13,290 research outputs found
Intra-group Light in Hickson Compact Groups
We have analyzed the intra-group light component of 3 Hickson Compact Groups
(HCG 79, HCG 88 and HCG 95) with detections in two of them: HCG 79, with
of the total band luminosity and HCG 95 with . HCG 88
had no component detected. This component is presumably due to tidally stripped
stellar material trapped in the group potential and represents an efficient
tool to determine the stage of dynamical evolution and to map its gravitational
potential. To detect this low surface brightness structure we have applied the
wavelet technique OV\_WAV, which separates the different components of the
image according to their spatial characteristic sizes.Comment: Small update on the associated institutions lis
Uterine Doppler studies: technology driven data, or answers to our pathophysiological queries?: Associate Editor's comment on ‘Endometrial and sub-endometrial perfusion are impaired in women with unexplained subfertility' by Raine-Fleming et al
Light Scattering on Random Dielectric Layers
Scattering of light by a random stack of dielectric layers represents a
one-dimensional scattering problem, where the scattered field is a
three-dimensional vector field. We investigate the dependence of the scattering
properties (band gaps and Anderson localization) on the wavelength, strength of
randomness and relative angle of the incident wave. There is a characteristic
angular dependence of Anderson localization for wavelengths close to the
thickness of the layers. In particular, the localization length varies
non-monotonously with the angle. In contrast to Anderson localization,
absorptive layers do not have this characteristic angular dependence.Comment: 14 pages, 11 figure
The Evolution of Spheroidal Galaxies in Different Environments
We analyse the kinematic and chemical evolution of 203 distant spheroidal
(elliptical and S0) galaxies at 0.2<z<0.8 which are located in different
environments (rich clusters, low-mass clusters and in the field). VLT/FORS and
CAHA/MOSCA spectra with intermediate-resolution have been acquired to measure
the internal kinematics and stellar populations of the galaxies. From HST/ACS
and WFPC2 imaging, surface brightness profiles and structural parameters were
derived for half of the galaxy sample. The scaling relations of the
Faber-Jackson relation and Kormendy relation as well as the Fundamental Plane
indicate a moderate evolution for the whole galaxy population in each density
regime. In all environments, S0 galaxies show a faster evolution than
elliptical galaxies. For the cluster galaxies a slight radial dependence of the
evolution out to one virial radius is found. Dividing the samples with respect
to their mass, a mass dependent evolution with a stronger evolution of
lower-mass galaxies (M<2x10^{11} M_{\sun}) is detected. Evidence for recent
star formation is provided by blue colours and weak OII emission or strong
H\delta absorption features in the spectra. The results are consistent with a
down-sizing formation scenario which is independent from the environment of the
galaxies.Comment: 4 pages, 2 figures, to be published in Astronomische Nachrichten
(proceedings of Symposium 6 of the JENAM 2008, Vienna
The cross helicity at the solar surface by simulations and observations
The quasilinear mean-field theory for driven MHD turbulence leads to the
result that the observed cross helicity may directly yield the
magnetic eddy diffusivity \eta_{T} of the quiet Sun. In order to model the
cross helicity at the solar surface, magnetoconvection under the presence of a
vertical large-scale magnetic field is simulated with the nonlinear MHD code
NIRVANA. The very robust result of the calculations is that \simeq 2
independent of the applied magnetic field amplitude. The
correlation coefficient for the cross helicity is about 10%. Of similar
robustness is the finding that the rms value of the magnetic perturbations
exceeds the mean-field amplitude (only) by a factor of five. The characteristic
helicity speed u_{\eta} as the ratio of the eddy diffusivity and the density
scale height for an isothermal sound velocity of 6.6 km/s proves to be 1 km/s
for weak fields. This value well coincides with empirical results obtained from
the data of the HINODE satellite and the Swedish 1-m Solar Telescope (SST)
providing the cross helicity component . Both simulations and
observations thus lead to a numerical value of \eta_{T} \simeq 10^12 cm^2 /s as
characteristic for the surface of the quiet Sun.Comment: 6 pages, 6 figure
Excitation of Slow-Modes in Network Magnetic Elements Through Magnetic Pumping
From radiation magnetohydrodynamic simulations of the solar atmosphere we
find a new mechanism for the excitation of longitudinal slow modes within
magnetic flux concentrations. We find that the convective downdrafts in the
immediate surroundings of magnetic elements are responsible for the excitation
of slow modes. The coupling between the external downdraft and the plasma
motion internal to the flux concentration is mediated by the inertial forces of
the downdraft that act on the magnetic flux concentration. These forces, in
conjunction with the downward movement, pump the internal atmosphere in the
downward direction, which entails a fast downdraft in the photospheric and
chromospheric layers of the magnetic element. Subsequent to the transient
pumping phase, the atmosphere rebounds, causing a slow mode traveling along the
magnetic flux concentration in the upward direction. It develops into a shock
wave in chromospheric heights, possibly capable of producing some kind of
dynamic fibril. We propose an observational detection of this process.Comment: 5 pages, 4 figures, accepted for publication in ApJ Lette
Closed Choice and a Uniform Low Basis Theorem
We study closed choice principles for different spaces. Given information
about what does not constitute a solution, closed choice determines a solution.
