Introduction

Abundance studies of the oldest stars provide critical clues to—and constraints upon—the characteristics of the earliest stellar populations in our Galaxy. Such constraints include those upon: light element production and BBN; the early star-formation and nucleosynthesis history of the Galaxy; the characteristics of heavy-element nucleosynthesis mechanisms; and the ages of early stellar populations from nuclear chronometers. Discussions of many of these issues are to be found in a number of review papers (Wheeler et al. 1989; McWilliam 1997; Truran et al. 2002; Gratton, Sneden, & Caretta 2004).

While much of the available data has been obtained with ground-based telescopes, there is much to learn with HST. Studies in the wavelength region accessible with HST can, in fact, address issues ranging from the origin of the light elements Li, Be, and B to the production mechanisms responsible for the synthesis of the heaviest elements through thorium and uranium. In the following two sections, we will review specifically first boron abundance studies at low Z and then abundances of the heavy elements Ge, Zr, Os, Pt, Au, and Pb, at low Z.

Boron abundances in halo stars

Knowledge of lithium, beryllium, and boron abundances in stars play a major role in our understanding of Big Bang nucleosynthesis, cosmic-ray physics, and stellar interiors.

In the standard model for the origin and evolution of the light elements, only 7Li is produced in significant amounts from Big Bang (primordial) nucleosynthesis.

Introduction

Since their discovery in 1962 (Giacconi et al. 1962), accreting compact objects in the Galaxy have offered unique insights into the astrophysics of the end stages of stellar evolution and the physics of matter at extreme physical conditions. During the first three decades of exploration, new phenomena were discovered and understood, such as the periodic pulsations in the X-ray lightcurve of spinning neutron stars (Giacconi et al. 1971) and the thermonuclear flashes on neutron-star surfaces that are detected as powerful X-ray bursts (see, e.g., Grindlay et al. 1976; Chapter 3). Moreover, the masses of the compact objects were measured in a number of systems, providing the strongest evidence for the existence of black holes in the Universe (McClintock & Remillard 1986; Chapter 4).

During the past ten years, the launch of X-ray telescopes with unprecedented capabilities, such as RXTE, BeppoSAX, the Chandra X-ray Observatory, and XMM-Newton opened new windows onto the properties of accreting compact objects. Examples include the rapid variability phenomena that occur at the dynamical timescales just outside the neutron-star surfaces and the black-hole horizons (van der Klis et al. 1996; Strohmayer et al. 1996; Chapters 2 and 4) as well as atomic lines that have been red- and blue-shifted by general relativistic effects in the vicinities of compact objects (Cottam et al. 2001; Miller et al. 2002b).

History of the term “plasma”

In the mid nineteenth century the Czech physiologist Jan Evangelista Purkinje introduced use of the Greek word plasma (meaning “formed” or “molded”) to denote the clear fluid that remains after removal of all the corpuscular material in blood. About half a century later, the American scientist Irving Langmuir proposed in 1922 that the electrons, ions, and neutrals in an ionized gas could similarly be considered as corpuscular material entrained in some kind of fluid medium and called this entraining medium plasma. However it turned out that, unlike blood where there really is a fluid medium carrying the corpuscular material, there actually is no “fluid medium” entraining the electrons, ions, and neutrals in an ionized gas. Ever since, plasma scientists have had to explain to friends and acquaintances that they were not studying blood!

Brief history of plasma physics

In the 1920s and 1930s a few isolated researchers, each motivated by a specific practical problem, began the study of what is now called plasma physics. This work was mainly directed towards understanding (i) the effect of ionospheric plasma on long-distance short-wave radio propagation and (ii) gaseous electron tubes used for rectification, switching, and voltage regulation in the pre-semiconductor era of electronics. In the 1940s Hannes Alfvén developed a theory of hydromagnetic waves (now called Alfvén waves) and proposed that these waves would be important in astrophysical plasmas.

Introduction

Solutions to Eq. (9.50), the Grad–Shafranov equation, (or to some more complicated counterpart in the case of non-axisymmetric geometry) provide a static MHD equilibrium. The question now arises whether the equilibrium is stable. This issue was forced upon early magnetic fusion researchers who found that plasma that was expected to be well confined in a static MHD equilibrium configuration would instead became violently unstable and crash destructively into the wall in a few microseconds.

The difference between stable and unstable equilibria is shown schematically in Fig. 10.1. Here a ball, representing the plasma, is located at either the bottom of a valley or the top of a hill. If the ball is at the bottom of a valley, i.e., a minimum in the potential energy, then a slight lateral displacement results in a restoring force, which pushes the ball back. The ball then overshoots and oscillates about the minimum with a constant amplitude because energy is conserved. On the other hand, if the ball is initially located at the top of a hill, then a slight lateral displacement results in a force that pushes the ball further to the side so that there is an increase in the velocity. The perturbed force is not restoring, but rather the opposite. The velocity is always in the direction of the original displacement; i.e., there is no oscillation in velocity.

