Fundamental Aspects of the ISM Fractality

The ubiquitous clumpy state of the ISM raises a fundamental and open problem of physics, which is the correct statistical treatment of systems dominated by long range interactions. A simple solvable hierarchical model is presented which explains why systems dominated by gravity prefer to adopt a fractal dimension around 2 or less, like the cold ISM and large scale structures. This has direct relation with the general transparency, or blackness, of the Universe.Comment: 6 pages, LaTeX2e, crckapb macro, no figure, uuencoded compressed tar file. To be published in the proceeedings of the "Dust-Morphology" conference, Johannesburg, 22-26 January, 1996, D. Block (ed.), (Kluwer Dordrecht

On the Exchange of Kinetic and Magnetic Energy Between Clouds and the Interstellar Medium

We investigate, through 2D MHD numerical simulations, the interaction of a uniform magnetic field oblique to a moving interstellar cloud. In particular we explore the transformation of cloud kinetic energy into magnetic energy as a result of field line stretching. Some previous simulations have emphasized the possible dynamical importance of a ``magnetic shield'' formed around clouds when the magnetic field is perpendicular to the cloud motion (Jones et al. 1996, Miniati et al. 1998). It was not clear, however, how dependent those findings were to the assumed field configuration and cloud properties. To expand our understanding of this effect, we examine several new cases by varing the magnetic field orientation angle with respect to the cloud motion (\theta), the cloud-background density contrast, and the cloud Mach number. We show that in 2D and with \theta large enough, the magnetic field tension can become dominant in the dynamics of the motion of high density contrast, low Mach number clouds. In such cases a significant fraction of cloud kinetic energy can be transformed into magnetic energy with the magnetic pressure at the cloud nose exceeding the ram pressure of the impinging flow. We derive a characteristic timescale for this process of energy ``conversion''. We find also that unless the cloud motion is highly aligned to the magnetic field, reconnection through tearing mode instabilities in the cloud wake limit the formation of a strong flux rope feature following the cloud. Finally we attempt to interpret some observational properties of the magnetic field in view of our results.Comment: 24 pages in aaspp4 Latex and 7 figures. Accepted for publication in The Astrophysical Journa

Interstellar Filaments and the Statistics of Galactic HI

This paper presents a statistical explanation of filament formation in the galactic atomic hydrogen. Recently developed technique allows to determine the 3D spectrum of random HI density. We claim that even in the absence of dynamical factors the Gaussian field corresponding to the measured values of the spectrum of HI density should exhibit filamentary structure, the existence of which has long been claimed.Comment: 14 pages, 5 figures (bitmapped). Original figures available from ftp://ftp.cita.utoronto.ca/ftp/cita/pogosyan/cita-96-17 (1.1MB

K–Te photocathodes: A new electron source for photoinjectors

K–Te photocathodes deposited on a Mo substrate have been successfully used as an electron source in the free electron laser of University of Twente. Long lifetimes have been measured: after more than 20 h of operation in the accelerator a K–Te cathode with 4.75% initial quantum efficiency still displays a 1.1% quantum efficiency at 259 nm. Moreover, the quantum efficiency of this cathode versus operation time can be fitted by an exponential decay curve, which saturates asymptotically to a 1.03% value, suggesting that a quantum efficiency close to 1% could be sustained for very long operation times. Films degraded by use can be recovered to a quantum efficiency which is close to the initial value, by heating the substrate at temperatures between 100 and 330 °C. A new procedure to obtain K–Te cathodes with high (up to 11%) quantum efficiencies is described

A neutron star candidate in the long-period binary 56 UMa

56 UMa is a wide binary system that contains a chemically peculiar red giant and a faint companion. Due to its surface chemical abundances, the red giant was classified as a barium (Ba) star. This implies that the companion has to be a white dwarf, since Ba stars form when mass is transferred to them from an s-process rich Asymptotic Giant Branch (AGB) star. However, in the case of 56 UMa, the companion might be too massive to be the progeny of an AGB star that efficiently produced s-process elements such as barium. In this Letter, we revisit the orbital parameters of the system and perform a full spectral analysis with the goal of investigating the Ba-star classification of the giant and unravelling the nature of its faint companion. We combined radial-velocity and astrometric data to refine the orbital parameters of the system, including the orbital inclination and the companion mass. Then, we re-determined the stellar parameters of the giant and its chemical abundances using high-resolution HERMES spectra. Finally, we investigated the morphology of the interstellar gas in the vicinity of the system. The faint component in 56 UMa has a mass of $1.31 \pm 0.12$ M$_{\odot}$, which, together with the mixed s+r abundance profile of the red giant, confirms that the giant is not a standard barium star. Additionally, the clear identification of a cavity surrounding 56 UMa could indicate that a supernova explosion occurred about 10 5 years ago in the system, suggesting that the faint companion might be a neutron star. However, finding an evolutionary scenario that explains all the observables is not trivial, so we discuss different possible configurations of the system and their respective merits.Comment: Accepted for publication in A&A

