This book deals with circumstellar dust shells. It is especially intended to provide a comprehensive presentation of the local and global aspects that determine the physical and chemical process constituting necessary ingredients of any conclusive description and hence of any reliable theoretical modeling of circumstellar dust shells. In this perspective, it puts forward a synthesis of all important observational, theoretical, and numerical aspects that have to be taken into account in any consistent modeling of such complex nonlinear dynamic systems.

Despite the impressive work dedicated to exploring the specific important processes taking place in cool circumstellar (dust) shells, hitherto there existed no publication on this subject based on an overall inclusive perspective. This situation urges the need for a detailed conclusive presentation covering the relevant complex processes and their intricate mutual interplay on which any realistic quantitative modeling of a circumstellar dust shell has to rely. Only such consistent models, based on first-principle physics and a realistic chemistry, are appropriate for being confronted with the observed facts by crucial tests. Thus, apart from inevitable simplifying assumptions, sometimes necessary for the basic physical or chemical characterization of certain processes, we always strive for a realistic approach in detail and generally for self-consistency – or at least for consistency – of the integral model description of a circumstellar dust shell comprising the essential local and global aspects: dynamics, thermodynamics, radiative transfer, chemistry, and grain growth.

The matter in circumstellar dust shells is a multicomponent mixture of various different gases with a small admixture of about 1 percent (by mass) of tiny solid particulates – the dust component of the circumstellar matter – which itself is a mixture of a number of condensed mineral phases that form particles of different sizes and shapes. The more formal aspects of the description of such a multicomponent mixture were discussed in Section 3.1. Now the details of the dynamic and thermal interaction between the different components are discussed, which will result in a specification of the corresponding terms in the general equations for the description of a gas-dust mixture and in identifying the important processes that have to be considered in models of circumstellar dust shells.

The most basic interaction processes in a mixture of gases are the permanent mutual collisions between the particles. These collisions result in an exchange of matter, momentum, and energy between the components of the mixture. In some cases they also result in chemical reactions that change the number densities of the components involved in the reactions. The most important process with respect to the stellar wind problem is the momentum exchange by collisions between particles from the different components that results in a very strong, dynamic coupling between the gaseous components and a close but not so strong dynamic coupling between the dust and gas components.

The existence of extended circumstellar dust shells is closely related to the process of mass loss during late stages of stellar evolution, either by strongly enhanced stellar winds or by explosive events. At the same time, products of nuclear burning processes deep in the stellar interior appear at the stellar surface. This changes the element mixture in the visible stellar atmosphere and in the ejected matter compared with the initial stellar composition. The abundance changes due to nucleosynthesis in evolved stars have strong implications for the nature of the condensates that may be formed in the stellar ejecta. For this reason, we start with a brief overview of stellar evolution before considering the dust-formation process in order to clarify which elemental compositions of the ejected material can be expected to exist in dust-forming objects.

Dust formation in nonexplosive events is observed to occur around stars that either are single stars or are members of a wide binary (multiple) system where the presence of the companion(s) does not significantly modify the evolution of the components. Dust formation in close binaries seems to be a rare process because of the hostile conditions for dust formation caused by mass transfer between the components and associated emission of energetic radiation generated by mass infall onto one of the components. The dust-forming late-type stars in binaries seemingly all are members of rather wide systems. Only a very small number of dusty symbiotic stars is known.

Effects of Black Hole Outbursts on the Galactic Center Gas

Some catastrophic events on Earth have a long-term drastic impact on the geology of the planet and biological evolution. Others have dramatic but short-lived effects. Examples of the first category would be volcanic eruptions that can create mountains or islands à la Pele, and collisions with astroids or comets that produce enormous craters and destroy many life forms; such episodes create relics that can persist longer than the time interval between these events. Examples of events with short-term effects would be earthquakes and tsunamis that can be devastating locally but with few long-lasting consequences on the structure of the Earth (apart from accumulative effects of many such events). While a tsunami creates a ripple on the surface of the ocean that can flood and destroy vast distant coastal regions, the long-term global effects of a single such event are negligible.

