Bøger i NATO Science Series C serien

X-ray astronomers discovered the diffuse gas in clusters of galaxies about 20 years ago. It was later realized that the central gas density in some clusters, and in elliptical galaxies, is so high that radiative cooling is a significant energy loss. The cooling time of the gas decreases rapidly towards the centre of the cluster or galaxy and is less than a Hubble time within the innermost few hundred kiloparsecs. This results in a cooling flow in which the gas density rises in order to maintain pressure to support the weight of the overlying gas. The rate at which mass is deposited by the flow is inferred to be several hundreds of solar masses per year in some clusters. The fraction of clusters in which cooling flows are found may exceed 50 per cent. Small flows probably occur in most normal elliptical galaxies that are not in rich clusters. The implications of this simple phenomenon are profound, for we appear to be witnessing the ongoing formation of the central galaxy. In particular, since most of the gas is undetected once it cools below about 3 million K, it appears to form dark matter. There is no reason why it should be detectable with current techniques if each cooling proton only recombines once and the matter condenses into objects of low mass.

Recent advances in the instrumentation used to observe star forming regions in both our own Milky Way and in external galaxies have transformed the subject from a phenomenological pursuit into an increasingly unified, physical science. High resolution centimetre, millimetre, infrared, and optical studies of local star forming clouds have allowed us to probe the physics of star formation down to spatial scales approaching those of the solar system. These developments make it possible to better constrain the basic physical processes underlying star formation itself. At the same time, these new instruments have placed extragalactic studies on a footing detailed enough to allow comparison with star forming regions within our own galaxy. This revolution means that we will soon be able to link the physics of local star forming regions to the global star forming properties of galaxies. The entire structure of this NATO Advanced Study Institute was designed to explore this new view of the subject. This Institute on "Galactic and Extragalactic Star Formation" was held from June 21 -July 4, 1987 at the Conference Centre in the village of Whistler, British Columbia, Canada. The informal atmosphere of this lovely mountain resort stim­ ulated many valuable scientific exchanges. The Institute was funded by a major grant from NATO Scientific Affairs. Additional financial and I.I1oral assistance was provided by the Canadian Institute for Theoretical Astrophysics (CITA) and Mc­ Master University.

This volume contains the lectures presented at the NATO Advanced Study Institute on "Frontiers of Laser Spectroscopy of Gases" held in the Hotel Golf Mar, Vimeiro, near Torres Vedras, Portugal from 30 March to 10 April 1987. The objective of the meeting was to take stock of the recent technological developments involving lasers and to assess their impact on spectroscopy. The whole range of wavelengths from the far infrared through to the extreme ultraviolet was covered. In addition, specific applications to both atoms and molecules were described. Indeed, one of the most successful and pleasant aspects of the Institute was the joint participation of atomic physicists and molecular spectro­ scopists, who meet all too rarely these days. The Institute also succeeded in covering a wide time span from the very earliest days of lasers to some of the very latest developments in both lasers and their applications to spectroscopy. There were 14 invited lecturers, giving a total of 40 lectures, and 89 other participants at the Institute. Each of the invited lecturers has contributed a chapter to this volume. In addition, on Thursday 2nd April a special one-day session was held in the Chemistry Department at the University of Coimbra to mark the retirement of Professor Dr. F.

Proceedings of the NATO Advanced Research Workshop, Castéra-Verduzan, France, June 1-5, 1987

This volume consists of invited talks and contributed papers presented at the NATO Advanced Study Institute "The Post Recombination Universe" which was held in Cambridge in the summer of 1987. There have, in recent years, been numerous meetings devoted to problems in observational cosmology. The attention given reflects the exciting rate of de­ velopment of the subject, and a survey of the proceedings from these symposia reveals that a great deal of emphasis has been given to consideration of the very early universe on the one hand, and to large scale structure in the universe at the present epoch on the other. The theme of this meeting was chosen to comple­ ment these efforts by focussing on the state of the universe at quite early times, but at those epochs which are still accessible to direct observations. The meet­ ing provided a broad coverage of the post recombination universe by drawing on experts from a wide variety of fields covering theory, background radiation fields and discrete sources at high redshift. Events in the moderately early universe will have left their mark in a great range of wavebands, from X-rays to the microwave region, and the evolution of the universe can be revealed by studies of the inter­ galactic medium, gravitational lensing and the abundance and clustering of high redshift sources. All of these subjects received much attention at the meeting, and the papers demonstrate the rich interplay between these areas in the rapidly expanding world of observational cosmology.

