10 micron feature to that in the 20 micron feature should be equal to that measured in other amorphous silicates (e.g., synthesized amorphous Mg2SiO4). It was found that T(sub cond) decreases with decreasing strength of the 10 micron feature, from T(sub cond) = 1000 K to 500 K (estimated error 20 percent). A value for the near-infrared dust absorptivity could not be determined. Although this parameter strongly affects the condensation radius, it hardly affects the shape of the LRS spectrum (as long as the optically thin approximation is valid), because it scales the spatial distribution of the dust. Information on the magnitude of the near-infrared dust absorptivity may be deduced from the unique carbon star BM Gem. This star has a LRS spectrum with silicate features indication an inner dust shell temperature of at least 1000 K. However, on the basis of observations in the 1920s-30s one may infer an inner dust shell radius of at least 6x10(exp 12)m. To have this high temperature at such a large distance, the near-infrared absorptivity of the dust must be high
Compact reflection nebulae, a transit phase of evolution from post-AGB to planetary nebulae
In a search of the optical counter-part of candidates of protoplanetary nebulae on the plates of UK Schmidt, ESO Schmidt, and POSS, five compact reflection nebulae associated with post-AGB stars were found. A simplified model (dust shell is spherical symmetric, expansion velocity of dust shell is constant, Q(sub sca)(lambda) is isotropic, and the dust grain properties are uniform) is used to estimate the visible condition of the dust shell due to the scattering of the core star's light. Under certain conditions the compact reflection nebulae can be seen of the POSS or ESO/SRC survey plates
Properties of galaxy clusters in N-body simulations: Intrinsic properties
We study the influence of the various parameters of scenarios of large-scale
structure formation on properties of galaxy clusters, and investigate which
cluster properties are most sensitive to these parameters. We present a set of
large N-body simulations and derive the intrinsic properties of galaxy clusters
in these simulations, which represent a volume of Mpc^3. The
cosmological scenarios studied differ in either the shape of the power spectrum
of initial fluctuations, its normalization, the density parameter ,
or the Hubble parameter . Between each of the simulations, only one
parameter is set differently, so that we can study the influence of that
parameter on the cluster properties. The cluster properties that are studied
are the mass, line-of-sight velocity dispersion, peculiar velocity, intrinsic
shape, and orientation with respect to its surroundings. has a
large impact on the cluster properties. The latter, viz. the cluster number
density, mass, line-of-sight velocity dispersion and peculiar velocity, are
also determined by , though somewhat less. Other parameters, such
as , the tilt of the initial fluctuation spectrum, and the exact shape
of this spectrum, are generally less important. Unlike the other cluster
properties studied, the peculiar velocity is found to depend on all parameters
of the formation scenario. Using scaling relations between the average
properties of the cluster sample and the parameters of the formation scenario,
one may try and interpolate between the scenarios studied here in order to find
the parameters of the scenario that is most consistent with the data.Comment: 21 pages, Latex (mn.sty), 16 figure
Het effect van direct beercontact
Het niet tijdig berig worden van gelten is op veel fok- en vermeerderingsbedrijven een probleem
Near infrared nadir retrieval of vertical column densities: methodology and application to SCIAMACHY
Nadir observations with the shortwave infrared channels of SCIAMACHY on-board the ENVISAT satellite can be used to derive information on atmospheric gases such as CO, CH<sub>4</sub>, N<sub>2</sub>O, CO<sub>2</sub>, and H<sub>2</sub>O. For the operational level 1b-2 processing of SCIAMACHY data, a new retrieval code BIRRA (Beer InfraRed Retrieval Algorithm) has been developed. BIRRA performs a nonlinear or separable least squares fit (with bound constraints optional) of the measured radiance, where molecular concentration vertical profiles are scaled to fit the observed data. Here we present the forward modeling (radiative transfer) and inversion (least squares optimization) fundamentals of the code along with the further processing steps required to generate higher level products such as global distributions and time series. Moreover, various aspects of level 1 (observed spectra) and auxiliary input data relevant for successful retrievals are discussed. BIRRA is currently used for operational analysis of carbon monoxide vertical column densities from SCIAMACHY channel 8 observations, and is being prepared for methane retrievals using channel 6 spectra. A set of representative CO retrievals and first CH<sub>4</sub> results are presented to demonstrate BIRRA's capabilities
Closed-form control with spike coding networks
Efficient and robust control using spiking neural networks (SNNs) is still an
open problem. Whilst behaviour of biological agents is produced through sparse
and irregular spiking patterns, which provide both robust and efficient
control, the activity patterns in most artificial spiking neural networks used
for control are dense and regular -- resulting in potentially less efficient
codes. Additionally, for most existing control solutions network training or
optimization is necessary, even for fully identified systems, complicating
their implementation in on-chip low-power solutions. The neuroscience theory of
Spike Coding Networks (SCNs) offers a fully analytical solution for
implementing dynamical systems in recurrent spiking neural networks -- while
maintaining irregular, sparse, and robust spiking activity -- but it's not
clear how to directly apply it to control problems. Here, we extend SCN theory
by incorporating closed-form optimal estimation and control. The resulting
networks work as a spiking equivalent of a linear-quadratic-Gaussian
controller. We demonstrate robust spiking control of simulated
spring-mass-damper and cart-pole systems, in the face of several perturbations,
including input- and system-noise, system disturbances, and neural silencing.
As our approach does not need learning or optimization, it offers opportunities
for deploying fast and efficient task-specific on-chip spiking controllers with
biologically realistic activity.Comment: Under review in an IEEE journa
Long-term analysis of GOME in-flight calibration parameters and instrument degradation
Since 1995, the Global Ozone Monitoring Experiment (GOME) has measured solar and backscattered
spectra in the ultraviolet and visible wavelength range. Now, the extensive data set of the most important
calibration parameters has been investigated thoroughly in order to analyze the long-term stability and
performance of the instrument. This study focuses on GOME in-flight calibration and degradation for the
solar path. Monitoring the sensor degradation yields an intensity decrease of 70% to 90% in 240–316nm
and 35% to 65% in 311–415 nm. The spectral calibration is very stable over the whole period, although a
very complex interaction between predisperser temperature and wavelength was found. The leakage
current and the pixel-to-pixel gain increased significantly during the mission, which requires an accurate
correction of the measured radiance and irradiance signals using proper calibration parameters. Finally,
several outliers in the data sets can be directly assigned to instrument and satellite anomalies
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