We show that with closed choice one can characterize several models of
hypercomputation in a uniform framework using Weihrauch reducibility. The
classes of functions which are reducible to closed choice of the singleton
space, of the natural numbers, of Cantor space and of Baire space correspond to
the class of computable functions, of functions computable with finitely many
mind changes, of weakly computable functions and of effectively Borel
measurable functions, respectively. We also prove that all these classes
correspond to classes of non-deterministically computable functions with the
respective spaces as advice spaces. Moreover, we prove that closed choice on
Euclidean space can be considered as "locally compact choice" and it is
obtained as product of closed choice on the natural numbers and on Cantor
space. We also prove a Quotient Theorem for compact choice which shows that
single-valued functions can be "divided" by compact choice in a certain sense.
Another result is the Independent Choice Theorem, which provides a uniform
proof that many choice principles are closed under composition. Finally, we
also study the related class of low computable functions, which contains the
class of weakly computable functions as well as the class of functions
computable with finitely many mind changes. As one main result we prove a
uniform version of the Low Basis Theorem that states that closed choice on
Cantor space (and the Euclidean space) is low computable. We close with some
related observations on the Turing jump operation and its initial topology
Critical rainfall conditions for the initiation of torrential flows: results from the Rebaixader catchment (Central Pyrenees)
Torrential flows like debris flows or debris floods are fast movements formed by a mix of water and different amounts of unsorted solid material. They generally occur in steep torrents and pose high risk in mountainous areas. Rainfall is their most common triggering factor and the analysis of the critical rainfall conditions is a fundamental research task. Due to their wide use in warning systems, rainfall thresholds for the triggering of torrential flows are an important outcome of such analysis and are empirically derived using data from past events.
In 2009, a monitoring system was installed in the Rebaixader catchment, Central Pyrenees (Spain). Since then, rainfall data of 25 torrential flows (“TRIG rainfalls”) were recorded, with a 5-min sampling frequency. Other 142 rainfalls that did not trigger torrential flows (“NonTRIG rainfalls”) were also collected and analyzed. The goal of this work was threefold: (i) characterize rainfall episodes in the Rebaixader catchment and compare rainfall data that triggered torrential flows and others that did not; (ii) define and test Intensity–Duration (ID) thresholds using rainfall data measured inside the catchment by with different techniques; (iii) analyze how the criterion used for defining the rainfall duration and the spatial variability of rainfall influences the value obtained for the thresholds.
The statistical analysis of the rainfall characteristics showed that the parameters that discriminate better the TRIG and NonTRIG rainfalls are the rainfall intensities, the mean rainfall and the total rainfall amount. The antecedent rainfall was not significantly different between TRIG and NonTRIG rainfalls, as it can be expected when the source material is very pervious (a sandy glacial soil in the study site). Thresholds were derived from data collected at one rain gauge located inside the catchment. Two different methods were applied to calculate the duration and intensity of rainfall: (i) using total duration, Dtot, and mean intensity, Imean, of the rainfall event, and (ii) using floating durations, D, and intensities, Ifl, based on the maximum values over floating periods of different duration. The resulting thresholds are considerably different (Imean = 6.20 Dtot-0.36 and Ifl_90% = 5.49 D-0.75, respectively) showing a strong dependence on the applied methodology.
On the other hand, the definition of the thresholds is affected by several types of uncertainties. Data from both rain gauges and weather radar were used to analyze the uncertainty associated with the spatial variability of the triggering rainfalls. The analysis indicates that the precipitation recorded by the nearby rain gauges can introduce major uncertainties, especially for convective summer storms. Thus, incorporating radar rainfall can significantly improve the accuracy of the measured triggering rainfall.
Finally, thresholds were also derived according to three different criteria for the definition of the duration of the triggering rainfall: (i) the duration until the peak intensity, (ii) the duration until the end of the rainfall; and, (iii) the duration until the trigger of the torrential flow. An important contribution of this work is the assessment of the threshold relationships obtained using the third definition of duration. Moreover, important differences are observed in the obtained thresholds, showing that ID relationships are significantly dependent on the applied methodology.Peer ReviewedPostprint (author's final draft
On the topological aspects of the theory of represented spaces
Represented spaces form the general setting for the study of computability
derived from Turing machines. As such, they are the basic entities for
endeavors such as computable analysis or computable measure theory. The theory
of represented spaces is well-known to exhibit a strong topological flavour. We
present an abstract and very succinct introduction to the field; drawing
heavily on prior work by Escard\'o, Schr\"oder, and others.
Central aspects of the theory are function spaces and various spaces of
subsets derived from other represented spaces, and -- closely linked to these
-- properties of represented spaces such as compactness, overtness and
separation principles. Both the derived spaces and the properties are
introduced by demanding the computability of certain mappings, and it is
demonstrated that typically various interesting mappings induce the same
property.Comment: Earlier versions were titled "Compactness and separation for
represented spaces" and "A new introduction to the theory of represented
spaces
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