Search results for " clusters"
showing 10 items of 1091 documents
Trumpler 14 and 16 in the Carina nebula
2017
We present the first extensive spectroscopic study of the global population in star clusters Trumpler 16, Trumpler 14, and Collinder 232 in the Carina nebula, using data from the Gaia-ESO Survey, down to solar-mass stars. In addition to the standard homogeneous survey data reduction, a special processing was applied here because of the bright nebulosity surrounding Carina stars. We find about 400 good candidate members ranging from OB types down to slightly subsolar masses. About 100 heavily reddened early-type Carina members found here were previously unrecognized or poorly classified, including two candidate O stars and several candidate Herbig Ae/Be stars. Their large brightness makes th…
How to boost clusters and regional change through cooperative social innovation
2020
The aim of this paper is to illustrate how social innovation is promoted and spread by cooperative clusters in order to develop regional change. This paper focuses on the main drivers of the spreading and exaptation processes of social innovation. The cooperative cluster model, the exaptation concept, the top-down approach, the meso-institutions concept and the meso-level perspective are used to capture the strategic approach of spreading social innovation. The study analyses two successful clusters: Mondragon, made up of industrial SMEs, and Anecoop, an agricultural cooperative group, both leading clusters in their respective region. Qualitative methodology is used to compare both case stu…
Reaching net zero energy at the neighbourhood scale: feasibility studies in the south of Italy
2020
Il concetto di “edificio a energia netta zero” è stato recentemente implementato in tutti i regolamenti dei paesi europei. Tuttavia, l’approccio dettato dall’Europa si concentra principalmente sulla scala dell’edificio singolo, tralasciando alcuni aspetti riguardanti ad esempio l’interazione di generazione e carico tra edifici e le interazioni energetiche tra edifici. L’obiettivo di questo studio consiste nell’applicazione della definizione di net zero energy building ad un gruppo di edifici ubicati nell’Italia meridionale e caratterizzati da una destinazione d’uso variegata. Lo status quo di una ventina di edifici è stato simulato in ambiente energy plus e sono state successivamente selezi…
Fluctuations of the Casimir-Polder force between an atom and a conducting wall
2007
We consider the quantum fluctuations of the Casimir-Polder force between a neutral atom and a perfectly conducting wall in the ground state of the system. In order to obtain the atom-wall force fluctuation we first define an operator directly associated to the force experienced by the atom considered as a polarizable body in an electromagnetic field, and we use a time-averaged force operator in order to avoid ultraviolet divergences appearing in the fluctuation of the force. This time-averaged force operator takes into account that any measurement involves a finite time. We also calculate the Casimir-Polder force fluctuation for an atom between two conducting walls. Experimental observabili…
Time-dependent Kohn-Sham approach to quantum electrodynamics
2010
We prove a generalization of the van Leeuwen theorem towards quantum electrodynamics, providing the formal foundations of a time-dependent Kohn-Sham construction for coupled quantized matter and electromagnetic fields. Thereby we circumvent the symmetry-causality problems associated with the action-functional approach to Kohn-Sham systems. We show that the effective external four-potential and four-current of the Kohn-Sham system are uniquely defined and that the effective four-current takes a very simple form. Further we rederive the Runge-Gross theorem for quantum electrodynamics.
Van der Waals and resonance interactions between accelerated atoms in vacuum and the Unruh effect
2017
We discuss different physical effects related to the uniform acceleration of atoms in vacuum, in the framework of quantum electrodynamics. We first investigate the van der Waals/Casimir-Polder dispersion and resonance interactions between two uniformly accelerated atoms in vacuum. We show that the atomic acceleration significantly affects the van der Waals force, yielding a different scaling of the interaction with the interatomic distance and an explicit time dependence of the interaction energy. We argue how these results could allow for an indirect detection of the Unruh effect through dispersion interactions between atoms. We then consider the resonance interaction between two accelerat…
Time-dependent Casimir-Polder forces and partially dressed states
2002
A time-dependent CasimirPolder force is shown to arise during the time evolution of a partially dressed two-level atom. The partially dressed atom is obtained by a rapid change of an atomic parameter such as its transition frequency, due to the action of some external agent. The electromagnetic field fluctuations around the atom, averaged over the solid angle for simplicity, are calculated as a function of time, and it is shown that the interaction energy with a second atom yields a dynamical CasimirPolder potential between the two atoms.
Analytical evaluation of integrals occurring in bound-free transitions.
1988
A class of three-dimensional integrals on spatial coordinates of the type occurring in treatments of multiphoton ionization of atoms and of other bound-free transitions is evaluated in closed form for both hydrogenic and Slater-type wave functions. The reported method of evaluation is illustrated with a number of examples, including the multiphoton ionization of hydrogen by a very intense laser field, when the ejected electrons have absorbed more photons than the minimum required to reach the continuum.
Field fluctuations near a conducting plate and Casimir-Polder forces in the presence of boundary conditions
2006
We consider vacuum fluctuations of the quantum electromagnetic field in the presence of an infinite and perfectly conducting plate. We evaluate how the change of vacuum fluctuations due to the plate modifies the Casimir-Polder potential between two atoms placed near the plate. We use two different methods to evaluate the Casimir-Polder potential in the presence of the plate. They also give new insights on the role of boundary conditions in the Casimir-Polder interatomic potential, as well as indications for possible generalizations to more complicated boundary conditions.
Dynamical Casimir-Polder force between an atom and a conducting wall
2008
The time-dependent Casimir-Polder force arising during the time evolution of an initially bare two-level atom, interacting with the radiation field and placed near a perfectly conducting wall, is considered. Initially the electromagnetic field is supposed to be in the vacuum state and the atom in its ground state. The analytical expression of the force as a function of time and atom-wall distance, is evaluated from the the time-dependent atom-field interaction energy. Physical features and limits of validity of the results are discussed in detail.