Motivation

Single particle motion in neutral gases is trivial – particles move in straight lines until they hit other particles or the wall. Because of this simplicity, there is no point in keeping track of the details of single particle motion in a neutral gas and instead a statistical averaging of this motion suffices; this averaging shows that neutral gases have Maxwellian velocity distributions and are in a local thermodynamic equilibrium. In contrast, plasma particles are nearly collisionless and typically have complex trajectories that are strongly affected by both electric and magnetic fields.

As discussed in the previous chapter, the velocity distribution in a plasma will become Maxwellian when enough collisions have occurred to maximize the entropy. However, since collisions occur infrequently in hot plasmas, many important phenomena have time scales shorter than the time required for the plasma velocity distribution to become Maxwellian. A collisionless model is thus required to characterize these fast phenomena. In these situations randomization does not occur, entropy is conserved, the distribution function need not be Maxwellian, and the plasma is not in thermodynamic equilibrium. Thermodynamic concepts therefore do not apply, and the plasma is instead characterized by concepts from classical mechanics such as momentum or energy conservation of individual particles. In these collisionless situations the complex details of single particle dynamics are not washed out by collisions but instead persist and influence the macroscopic scale.

Introduction

When both the photometric transits and the radial velocity variations due to an extrasolar planet are observed, we are granted access to key quantities of the object that Doppler monitoring alone cannot provide. In particular, precise measurements of the planetary mass and radius allow us to calculate the average density and infer a composition.

Introduction

Regarded as an astrophysical mystery and a curiosity for decades, cosmic gammaray bursts are finally entering the mainstream of astronomy and astrophysics. In the past few years, we have learned that they lie at cosmological distances, and are probably caused, possibly among other things, by the collapses and subsequent explosions of massive stars. Energetically they are roughly analogous to supernovae, to which they may indeed be related in some cases; no new physics needs to be invented to explain their prodigious luminosities. Unlike supernovae, however, they are relatively rare, and their energy output is distributed quite differently over wavelength and time. They can probably be observed out to distances comparable to, or even farther than, those of the most distant quasars, which makes them useful to cosmologists as lighthouses to the early Universe. Finally, too, they hold the promise of revealing properties of early galaxies such as star formation rates and metallicities in ways that are unique. For all of these reasons, in addition to the facts that they signal the formation of black holes and drive ultra-relativistic winds, they have begun to attract the attention of people working in very diverse disciplines. The words “gamma-ray burst” have even begun to enter the vocabulary of the general public, which regards them with a certain morbid fascination.

It was not at all clear a decade ago that the study of gamma-ray bursts (GRBs) had such a promising future.

Introduction

Cataclysmic variables (CVs) are a distinct class of interacting binaries, transferring mass from a donor star to a degenerate accretor, a white dwarf (WD). In all observational determinations, and as is required by theory for stable mass transfer, the donor star is of lower mass than the accretor. For comprehensive overviews on the subject of CVs we refer to Hack & La Dous (1993) and Warner (1995).

The majority of CVs have orbital periods, Porb, between 75 min and 8 h (see Ritter & Kolb 2003) and consist of Roche lobe-filling main sequence donors and WDs. These are WD analogues of the low-mass X-ray binaries (LMXBs; see Chapter 1). In the period range 8 h–3 d the donors must have larger radii than dwarfs in order to fill their Roche lobes and are therefore evolved subgiants. A few CVs are found with Porb ∼ 200 d, which require giant donors for them to be lobe-filling. The absence of evolved CVs with periods ∼3 to ∼200 d is connected with the dynamical instability that results from an initial donor that had a mass larger than about 67% of that of the WD; such binaries will have experienced rapid mass transfer and shortened their periods during a common envelope phase (e.g., Iben & Livio 1993; see also Chapter 16). Beyond Porb ∼ 200 d, mass-transferring systems also exist.

The Space Telescope Science Institute Symposium on Planets to Cosmology: Essential Science in the Final Years of the Hubble Space Telescope took place during 3–6 May 2004.

These proceedings represent only a part of the invited talks that were presented at the symposium. We thank the contributing authors for preparing their manuscripts.

With some uncertainty concerning Hubble's next Servicing Mission still hanging, identifying the most crucial science to be performed by this superb telescope has become of paramount importance. With this goal in mind, the symposium examined a wide range of topics at the forefront of astronomy and astrophysics. The result is a magnificent collection of results, with a special emphasis on future research.

We thank Sharon Toolan of ST ScI for her help in preparing this volume for publication.

