I review our understanding of the structure and kinematics of the Large Magellanic Cloud (LMC), with a particular focus on recent results. This is an important topic, given the status of the LMC as a benchmark for studies of microlensing, tidal interactions, stellar populations, and the extragalactic distance scale. I address the observed morphology and kinematics of the LMC; the angles under which we view the LMC disk; its in-plane and vertical structure; the LMC self-lensing contribution to the total microlensing optical depth; the LMC orbit around the Milky Way; and the origin and interpretation of the Magellanic Stream. Our understanding of these topics is evolving rapidly, in particular due to the many large photometric and kinematic datasets that have become available in the last few years. It has now been established that: the LMC is considerably elongated in its disk plane; the LMC disk is thicker than previously believed; the LMC disk may have warps and twists; the LMC may have a pressure-supported halo; the inner regions of the LMC show unexpected complexities in their vertical structure; and precession and nutation of the LMC disk plane contribute measurably to the observed line-of sight velocity field. However, many open questions remain and more work is needed before we can expect to converge on a fully coherent structural, dynamical and evolutionary picture that explains all observed features of the LMC.

Planetary Nebulae (PNs) in the Magellanic Clouds offer the unique opportunity to study both the population and evolution of low- and intermediate-mass stars, in an environment that is free of the distance scale bias and the differential reddening that hinder the observations of the Galactic sample. The study of LMC and SMC PNs also offers the direct comparison of stellar populations with different metallicity. The relative proximity of the Magellanic Clouds allows detailed spectroscopic analysis of the PNs therein, while the Hubble Space Telescope (HST) is necessary to obtain their spatially-resolved images. In this paper we discuss the history and evolution of this relatively recent branch of stellar astrophysics by reviewing the pioneering studies, and the most recent ground- and space-based achievements. In particular, we present the results from our recent HST surveys, including the metallicity dependence of PN identification (and, ultimately, the metallicity dependence of PN counts in galaxies); the morphological analysis of Magellanic PNs, and the correlations between morphology and other nebular properties; the relations between morphology and progenitor mass and age; and the direct analysis of Magellanic central stars and their importance to stellar evolution. Our morphological results are broadly consistent with the predictions of stellar evolution if the progenitors of asymmetric PNs have on average larger masses than the progenitors of symmetric PNs, without any assumption or relation to binarity of the stellar progenitors.

Introduction

Planetary Nebulae (PNs) are the gaseous relics of the envelopes ejected by low- and intermediate-mass stars (1 < M < 8 M⊙) at the tip of the asymptotic giant branch (AGB), thus they are important probes of stellar evolution, stellar populations, and cosmic recycling.

Recent progress on the astrophysics of globular clusters is discussed. Highlights are (a) developments in color-magnitude survey work, (b) globular cluster structures and the “fundamental plane,” and (c) the relation between globular clusters and the halo field stars in the same host galaxy.

Color-Magnitude studies: The beginning and end of an era

Above almost all other types of astronomical systems, globular clusters offer the chance to take a broad historical perspective. Much of the history of astrophysics in the twentieth century—stellar structure, stellar evolution, the distance scale, galactic structure and evolution, stellar populations, variable stars, high-energy sources—was driven by the need to understand the complex array of phenomena taking place inside these dense stellar systems.

No review of this kind should fail to mention the continuing efforts to understand the stellar content of these elegant systems through color-magnitude studies (CMDs), which are penetrating to ever-greater detail and depth. The very first such studies (see, for example, Shapley & Davis 1920) barely showed the red-giant stars and brightest horizontal-branch stars for the nearest clusters. This past year, a watershed in this kind of basic color-magnitude survey work was reached with the publication of the monumental survey project of Piotto et al. (2002), who used the WPFC2 camera on board HST to obtain (B, V) CMDs for 74 globular clusters, very nearly half of the entire Milky Way globular cluster population. The objects range from nearby, high-latitude clusters with beautifully precise, classic CMD sequences, down to sparse objects deeply embedded in the heavy field contamination and differential reddening of the Galactic bulge.