This volume contains all but one of the lectures and seminars presented at the NATO Advanced Study Institute on HOI Thin Plasmas in Astrophysics held in Cargese, Corsica, from September 8 to 18, 1987. The meeting was planned in collaboration with the members of the Scientific Organizing Committee, 10 whom I am grateful for suggesting a comprehensive and well balanced program. The SOC was comprised of Prof. J. Bleeker (Space Research Institute. Utrecht, The Netherlands), Dr. C. Cesarsky (CEN Saclay, France), Dr. R. Mushotzky (GSFC, USA), Prof. K. Pounds (University of Leicester, UK), Prof. H. Schnopper (Danish Space Research Laboratory, Denmark), Dr. H. Tananbaum (Center for Astrophysics, USA), Dr. G. Trinchieri (Arcetri Observatory, Italy), and Prof. 1. Truemper (MPE, Garching, Germany). The ASI, fully supported by the NATO Scientific Affairs Division, was organized with the intent of providing a critical and up-to-date overview of our present kowledge and understanding of the properties of hot thin plasmas in astrophysics as they are revealed by X-ray observations from space. The X-ray and UV emission from optically thin thermal plasmas is a common feature of many astrophysical systems. This type of emission occurs in the solar corona and in the coronae of other stars, in supernova remnants and in the hot interstellar medium, in normal galaxies and galactic halos, and in the intergalactic gas in clusters.

The articles in this book represent the major contributions at the NATO Advanced Research Workshop that was held from 6 to 9 July 1987 in the magnificent setting of Dyffryn House and Gardens, in St. Nicholas, just outside Cardiff, Wales. The idea for such a meeting arose in discussions that I had in 1985 and 1986 with many of the principal members of the various groups building prototype laser-interferometric gravitational wave detectors. It became clear that the proposals that these groups were planning to submit for large-scale detectors would have to address questions like the following: . What computing hardware might be required to sift through data corning in at rates of several gigabytes per day for gravitational wave events that might last only a second or less and occur as rarely as once a month? . What software would be required for this task, and how much effort would be required to write it? . Given that every group accepted that a worldwide network of detectors operating in co­ incidence with one another was required in order to provide both convincing evidence of detections of gravitational waves and sufficient information to determine the amplitude and direction of the waves that had been detected, what sort of problems would the necessary data exchanges raise? Yet most of the effort in these groups had, quite naturally, been concentrated on the detector systems.

The book you are now holding represents the final step in a long process for the editors and organizers of the Advanced Study Institute on hard magnetic materials. The editors interest in hard magnetic materials began in 1985 with an attempt to better understand the moments associated with the different iron sites in Nd Fe B. These 14 moments can be obtained from neutron diffraction studies, but we qUickly realized that iron-57 Mossbauer spectroscopy should lead to a better determination of these moments. However, it was also realized that the complex Mossbauer spectra obtained for these hard magnetic materials could not be easily understood without a broad knowledge of their various structural, electronic, and magnetic properties. Hence it seemed useful to the editors to bring together scientists and engineers to discuss, in a tutorial setting, the various properties of these and future hard magnetic materials. We believe the inclusion of engineers as well as scientists in these discussions was essential because the design of new magnetic materials depends very much upon the mode in which they are used in practical devices.

This interdisciplinary meeting has brought together a group of astrophysicists with hands-on experience in the numerical computation of astrophysical fluid dynamics, in particular nonlinear stellar pulsations, and a group of applied mathematicians who are actively engaged with the development of novel and improved numerical methods. The goal of the workshop has been for the astrophysicists to discuss in detail the numerical problems encountered in the modelling of stellar pulsations and for the mathematicians to present a survey of recent developments in numerical techniques. This astrophysical-mathematical intercourse will help the astrophysicists in the future development of more reliable and efficient codes, on the one hand, and it has introduced the mathematicians to an unfamiliar area which is a tough testing ground for their techniques. Since the difficulties encountered are common to other fluid dynamics problems, and are in fact perhaps more severe, fluid dynamicists in other research areas may fmd the results of this workshop of interest as well. Much of our theoretical understanding of the intricate and interesting behavior of variable stars rests on our ability to perform accurate numerical hydrodynamical computations of stellar models. Extensive calculations of nonlinear radial stellar pulsations with the use of increasingly powerful computers are showing more and more clearly that the numerical codes in current use have serious deficiencies.

Audouin Dollfus Observatoire de Paris, Section de Meudon, 92195 Meudon, FRfu~CE The North Atlantic Treaty Organization (NATO) and, in particular, its Department of Scientific Affairs headed by Dr. C. Sinclair, actively supports new fields of science. The recent exploration of the outer parts of the Solar System by spacecraft focused the attention of a large community of scientists on the problem of ices, which playa major role in the accretionary processes in space except for the close neighborhood of the Sun and of other stars. NATO responded to this new interest by agreeing to sponsor an Advanced Research Workshop "Ices in the Solar System", provided a proper organizing body could be set up. It was a pleasure to organize such a workshop jointly with Profes­ sor Roman Smoluchowski who had earlier organized similar conferences. I knew from the experience of others who managed such meetings in the past that there would be much work, but the opportunity of cooperating with Smoluchowski was very attractive and convinced me to agree. If well organized, the whole project promised to be more than rewarding for a large community of scientists, both in the short run and in the long run, by clarifying certain outstanding questions in astrophysics. It became clear that a well-organized international conference would attract top scientists and help unravel many fundamental problems.

Proceedings of the NATO Advanced Study Institute, Erice, Italy, May 11-23, 1986