Extra-solar X-ray astronomy began with the historical paper in Physical Review Letters by Giacconi, Gursky, Paolini, and Rossi (1962). Now, more than four decades later, X-ray astronomy is central to many aspects of astronomy. In 2002, Riccardo Giacconi was awarded the Nobel Prize in Physics “for pioneering contributions to astrophysics, which have led to the discovery of cosmic X-ray sources”. In the decade since the publication of X-ray Binaries – the predecessor of the present book – the study of compact stellar X-ray sources has received enormous impetus from observations with the BeppoSAX, Rossi X-ray Timing Explorer (RXTE), Chandra, and XMM-Newton X-ray observatories. In addition, many exciting new results on these X-ray sources have also been produced in the radio, infrared, optical and ultraviolet bands. Highlights include the discovery in low-mass X-ray binaries of millisecond X-ray pulsations, confirming the connection with the millisecond radio pulsars. Millisecond and sub-millisecond quasi-periodic oscillations (QPO) were discovered that are thought to provide a direct view of regions of strong-field gravity near neutron stars and black holes. The discovery of X-ray, optical and radio afterglows of gamma-ray bursts (GRB) firmly established their long-suspected cosmological distances. Super-luminal motion of radio jets was discovered in accreting black-hole binaries. Dozens of ultra-luminous X-ray sources (ULX) have been detected in many galaxies. Their origin is still not clear; some may be accreting intermediate-mass (i.e., of order 103 M⊙) black holes (IMBH).

Introduction

Many accreting neutron stars erupt in spectacular thermonuclear conflagrations every few hours to days. These events, known as Type I X-ray bursts, or simply X-ray bursts, are the subject of our review. Since the last review of X-ray burst phenomenology was written (Lewin, van Paradijs & Taam 1993; hereafter LVT), powerful new X-ray observatories, the Rossi X-ray Timing Explorer (RXTE), the Italian–Dutch BeppoSAX mission, XMM-Newton and Chandra have enabled the discovery of entirely new phenomena associated with thermonuclear burning on neutron stars. Some of these new findings include: (i) the discovery of millisecond (300–600 Hz) oscillations during bursts, so-called “burst oscillations”; (ii) a new regime of nuclear burning on neutron stars which manifests itself through the generation of hours-long flares about once a decade, now referred to as “superbursts”; (iii) discoveries of bursts from low accretion rate neutron stars; and (iv) new evidence for discrete spectral features from bursting neutron stars.

It is perhaps surprising that nuclear physics plays such a prominent role in the phenomenology of an accreting neutron star, as the gravitational energy released per accreted baryon (of mass mp), GMmp/R ≈ 200 MeV, is so much larger than the nuclear energy released by fusion (≈5 MeV when a solar mix goes to heavy elements). Indeed, if the accreted fuel was burned at the rate of accretion, any evidence of nuclear physics would be swamped by the light from released gravitational energy.

Introduction

HST and quasar host galaxy studies have grown up together over the past 30 years. Indeed, “the imaging of low-redshift quasars at high angular resolution (∼0″.1) is one of the principal scientific goals for which the Hubble Space Telescope was designed” (Bahcall, Kirhakos, & Schneider 1994). The nice demonstration by Kristian (1973) that nearby quasars are, in fact, surrounded by “fuzz” in deep 200-inch photographs provided timely input for the design of HST and its instruments, the specifications for which were outlined by the Large Space Telescope Science Working Group in 1974 (HST website). While HST has changed the way we look at quasar hosts, the ultimate goal of our studies has not changed over the decades. Then, as now, we strive to understand the roles played by quasars in galaxy evolution.

Introduction

This chapter deals with X-ray emission from isolated neutron stars for which the energy for the observed X-rays is thought to originate from the rotation of the neutron star, or from an internal heat reservoir following formation. Rotation power can manifest itself as pulsed emission, or as nebular radiation produced by a relativistic wind of particles emitted by the neutron star. Residual heat of formation is observed as soft X-ray emission from young neutron stars. Such thermal radiation, however, can also be produced as a result of reheating from internal or external sources. Rotation-powered pulsed and nebular X-ray emission, as well as thermal emission, can often be observed in a single object simultaneously; this is both fascinating and annoying, as one invariably contaminates the study of the other. There are also a handful of neutron stars for which the origin of the observed X-ray emission is unclear but may be related to the above processes; we will discuss those as well.

Rotation-powered neutron stars are generally referred to as “radio pulsars” since it is at radio wavelengths that the vast majority of the catalogued population (currently numbering ∼1400) is observed. However, the radio emission is energetically unimportant, and we now know of several rotation-powered neutron stars that are not detected as radio sources in spite of deep searches (e.g. Crawford et al. 1998; McLaughlin et al. 2001). We therefore use the more physically motivated term “rotation-powered.”