I present the preliminary results of a program to measure the star formation history in the halo of the Andromeda galaxy. Using the Advanced Camera for Surveys (ACS) on the Hubble Space Telescope, we obtained the deepest optical images of the sky to date, in a field on the southeast minor axis of Andromeda, 51′ (11 kpc) from the nucleus. The resulting color-magnitude diagram (CMD) contains approximately 300,000 stars and extends more than 1.5 mag below the main sequence turnoff, with 50% completeness at V = 30.7 mag. We interpret this CMD using comparisons to ACS observations of five Galactic globular clusters through the same filters, and through χ2-fitting to a finely-spaced grid of calibrated stellar population models. We find evidence for a major (∼30%) intermediate-age (6–8 Gyr) metal-rich ([Fe/H]> −0.5) population in the Andromeda halo, along with a significant old metal-poor population akin to that in the Milky Way halo. The large spread in ages suggests that the Andromeda halo formed as a result of a more violent merging history than that in our own Milky Way.

Introduction

One of the primary quests of observational astronomy is understanding the formation history of galaxies. An impediment to this research is the relative paucity of galaxies in the Local Group, which contains no giant ellipticals, and only two giant spirals-our own Milky Way and Andromeda. Fortunately, Andromeda (M31, NGC 224) is well situated for studying the formation of giant spiral halos, due to its proximity (770 kpc; Freedman & Madore 1990), small foreground reddening (EB-V = 0.08 mag; Schlegel, Finkbeiner, & Davis 1990), and low inclination (i ≍ 12.50°; de Vaucouleurs 1958).

The galaxies of the Local Group that are currently forming stars can serve as our laboratories for understanding star formation and the evolution of massive stars. In this talk I will summarize what I think we've learned about these topics over the past few decades of research, and briefly mention what I think needs to happen next.

Introduction

My talk today will be restricted to giving a brief introduction to the study of massive stars in the Local Group; I'll begin by discussing why I think the subject is important, and giving you a few of the complications and caveats. I'll spend most of my time then talking about what I think we've learned, first about star formation (stories of star formation, the initial mass function, and the upper mass cut-off), and second about the evolution of massive stars (including Luminous Blue Variables, Wolf-Rayet stars, and red supergiants). Finally I'll conclude with a brief discussion of what I think we need to do next. This talk is based in large part on an Annual Reviews of Astronomy & Astrophysics paper that I have coming out in October (Massey 2003), and the reader is referred there for a more in-depth analysis. I have used this opportunity to update some of the figures and thoughts from that, so hopefully the two will be somewhat complementary.

It is suggested that M31 was created by the early merger and subsequent violent relaxation of two or more massive metal-rich ancestral galaxies within the core of the Andromeda subgroup of the Local Group. On the other hand, the evolution of the main body of the Galaxy appears to have been dominated by the collapse of a single ancestral object that subsequently evolved by capturing a halo of small metal-poor companions. It remains a mystery why the globular cluster systems surrounding galaxies like M33 and the LMC exhibit such striking differences in evolutionary history. It is argued that the first generation of globular clusters might have been formed nearly simultaneously in all environments by the strong pressure increase that accompanied cosmic reionization. Subsequent generations of globulars may have formed during starbursts that were triggered by collisions and mergers of gas-rich galaxies.

The fact that the [G]alactic system is a member of a group is a very fortunate accident. Hubble (1936, p. 125)

Introduction

According to Greek mythology, the goddess of wisdom, Pallas Athena, clad in full armor, emerged from Zeus's head after Hephaestus split it open. In much the same way the Local Group sprang forth suddenly, and almost complete, in Chapter VI of The Realm of the Nebulae (Hubble 1936, pp. 124–151). Hubble describes the Local Group as “a typical small group of nebulae which is isolated in the general field.” He assigned (in order of decreasing luminosity) M31, the Galaxy, M33, the Large Magellanic Cloud, the Small Magellanic Cloud, M32, NGC 205, NGC 6822, NGC 185, IC 1613 and NGC 147 to the Local Group, and regarded IC 10 as a possible member.

Introduction

In a compact binary a black hole, neutron star, or white dwarf accretes from a companion star. These systems have long been a paradigm for accretion theory. Much of our present view of how accretion occurs comes directly from the comparison of theory with observations of these sources. Since theory differs little for other objects such as active galaxies, increasing efforts have recently gone into searching for correspondences in observed behavior. This chapter aims at giving a concise summary of the field, with particular emphasis on new developments since the previous edition of this book.

