Introduction

Despite considerable progress in our understanding of the formation of galaxies, the origin of the Hubble sequence remains a major unsolved problem. The main morphological parameter that sets the classification of galaxies in the Hubble diagram is the disk-to-bulge ratio (D/B).

Introduction

Statistics of boxy- and peanut-shaped (b/p) bulges in edge-on galaxies show (Shaw 1987, Dettmar 1989) that such bulges are not really that peculiar as it seemed in the past and very common processes are required to explain the high frequency. At present several mechanisms for their origin are discussed. Binney & Petrou (1985) and Whitmore & Bell in their paper on IC 4767 (1988) suggested that these structures may result from material accreted from infalling satellite companions (soft merging). An alternative mechanism for forming boxy bulges are instabilities or resonances animated by bars (Combes et al. 1990; Raha et al. 1991). N-body simulations for stars in barred potentials have demonstrated that with regard to the shape of bulges this theory and observational evidence are consistent.

Introduction

It is now widely believed that the effects of central black holes and cusps on the dynamics of triaxial galaxies are well understood: the box orbits which form the back bone of triaxial elliptical galaxies become chaotic due to scattering by the divergent central force (e.g. Gerhard & Binney 1985). The scattering of these orbits then results in the evolution of the triaxial galaxy to an axisymmetric one whose dynamics is dominated by well behaved families of regular orbits.

Starburst Models and the Formation of Bulges

If the duration of bulge formation is as short as some recent data suggest (e.g., Renzini 1999, these proceedings), then star formation in the bulge must have occurred very rapidly, perhaps in only a few internal crossing times. This implies a star formation rate of several hundred M⊙ yr−1 for less than 108 years. Such an event would be called a starburst if viewed in a primordial galaxy, so it is natural to wonder if any of the starburst regions that are observed today could be undergoing processes similar to what happened in bulges in the early Universe.

Wada, Habe & Sofue (1995) suggested that a starburst in the nuclear disk of a galaxy could generate an expanding shell of gas because of pressures from supernovae and winds. They proposed such a shell would turn into stars and mix, forming a bulge in only a few orbits.

Introduction

Much on our speculations on how bulges originated depends on what we believe about when they formed. Some scenarios prefer bulges to be young, or middle age, late comers anyway.

Introduction

A Galactic Soft X-ray Diffuse Background (SXDB) below ∼2 keV was discovered by Bowyer, Field & Mack (1968). Four soft X-ray all-sky surveys produced maps of this SXDB (McCammon et al. 1983; Marshall & Clark 1984; Garmire et al. 1992; Snowden et al. 1995, 1997) which show complex features, indicating that the SXDB must be made up of several components. However, Snowden et al. (1997) established that the SXDB maps above 0.5 keV are smooth on the south side of the plane, which can be reproduced with only one component: a hot gas in the bulge with a scale height of ∼1.9 kpc. Thus they named this component as the ‘bulge’ emission. The typical temperature was estimated to be ∼0.3 keV.

The ASCA satellite has the capability to observe the SXDB with a reasonable energy resolution (Tanaka et al. 1994), which allows an improved study of line emission. We present here results of our initial analysis of the ASCA spectrum and discuss the bulge emission.

This cycle of lectures presents a self consistent sketch of current understanding about chemical composition of globular clusters and its aftermaths. The first two lectures give basic about nucleosynthesis, chemical models, and abundance determinations. Main results for globular clusters are presented in the next two lectures. In the final lecture I review various indices used to derive abundances from photometry and low dispersion spectroscopy.

Early Nucleosynthesis and models of galactic chemical evolution

In this first lecture I will briefly present the fundamentals of nucleosynthesis and chemical evolution. Owing to lack of time, only few sketches can be given.

The basic observation that we live in an environment rather rich in heavy elements (hereinafter metals) that could not be produced by Big Bang leads us to try to describe the mechanisms of formation of these elements. There is a close interaction between chemical and dynamical evolution of stellar systems; chemical abundances provide then a basic diagnostic for models of galactic evolution.

Figure 1 sketches the most important features to be introduced in this picture. Stars form from condensation of the most dense clouds within the interstellar medium (ISM). Metals are produced by nucleosynthesis processes within the stellar interiors. Stars lose part of their metal-enriched material either through more or less quiescent stellar winds, or through explosive events (SNe) at the end of their lives: the amount of each element produced within stars and returned to the ISM depends on the stellar masses and in some case on the presence of close companions.

For a decade now, the Instituto de Astrofisica de Canrias (IAC) has hosted the Canary Islands Winter School of Astrophysics in which young astrophysicists from all over the world have the opportunity of meeting accredited specialists to study the topics of most active concern in present-day astronomy. During these ten years 80 lecturers and more than 600 students have attended the Winter School, an even higher number not being able to come due to the limited number of places available.

