43,108 research outputs found
NICEST, a near-infrared color excess method tailored for small-scale structures
Observational data and theoretical calculations show that significant
small-scale substructures are present in dark molecular clouds. These
inhomogeneities can provide precious hints on the physical conditions inside
the clouds, but can also severely bias extinction measurements. We present
NICEST, a novel method to account and correct for inhomogeneities in molecular
cloud extinction studies. The method, tested against numerical simulations,
removes almost completely the biases introduced by sub-pixel structures and by
the contamination of foreground stars. We applied NICEST to 2MASS data of the
Pipe molecular complex. The map thereby obtained shows significantly higher (up
to 0.41 mag in A_K) extinction peaks than the standard NICER (Lombardi et al.
2001) map. This first application confirms that substructures in nearby
molecular clouds, if not accounted for, can significantly bias extinction
measurements in regions with A_K > 1 mag; the effect, moreover, is expected to
increase in more distant molecular cloud, because of the poorer physical
resolution achievable.Comment: 13 pages, A&A in pres
Larson's third law and the universality of molecular cloud structure
Larson (1981) first noted a scaling relation between masses and sizes in
molecular clouds that implies that these objects have approximately constant
column densities. This original claim, based upon millimeter observations of
carbon monoxide lines, has been challenged by many theorists, arguing that the
apparent constant column density observed is merely the result of the limited
dynamic range of observations, and that in reality clouds have column density
variations over two orders of magnitudes. In this letter we investigate a set
of nearby molecular clouds with near-infrared excess methods, which guarantee
very large dynamic ranges and robust column density measurements, to test the
validity of Larson's third law. We verify that different clouds have almost
identical average column densities above a given extinction threshold; this
holds regardless of the extinction threshold, but the actual average surface
mass density is a function of the specific threshold used. We show that a
second version of Larson's third law, involving the mass-radius relation for
single clouds and cores, does not hold in our sample, indicating that
individual clouds are not objects that can be described by constant column
density. Our results instead indicate that molecular clouds are characterized
by a universal structure. Finally we point out that this universal structure
can be linked to the log-normal nature of cloud column density distributions.Comment: 5 pages, 4 figures, A&A in press (letter
A Diffusion Model for Classical Chaotic Compound Scattering
We consider the classical map proposed previously to be the exact classical
analogue of Rydberg Molecules calculated with the approximations relevant to
the multi-channel quantum defect theory. The resulting classical map is
analyzed at energies above the threshold for the Rydberg electron. At energies
very near to this threshold we find the possibility of bounded motion for
positive energy due to conserved tori as well as the possibility of forming a
compound system, i.e. a system where the particle is trapped for long times
before emerging again to the continuum. The compound scattering displays
unusual features for short time behavior. A diffusion model explains these
features.Comment: 29 pages, 16 eps figures, LaTeX (elsart), introduction and background
info improve
The astroclimatological comparison of the Paranal Observatory and El Roque de Los Muchachos Observatory
The new extremely large telescope projects need accurate evaluation of the
candidate sites. In this paper we present the astroclimatological comparison
between the Paranal Observatory, located on the coast of the Atacama Desert
(Chile), and the Observatorio del Roque de Los Muchachos (ORM), located in La
Palma (Canary Islands). We apply a statistical analysis using long term
databases from Paranal and Carlsberg Meridian Telescope (CAMC) weather
stations. Significant differences between the two analyzed sites have been
found.Comment: Accepted for publication in MNRAS. 11 pages, 12 figures, 12 table
Hermetic seal for a shaft
An hermetic seal for a linear rod having a portion thereof projected axially through a port defined in a wall for a pressure chamber and supported thereby for omni-directional motion is described. The seal is characterized by a resilient, impervious, cylindrical body having a first section concentrically related to the shaft and integrally affixed thereto comprising a linear ordered array of annular flutes. A second section integrally is affixed to the wall of the chamber and concentrically related to the port comprising a second linear ordered array of annular flutes. A third section is interposed between the first and second sections and integrally affixed in coaxial alignment therewith comprising an annular ordered array of linear flutes concentrically related to the shaft, whereby axial, angular, and pivotal motion of the rod is accommodated
Implementation of Higher-Order Two-Dimensional Singular Elements in FEM codes and Numerical Results problems
This paper presents strategies and schemes to implement higher-order two dimensional singular elements in Finite Elements codes. Accurate numerical results are presented and the efficiency of the schemes is discusse
Head for high speed spinner having a vacuum chuck
The head for a high-speed spinner is characterized by a substantially cylindrical body adapted to be mounted at the distal end of a vertically oriented drive shaft. A vacuum chuck with an upwardly facing chamber is circumscribed by an annular surface for receiving in supported relation a silicon chip. An ordered array of low-pressure cavities is defined about the periphery of the body and connected in communication with the chamber via radially extended bores in order to translate low pressures to the chamber as the head is angularly displaced. A pressure differential is thereby established across the chip for securing it to the head
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