Into which category would the hypothetical occasional eruptions at the Galactic Center fall? What is the effect of outbursts of the black hole on the surrounding environment? How long do such effects last and how far do they extend into the Milky Way? It is certain that the black hole is presently inactive – in fact, almost embarrassingly so. In 2001, using the Chandra satellite, F.K. Baganoff and collaborators detected X-ray emission from Sgr A*, quite possibly due to thermal emission from hot gas in the near vicinity of the black hole.

The Near Infrared and the Distribution of Stars in the Galactic Center

At 26000 light years the center of the Milky Way Galaxy is the closest galactic nucleus. At that distance one light year corresponds to about 8 arc seconds, which is easily resolvable with large optical telescopes; in fact, such instruments can resolve one arc second, which would correspond to one and one-half light months at the distance to the Galactic Center. Even the Schwarzschild diameter of a four-million solar mass black hole would have an angular size of 20 micro-arc seconds (20 millionths of an arc second), which can be resolvable by VLBI (very long baseline interferometry) at a wavelength of 1 millimeter. So if one wishes to observe a galactic nucleus in fine detail, the center of the Milky Way is the place to look.

This was realized by Jan Oort, who in the 1970s became intensely interested in the Galactic Center and the processes that might be going on there. Oort was impressed by the number of gas features observed in the 21-cm line of neutral hydrogen in the general direction of the center that appeared to be expanding away from the center - features such as the 3-kiloparsec arm, seen in absorption at -53 km/s (motion toward the Sun and away from the center), and a less prominent feature apparently on the opposite side of the center but moving away at 135 km/s.

The View from Groningen, 1983

In the late spring of 1983 the more observant residents of Groningen might have noticed groups of unusual dreamy-looking people wandering their streets and squares muttering among themselves about spirals, gas, stars, and bars (but not the usual sort which are in abundant supply in the town). They might have even heard the occasional mention of a black hole, although they probably would have thought that the visitors were discussing the large and unpleasant underground public toilet on the Grote Markt (the central market square).

Most Groningers do not realize (or much care) that their town is the birthplace of the modern study of the structure the Galaxy – that this is where, 100 years earlier, Jacobus Kapteyn began his measurements of the images of tens of thousands of stars on photographic plates and devised his model of the Milky Way – the Kapteyn Universe - which placed the Sun near the center of the great star system. It is the university town where Jan Oort defended his thesis in 1926 before moving on to the more cosmopolitan center of Leiden. And now Oort was back in town, along with about 200 other astronomers for an international symposium at the Kapteyn Institute appropriately entitled The Milky Way Galaxy.

This meeting attracted the most prominent astronomers who were working or had worked on various aspects of galactic structure and dynamics. Included among these were Donald Lynden-Bell, Martin Rees, Jerry Ostriker, and Martin Schmidt. There were even several eminent historians of science, Owen Gingerich, for example, who discussed various developments related to the discovery of the Milky Way as a spiral galaxy.

The Jet Set

In the late 1970s, confronted by a more plausible explanation for noncircular gas motions observed toward the center of the Galaxy, the idea of impulsive ejections or explosions was falling out of favor. At the same time, the nearest example of an “exploding galaxy,” M82, appeared to be a less dramatic phenomenon – spectacular but not a single energetic event. Reinterpretation of the geometry of the filaments above and below the plane of M82 suggested two expanding bubbles and not motion confined to be parallel to the rotation axis of the galaxy – almost perpendicular to the line of sight. This means the true three-dimensional velocity of the filaments is closer to the observed line-of-sight velocities of 200 to 300 km/s rather than several thousand kilometers per second as originally thought; then the kinetic energy in gas motion is almost 100 times lower than in the original model. In addition, accumulating observational evidence of furious star formation in the plane of the M82 (such as the presence of a number of remnants of massive short-lived stars – supernova remnants) suggested that the outward moving filaments were in fact a large-scale galactic wind fueled by a burst of star formation in the disk of the galaxy – certainly not a massive single explosion.