Vrije-Elektronen-Laser werk aan de Universiteit van Twente in samenwerking met het Nederlands Centrum voor Laser Research

In de vakgroep Quantum Elektronica van de Faculteit Technische Natuurkunde aan de Universiteit Twente zijn momenteel drie Vrije-Elektronen-Laser projecten operationeel. De projecten worden uitgevoerd in nauwe samenwerking met het Nederlands Centrum voor Laser Research. Het FEL-onderzoek binnen de vakgroep richt zich met name op de fysische aspecten van het FEL-mechanisme en de technologische ontwikkeling van componenten voor een effectieve produktie van straling. Ook wordt veel aandacht besteed aan de ontwikkeling van diagnostiek, zowel voor de elektronenbundel als voor de optische bundel. Veel componenten zijn in huis ontwikkeld en gebouwd. Op deze wijze wordt getracht FEL’s aantrekkelijk te maken voor industriële toepassingen. Dit laatste aspect krijgt met name veel aandacht in de samenwerking met het NCLR. Hieronder volgt een korte beschrijving van de verschillende projecten

Introduction to Magnetic Monopoles

One of the most basic properties of magnetism is that a magnet always has two poles, north and south, which cannot be separated into isolated poles, i.e., magnetic monopoles. However, there are strong theoretical arguments why magnetic monopoles should exist. In spite of extensive searches they have not been found, but they have nevertheless played a central role in our understanding of physics at the most fundamental level.Comment: 22 pages, 7 figures. To be published in Contemporary Physic

Climate threats to coastal infrastructure and sustainable development outcomes

Data availability: Data used in this study can be accessed at https://doi.org/10.5281/zenodo.10554713.Code availability: Code relevant to the analysis can be accessed at https://www.dropbox.com/scl/fi/tpjcxtl4j9m9ht0tl0ocq/NCLIM-23071599-code_final.zip?rlkey=ux7du7k4rkru352moob6quwwu&dl=0.Change history: 11 March 2024A Correction to this paper has been published: https://doi.org/10.1038/s41558-024-01974-8Acknowledgements: We acknowledge the Bangladesh Climate Change and Disaster Risk Management Team at the World Bank, in particular S. Kazi and I. Urrutia, for providing the synthetic household data and general support throughout the project. Any views expressed are not necessarily those of or endorsed by the World Bank. We also acknowledge support from the United Nations Office for Project Services (UNOPS), the Global Center on Adaptation (GCA), the Government of Bangladesh, and the Center for Environmental and Geographic Information Services (CEGIS) for assisting with access to data and in-country facilitation. We acknowledge imagery courtesy of the United Nations Sustainable Development Goals (https://www.un.org/sustainabledevelopment), although the content of this publication is not endorsed by the United Nations or its officials or the Member States.Climate hazards pose increasing threats to development outcomes across the world’s coastal regions by impacting infrastructure service delivery. Using a high-resolution dataset of 8.2 million households in Bangladesh’s coastal zone, we assess the extent to which infrastructure service disruptions induced by flood, cyclone and erosion hazards can thwart progress towards the Sustainable Development Goals (SDGs). Results show that climate hazards potentially threaten infrastructure service access to all households, with the poorest being disproportionately threatened in 69% of coastal subdistricts. Targeting adaptation to these climatic threats in one-third (33%) of the most vulnerable areas could help to safeguard 50–85% of achieved progress towards SDG 3, 4, 7, 8 and 13 indicators. These findings illustrate the potential of geospatial climate risk analyses, which incorporate direct household exposure and essential service access. Such high-resolution analyses are becoming feasible even in data-scarce parts of the world, helping decision-makers target and prioritize pro-poor development.Open access funding provided by Royal Institute of Technology