The X Canary Islands Winter School on Astrophysics was dedicated to Globular Clusters, one of the basic sources of our knowledge concerning the lives of the stars and the physics of their evolution.

The School intended to portray a thorough review of research in this field, covering all the relevant disciplines with the aid of the best possible international team of specialists (Canada, Italy, South Africa, Spain, the United Kingdom and the United States), including the theoretical and observational aspects of stellar populations, stellar evolution and chemical abundances, dynamics, variable stars, X-ray sources and the globular clusters of other galaxies.

We take the opportunity to thank local Canarian authorities - Cabildo Insular de La Palma, and Cabildo Insular de Tenerife, as well as the Town Hall of La Laguna, for their continuous support during this and also previous editions of the School.

This tenth Winter School marks a milestone on a long but gratifying journey, in spite of occasional difficulties.

The study of globular clusters has been and still is essential for furthering our knowledge of such astrophysical phenomena as stellar and galactic evolution, variable and X-ray emission stars, chemical abundances (primordial nucleosynthesis), etc. Globular clusters are ideal laboratories for testing theories of stellar evolution, the chemical evolution of the Universe and the dynamics of N-body systems. They are the oldest known objects whose ages can be independently determined, the closest in proximity to the origin of the Universe and the sole surviving structures of the first stages in the formation of the Galaxy. They provide us with important evidence concerning on the age and formation processes of the Galaxy. Globular Clusters are a fundamental unit of the known Universe, they are also found in all other galaxies within our observational grasp. They are possibly a necessary stage in the formation of galaxies.

Research on Globular Clusters covers a vast amount of territory that was reviewed and collected in the present book. From the photographic plate to the HST most recent results, the field of Globular Clusters was actualised and presented by Ivan R. King, with an interesting Observational Approach to Populations in Globular Clusters, where discusses the observations on which our understanding of globular clusters lies. Steven Majewski, reviews the Stellar Populations and Formation of the Milky Way, with particular emphasis on the role of globular clusters in tracing stellar populations and unravelling the Galactic history.

The total mass of the globular cluster system of our Galaxy makes only ∼10−3 the mass of the Galactic disk. It contains, however, ∼20% of all known low-mass binary X-ray sources, about one half of all binary pulsars, and more than a half of the millisecond pulsars in the Galaxy. Close binary systems containing neutron stars should thus form much more easily in the dense stellar environment of globular clusters than elsewhere in the Galaxy. In these lectures we first review the formation mechanism of neutron stars. Then, we present the evolutionary scenarios leading to the formation of binary X-ray sources and binary and millisecond pulsars in the Galactic disk and the Galactic bulge. We later discuss the specific mechanisms to form neutron star binaries in globular clusters. We end by discussing the open issues concerning the origin and evolution of X–ray sources and millisecond pulsars in globular clusters, and their relationship with the structure, dynamics and evolution of the clusters themselves.

Low–mass binary X–ray sources and millisecond pulsars

An early, unexpected result of X–ray astronomy was the discovery of several bright X–ray sources in globular clusters. Later on, searches for radio pulsars have produced many detections, especially of short period pulsars. We begin these lectures with a very schematic presentation of those two kinds of objects.

X–ray binaries

Luminous Galactic binary X–ray sources provided the first evidence of neutron star binaries, that is binary star systems containing neutron stars (Giacconi et al. 1971; Lewin et al. 1971; Schreier et al. 1972; Tananbaum et al. 1972).

Current theoretical predictions concerning the evolution of old, metal-poor stars in galactic globulars are revisited in the light of recent improvements in the input physics. After a short introduction, the role of fundamental physics in constraining stellar structure all along the various evolutionary phases is shortly recalled, together with the additional role played by the evaluation of some macroscopic mechanisms, like convection and diffusion (Sect. 2). Theoretical predictions concerning the CM-diagram location of the best-known evolutionary phases are discussed in some detail, with particular regard to the existing uncertainties in modeling stellar structure as well as in handling observational data (Sect. 3). This discussion is thus extended to the faint stars recently revealed by HST, either at the faint end of the MS or along the WD cooling sequence (Sect. 4). Additional theoretical constraints given by the pulsational properties of RR Lyrae pulsators are recalled (Sect. 5) and the case of extragalactic globulars in the Local Group are briefly discussed (Sect. 6). Some general and methodological considerations close the paper.

The CM diagram: an introduction

The birth of modern physics dates back to the time when Galileo Galilei stated that any attempt to understand the world around us must—first of all—save the phenomena (“salvare i fenomeni”). In modern words, we say that physics is studying relations between observable quantities, so that the identification of suitable “observables” is a first-priority step in any physical investigation.