Even for obviously active galaxies, radio galaxies, the original model of sudden expulsions of enormous clouds of ionized gas and relativistic particles was severely challenged by a new model – a model involving the more steady ejection of relativistic particles from the nucleus to the radio emitting lobes – the jet model.

Nothing can be more spectacular than the nocturnal moonless sky, far from big city lights and haze. This is especially true in the summer, or so it seemed to me in the small Texas town of my childhood. Even in that soupy semitropical climate near the Gulf of Mexico there were nights when the sky was brilliant with stars of all brightness and colors. It appeared to me not as a flat canopy but as a void with depth – three-dimensional – the brighter objects so close that I could almost touch them and the fainter ones fading away to infinity. And through it all ran the luminous band of the Milky Way flowing north to south from Cassiopeia down to Sagittarius with a conspicuous bifurcation halfway between in Cygnus. What was it? What comprised this shining ribbon, present in the same form night after night in the summer sky? I think that it was the appearance of the Milky Way that stimulated my early obsession with astronomy, an obsession that I never outgrew.

Primitive peoples gazed upon this same celestial spectacle and, given their intimate proximity to nature and the absence of interfering human sources of light, were certainly more aware of its appearance and constancy than are we. Although we have no idea of the mystical or superstitious or anthropomorphic associations prehistoric humans assigned to this phenomenon, the mythology of ancient peoples does present several consistent images invoked to explain, or describe, the Milky Way.

Ejections and Explosions

The discipline of astronomy is replete with unconventional personalities. The Armenian astrophysicist Victor Ambartsumian (Figure 7.1), who had a highly respectable career behind him by the mid-twentieth century, was certainly one such character. In 1958 he presented a paper at the prestigious Solvay conference in Belgium on the role of activity in galactic nuclei in shaping the structure and evolution of the surrounding galaxy; he proposed that spiral structure is formed by ejections from the nuclei of galaxies and that, in extreme cases, new blue galaxies are “born” (ejected) from the centers of giant galaxies. This all seemed quite bizarre at the time but the concurrent discovery of radio galaxies and, several years later, of quasars gave support to the less radical idea that many galaxies appear to undergo sudden impulsive events involving mass ejection, often in two opposite directions.

This was an initial interpretation of the morphology of radio galaxies, where two enormous lobes of radio emission are often observed beyond and on opposite sides of the visible object, generally an elliptical galaxy (see Figure 5.4 of Cygnus A). An early model involved the explosive ejection of two oppositely directed clouds of ionized gas and relativistic particles that are confined by ram pressure of supersonic motion through an ambient intergalactic medium. The amount of energy involved in such events would be huge, much larger than the rest mass energy of one million suns directly observed as relativistic particles.

Two Mountains, Two Legends

The Hawaiian islands in the mid-Pacific are quite young on geological timescales; in fact they are still forming. The volcanic island chain runs from northwest to southeast; as the Pacific plate drifts northwestward over a weak spot in the underlying mantel, magma, hot molten basalt, occasionally bubbles up to the seabed and onto the surface forming a new island. The most recent consequence of this process is the big island of Hawaii which is less than 400 thousand years old. Its principal volcano, Mauna Kea, stands at 4207 meters (13 803 feet) above the sea, but measured from its base at the bottom of the sea it is 10 200 meters (33 500 feet), taller than Mount Everest at 8848 meters. It is a significant mountain. Although the climate of the island is wet, the summit of Mauna Kea stands above moisture in the atmosphere. It is above 90% of the water vapor and above 40% of the atmosphere itself. This makes the site ideal for astronomical observations, and since the 1950s when the first solar observatories were constructed, there has been continuing development of the summit for astronomy.

Mauna Kea is sacred for the indigenous Hawaiian people; the ancient belief is that many of their principal deities live at the summit, which plays a role rather like Mt. Olympus in Greek mythology. In the past, major chieftains ascended to the top to commune with the deities. Continuing reverence for the mountain has made the issue of observatory development quite sensitive.