6,287 research outputs found
Steady-state differential calorimeter measures gamma heating in reactor
Steady-state differential calorimeter, which displays good accuracy and reproducibility of results, is used to measure gamma heating in a reactor environment. The calorimeter has a long life expectancy since it is virtually unharmed by the reactor environment
Data-driven Design of Engineering Processes with COREPROModeler
Enterprises increasingly demand IT support for the coordination of their engineering processes, which often consist of hundreds up to thousands of sub-processes. From a technical viewpoint, these sub-processes have to be concurrently executed and synchronized considering numerous interdependencies.
So far, this coordination has mainly been accomplished manually, which has resulted in errors and inconsistencies. In order to deal with this problem, we have to better understand the interdependencies between the subprocesses to be coordinated. In particular, we can benefit from the fact that sub-processes are often correlated to the assembly of a product (represented by a product data structure). This information can be utilized for the modeling and execution of so-called data-driven process structures. In this paper, we present the COREPRO demonstrator that supports the data-driven modeling of these process structures. The approach explicitly establishes a close linkage between product data structures and engineering processes
Age spreads in star forming regions?
Rotation periods and projected equatorial velocities of pre-main-sequence
(PMS) stars in star forming regions can be combined to give projected stellar
radii. Assuming random axial orientation, a Monte-Carlo model is used to
illustrate that distributions of projected stellar radii are very sensitive to
ages and age dispersions between 1 and 10 Myr which, unlike age estimates from
conventional Hertzsprung-Russell diagrams, are relatively immune to
uncertainties due to extinction, variability, distance etc. Application of the
technique to the Orion Nebula cluster reveals radius spreads of a factor of
2--3 (FWHM) at a given effective temperature. Modelling this dispersion as an
age spread suggests that PMS stars in the ONC have an age range larger than the
mean cluster age, that could be reasonably described by the age distribution
deduced from the Hertzsprung-Russell diagram. These radius/age spreads are
certainly large enough to invalidate the assumption of coevality when
considering the evolution of PMS properties (rotation, disks etc.) from one
young cluster to another.Comment: To appear in "The Ages of Stars", E.E. Mamajek, D.R. Soderblom,
R.F.G. Wyse (eds.), IAU Symposium 258, CU
Observations of pre-stellar cores
Our understanding of the physical and chemical structure of pre-stellar
cores, the simplest star-forming sites, has significantly improved since the
last IAU Symposium on Astrochemistry (South Korea, 1999). Research done over
these years has revealed that major molecular species like CO and CS
systematically deplete onto dust grains at the interior of pre-stellar cores,
while species like N2H+ and NH3 survive in the gas phase and can usually be
detected towards the core centers. Such a selective behaviour of molecular
species gives rise to a differentiated (onion-like) chemical composition, and
manifests itself in molecular maps as a dichotomy between centrally peaked and
ring-shaped distributions. From the point of view of star-formation studies,
the identification of molecular inhomogeneities in cores helps to resolve past
discrepancies between observations made using different tracers, and brings the
possibility of self-consistent modelling of the core internal structure. Here I
present recent work on determining the physical and chemical structure of two
pre-stellar cores, L1498 and L1517B, using observations in a large number of
molecules and Monte Carlo radiative transfer analysis. These two cores are
typical examples of the pre-stellar core population, and their chemical
composition is characterized by the presence of large freeze out holes in most
molecular species. In contrast with these chemically processed objects, a new
population of chemically young cores has started to emerge. The characteristics
of its most extreme representative, L1521E, are briefly reviewed.Comment: 10 pages, 5 figures. To appear in IAU 231 conf. proc.
"Astrochemistry: Recent Successes and Current Challenges," eds. D.C. Lis,
G.A. Blake, and E. Herbs
Boundary states, matrix factorisations and correlation functions for the E-models
The open string spectra of the B-type D-branes of the N=2 E-models are
calculated. Using these results we match the boundary states to the matrix
factorisations of the corresponding Landau-Ginzburg models. The identification
allows us to calculate specific terms in the effective brane superpotential of
E_6 using conformal field theory methods, thereby enabling us to test results
recently obtained in this context.Comment: 20 pages, no figure
A vanishing theorem for operators in Fock space
We consider the bosonic Fock space over the Hilbert space of transversal
vector fields in three dimensions. This space carries a canonical
representation of the group of rotations. For a certain class of operators in
Fock space we show that rotation invariance implies the absence of terms which
either create or annihilate only a single particle. We outline an application
of this result in an operator theoretic renormalization analysis of Hamilton
operators, which occur in non-relativistic qed.Comment: 14 page
Effective superpotentials for B-branes in Landau-Ginzburg models
We compute the partition function for the topological Landau-Ginzburg B-model
on the disk. This is done by treating the worldsheet superpotential
perturbatively. We argue that this partition function as a function of bulk and
boundary perturbations may be identified with the effective D-brane
superpotential in the target spacetime. We point out the relationship of this
approach to matrix factorizations. Using these methods, we prove a conjecture
for the effective superpotential of Herbst, Lazaroiu and Lerche for the A-type
minimal models. We also consider the Landau-Ginzburg theory of the cubic torus
where we show that the effective superpotential, given by the partition
function, is consistent with the one obtained by summing up disk instantons in
the mirror A-model. This is done by explicitly constructing the open-string
mirror map.Comment: 57p, 7 figs, harvma
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