Globular cluster systems represent only a small fraction of the total stellar mass of galaxy halos, but provide unique tracers which can be used to address models of galaxy formation. Several “case studies” of individually important galaxies are presented, in which we look at the characteristics of their globular clusters including the metallicity distributions, specific frequencies, luminosity (mass) distributions, and kinematics. Among these galaxies are the Milky Way, the nearby giant elliptical NGC 5128, the Virgo ellipticals NGC 4472 and M87, and the supergiant cD galaxies at the centers of rich clusters. In each case the possible roles of mergers, small-satellite accretions, and in situ formation in the growth of the galaxy are discussed. We also briefly touch on the connection between the globular clusters and the much more numerous field-halo stars. We conclude that in all formation scenarios, the presence or absence of gas at any stage of the galaxy's evolution plays a crucial role in determining the total cluster population, the number of distinguishable subpopulations, and the metallicity distribution of the clusters.

The analysis of globular cluster systems (GCSs) in other galaxies is starting to fulfil its long-held promise of informing us about galaxy formation in ways that are unique. The more we learn about GCSs, the more we realize that their role in galaxy formation is an intricate and varied process – yet with common themes that apply particularly to the old-halo population that is found in every type of galaxy.

About Pulsating Stars

Introduction

The discussions in the following sections are limited to pulsating variables and thus omit such objects as eclipsing and cataclysmic variables. Rather than try to cover every conceivable aspect of the subject an attempt is made to discuss in detail a few problems of current interest. This has meant that some types of pulsating variable are not dealt with at all, e.g. Type II Cepheids (including RV Tau stars), SX Phe variables and the recently discovered pulsating K giants in globular clusters (Edmonds and Gilliland 1996). In several of the areas covered strongly divergent views are held by different workers. In such cases an attempt is made to summarize the arguments of the various groups whilst at the same time indicating what in the present writer's opinion seems most likely to be the correct interpretation.

Pulsating stars are of importance for a variety of reasons. First, a study of their light, colour, and radial velocity, changes through the pulsation cycle tell us a great deal about the stars themselves – about their structure – which we cannot easily learn in other ways. Secondly, because pulsating variables are rather easily classified into groups with homogeneous properties it is possible to use them, provided their absolute magnitudes can be calibrated, to derive distances. Pulsating stars are at the basis of the galactic and extragalactic distance scales and are important in determining the distances and ages of classical, old, globular clusters.

Globular clusters provide ideal laboratories for studying the dynamical behaviour of N-body systems. If one includes in one's definition of ‘globular cluster’ the young and intermediate age rich star clusters in the Magellanic Clouds, then one has at hand a set of objects that can serve as a testing ground for theories that describe self-gravitating systems of point masses at any stage in their evolution. These different stages include violent relaxation, a gradual approach to quasi-static equilibrium through two-body relaxation, the dramatic collapse, probably followed by oscillations, of the cluster core, and ultimately dissolution of the cluster as it contributes its stars to the parent/host galaxy's field (usually halo) population. Understanding the mechanisms that hasten the dissolution of a cluster can help us reconstruct the original population of clusters in a given galaxy. This in turn can guide theories of globular cluster formation, and, to the extent that globular clusters trace the early stages of galaxy evolution, the formation of galaxies themselves. This chapter provides an overview of the life of a globular cluster (Section 1), derives the time scales relevant to various stages of cluster evolution (Section 2), and discusses the main observable qualities of clusters relevant to their dynamical evolution: their surface brightness profiles (Section 3) and their internal velocity dispersions (Section 4). In Section 5 some recent results from a large HST project to study the formation and evolution of rich star clusters in the Large Magellanic Cloud are described.

This introductory chapter discusses the observations on which our understanding of globular clusters lies. Successive sections deal with photometry, chemical abundances, the details of color-magnitude diagrams, the distance scale, luminosity and mass functions, and the lower end of the main sequence. An appendix treats the dynamical role of binaries in globular clusters.

Astronomy aims at an understanding of the facts and phenomena that we see, and the processes by which they came about—and in the best of possible cases, the recognition of why they had to be this way and could not have been otherwise. The first stage in this endeavor is to see what is there, and, to the extent that we can, how it became that way.

Globular clusters can in many ways be considered the crossroads of astronomy. They have played a central role in the unfolding of our astronomical understanding, to which they bring two singular advantages: first, each cluster (with a possible rare exception) is a single and specific stellar population, stars born at the same time, in the same place, out of the same material, and differing only in the rate at which each star has evolved. Such a group is much easier to study than the hodgepodge that makes up the field stars of the Milky Way. Second, globular clusters are made up of nearly the oldest—perhaps the very oldest—stars of the Universe, and as such they give us an unparalleled opportunity to probe the depths of time that are the remotest to reach.