These developments have been significant. Much of the earlier literature implicitly assumed that accreting binaries were fairly steady sources accreting most of the mass entering their vicinity, often with main-sequence companions, and radiating the resulting accretion luminosity in rough isotropy. We shall see that in reality these assumptions fail for the majority of systems. Most are transient; mass ejection in winds and jets is extremely common; a large (sometimes dominant) fraction of even short-period systems have evolved companions whose structure deviates significantly from the zero-age main sequence; and the radiation pattern of many objects is significantly anisotropic. It is now possible to give a complete characterization of the observed incidence of transient and persistent sources in terms of the disc instability model and formation constraints. X-ray populations in external galaxies, particularly the ultraluminous sources, are revealing important new insights into accretion processes and compact binary evolution.

Introduction

A census of baryons in the local universe indicates that the majority of this normal matter is undetected, or “missing.” At high redshifts (z ∼ 3), big-bang nucleosynthesis models and QSO absorption line observations indicate a baryon mass fraction of Ωb ∼ 0.04 (e.g., Fukugita, Hogan & Peebles 1998). The stars and gas detected in local galaxies account for only 20% of this (Ωb ∼ 0.008). The absence of a local Lyα forest indicates that these baryons are likely in a hot (T > 105 K), diffuse medium which has heretofore remained undetectable (e.g., Fukugita, Hogan & Peebles 1998; Cen & Ostriker 1999a; Davé et al. 2001).

Wave propagation in inhomogeneous plasmas

Thus far in our discussion of wave propagation it has been assumed that the plasma is spatially uniform. While this assumption simplifies analysis, the real world is usually not so accommodating and it is plausible that spatial non-uniformity might modify wave propagation. The modification could be just a minor adjustment or it could be profound. Spatial non-uniformity might even produce entirely new kinds of waves. As will be seen, all these possibilities can occur.

To determine the effects of spatial non-uniformity, it is necessary to re-examine the original system of partial differential equations from which the wave dispersion relation was obtained. This is because the technique of substituting ik for ∇ is, in essence, a shortcut for spatial Fourier analysis, and so is mathematically valid only if the equilibrium is spatially uniform. The criteria for whether or not ∇ can be replaced by ik can be understood by considering the simple example of a high-frequency electromagnetic plasma wave propagating in an unmagnetized three-dimensional plasma having a gentle density gradient. The plasma frequency will be a function of position for this situation. To keep matters simple, the density non-uniformity is assumed to be in one direction only, which will be labeled the x direction. The plasma is thus uniform in the y and z directions, but non-uniform in the x direction.

Introduction

The X-ray sky is extremely variable. Transient sources occur on probably all timescales. Historically, the easiest timescales to study are seconds to minutes, and days to months. Most of these transients are Galactic in origin and have been shown to be powered by gravitational energy release during accretion of matter onto a compact object or by thermonuclear runaway processes on neutron stars (see Chapter 3 by Strohmayer and Bildsten). Less well understood are the transients with timescales between a minute and a day, the so-called “fast X-ray transients” (FXTs). These often occur off the Galactic plane and are, therefore, sometimes also referred to as “high latitude transients” (see Fig. 6.1). In general an FXT is loosely defined as a new temporary X-ray source that disappears on a timescale of less than a day and is not related to a known persistent X-ray source. The term, however, has been used as a repository for all sorts of ill-understood flares, bursts, flashes and related phenomena.

The early detections of fast X-ray transients

The very first X-ray satellite, Uhuru, detected fast transient X-ray sources (Forman et al. 1978), but it was through studies with Ariel-V (Pye & McHardy 1983) and the High Energy Astronomy Observatory-1 (HEAO-1) (Ambruster & Wood 1986; Connors et al. 1986) that FXTs came to be recognized as a separate class of transients.

General method for analyzing small-amplitude waves

All plasma phenomena can be described by combining Maxwell's equations with the Lorentz force equation where the latter is represented by the Vlasov, the two-fluid, or the MHD approximation. The subject of linear plasma waves provides a good introduction to the study of plasma phenomena because linear waves are relatively simple to analyze and yet demonstrate many essential features of plasma behavior.

Linear analysis, a straightforward method applicable to any set of partial differential equations describing a physical system, reveals the physical system's simplest non-trivial, self-consistent dynamical behavior. In the context of plasma dynamics, the method is as follows:

1. By making appropriate physical assumptions, the general Maxwell–Lorentz system of equations is reduced to the simplest set of equations characterizing the phenomena under consideration.

