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.

We have seen in preceding chapters that molecular lines are excellent tracers of interstellar gas, of star-forming regions, and of the interactions of stars on their environments in the Milky Way Galaxy and in external galaxies. Observations of molecular emissions, supported by detailed modelling, allow a rather complete physical description to be made of the regions where these molecules are located, even when the galaxies are not spatially resolved. But what about pregalactic situations in the Universe? These include some of the most active areas of research in modern astronomy. Did molecules have a role to play in pregalactic astronomy, and if so could molecular emissions help to trace processes occurring very early in the evolution of the Universe? When did molecular processes begin to play an important role? What are the best tracers of the first galaxies in the Universe?

In this chapter we show that molecules were likely to be present from the era of recombination after the Big Bang and certainly played an important role in the formation of protogalaxies and of the first stars. Whether molecules generated detectable signatures of those very early events is problematic, at least with our present range of astronomical instrumentation. However, it seems likely that we shall soon find molecular signatures of the post-recombination era. Once the first stars appeared and seeded their environments with metallicity, the formation of the first galaxies was modulated by molecules and it should certainly be possible to trace their formation using molecular emissions.

The preceding chapters in this book have demonstrated that to trace particular astronomical features in the Milky Way or in external galaxies by using molecular line emissions, the astronomer needs to choose lines corresponding to appropriate transitions. The transitions to use will, obviously, be those whose upper levels are readily populated in the gas that is to be observed. In many situations, the most important excitation mechanism to the upper level is collisional, and H2 is often the main collisional partner.

For example, we have seen that the CO(1–0) transition is appropriate for searching for and detecting cold neutral gas with a kinetic temperature of ∼10 K, where the number density of hydrogen molecules is ∼103 cm−3. However, observations of radiation emitted in this transition cannot reveal, say, the presence of either cold or warm gas at a density of, say, ∼105 cm−3, because collisional de-excitation of the upper level occurs before radiation in the (1–0) line can occur. Therefore, to observe gas at higher densities, observers must use more highly excited CO lines that have larger spontaneous radiation probabilities (assuming that these highly excited levels are sufficiently populated at the prevailing temperature). Alternatively, observers may use a line from some other molecular species that has more appropriate fundamental properties for the physical conditions in the gas to be observed. Of course, as we have seen in Chapters 8 and 9, complications introduced by high optical depths in the lines observed may also make it difficult to infer physical properties in the observed regions. The simple physics in the above arguments is encapsulated in the concept of critical density (see Section 2.3).

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.

Studying the interstellar medium of the Milky Way Galaxy gives us the opportunity of identifying in detail the various components of the medium. The equivalent components in distant galaxies may be unresolved, but contribute to the overall emission. We show in Chapter 6 how to deal with emission from unresolved regions. In this chapter we consider the various distinct types of region in the Milky Way that can be explored through molecular line absorptions and emissions. We show that the chemistry in each of these molecular regions is dominated by one or more of the chemical drivers discussed in Chapter 3. The sensitivity of the chemistry to particular physical parameters, discussed in Chapter 4, may be an important concern in some cases. For most molecular regions, we identify a well-known example of each type, which is not necessarily typical but is one in which the consequences of the chemical driver are prominently displayed. We also list some molecular tracers useful in describing the physical conditions in these different situations. We emphasise in particular the tracers of density and temperature for Milky Way conditions. The aim of this chapter is to show how tracer molecules can reveal the nature, origin, and evolution of many types of region in the Milky Way. Tracers of conditions in galaxies external to the Milky Way are considered in Chapter 6.