2. An equilibrium solution is determined for this set of equations. The equilibrium might be trivial such that densities are uniform, the plasma is neutral, and all velocities are zero. However, less trivial equilibria could also be invoked where there are density gradients or flow velocities. Equilibrium quantities are designated by the subscript 0, indicating “zero-order” in smallness.

3. […]

Introduction

Starbursts are not simply scaled-up versions of the disks of normal spiral and irregular galaxies. The composite HST WFPC2 image of the classic starburst galaxy M82 in Figure 1 illustrates some of the differences. Star formation is localized in a well-defined central zone, where it is concentrated in clumps, beyond which there is virtually no star-forming activity (O'Connell & Mangano 1978). The well-known superwind extends above and below the plane out to kiloparsecs beyond the main starburst zone (Shopbell & Bland-Hawthorn 1998 and references therein). In M82 we can observe the combined effects of stellar feedback and a weak interaction with M81 in sufficient detail to test our models of galactic star formation. This is critical for understanding how the cycling of baryonic matter through stars relates to the overall structure of a galaxy, including its dark matter halo; e.g., through its influence in varying the luminosity part of the Tully–Fisher relationship (van Driel, van den Broek & Baan 1995).

Introduction

Logically, this chapter ought to be located at the beginning of Chapter 2, just after the discussion of phase-space concepts. This chapter is not located there because the theory in this chapter is too advanced to be so close to the beginning of the book and its location near the beginning would have delayed the introduction of other important topics that do not need the detail of this chapter.

The discussion of collisions in Chapter 1 was very approximate. Collisions were shown to scale as an inverse power of temperature, but this was based on a “one size fits all” analysis since it was assumed that collision frequencies of slow and fast particles were nominally the same as that of a particle moving at the thermal velocity. Because the collision frequency scales as v−3, it is quite dubious to assume that the collision rates of both super-thermal and sub-thermal particles can be well represented by a single collision frequency and a more careful averaging over velocities is clearly warranted. This careful averaging is provided by a Fokker–Planck analysis due to Rosenbluth, Macdonald, and Judd (1957). If this much more detailed analysis simply provided more accuracy, it would not be worth the considerable effort it requires except for occasional situations where high accuracy is important. However, the Fokker–Planck theory not only provides more accuracy, but also reveals new and important phenomena and, in particular, indicates when resistive MHD fails.

Introduction

The previous chapter introduced the concept of magnetic helicity via the energy principle and showed that total helicity K = ∫ d3rA · B is a conserved quantity in an ideal plasma. This chapter shows that helicity can be interpreted in a topological sense as a count of the linkages of magnetic flux tubes with each other. Furthermore, it will be shown that when the plasma is not ideal so energy is not conserved, helicity conservation remains a rather good approximation.

The greater robustness of magnetic helicity compared to magnetic energy in the presence of dissipation leads to the Woltjer–Taylor relaxation theory, which shows that a dissipative plasma will spontaneously relax from an arbitrary initial state to a specific final state. Relaxation theory has two remarkable features, namely (i) it sidesteps describing the actual MHD dynamics and simply predicts the end state after all dynamics is over, and (ii) it thrives on complexity so the more complicated the dynamics, the more applicable is the theory. The second feature results because increased complication simply provides more channels whereby the plasma can relax to the specific final state. Relaxation theory has been very successful at predicting the approximate behavior of many laboratory, space, and astrophysical plasmas.

This chapter concludes by showing that magnetic helicity can be manifested in different forms. In particular, the kink instability will be shown to be a mechanism that converts helicity from one of these forms (twist) to another (writhe).

Introduction: The inner structure of AGNs

There is now general consensus that the long-standing paradigm for active galactic nuclei (AGNs) is basically correct, i.e., that AGNs are fundamentally powered by gravitational accretion onto supermassive collapsed objects. Details of the inner structure of AGNs, however, remain sketchy, although both emission lines and absorption lines reveal the presence of large-scale gas flows on scales of hundreds to thousands of gravitational radii. The accretion disk produces a time-variable high-energy continuum that ionizes and heats this nuclear gas, and the broad emission-line fluxes respond to the changes in the illuminating flux from the continuum source. The geometry and kinematics of the broad-line region (BLR), and fundamentally, its role in the accretion process, are not understood. Immediate prospects for understanding this key element of AGN structure do not seem especially promising with the realization that the angular size of the nuclear regions projects to only microarcsecond scales even in the case of the nearest AGNs.