More and more investigators are being attracted to work in the infrared spectral region. My intention in this book is to introduce the infrared, its peculiarities and its special techniques, to a new audience that may include established astronomers as well as new students. Basic facts and figures are emphasized and I have not tried to cover all current research. The approach usually avoids the historical and the reader is referred instead to recent books and papers. The origins of ideas are always complex and, with something like 104 papers published, it is impossible to do justice to every individual in the field.

The first chapter deals with the basic facts of blackbody radiation, which plays a dominant role in infrared astronomy. A short review of atomic and molecular physics follows in order to provide a convenient reminder of the meaning of spectroscopic nomenclature. The second chapter covers the general properties of the Earth's atmosphere and outlines the main infrared surveys that have taken place or will shortly do so. Chapter 3 is devoted to photometry, emphasizing its fundamental aspects and the importance of traceability and calibration. Chapter 4 is an introduction to spectroscopy, treating firstly the stars. Photodissociation regions, which are of great interest in the infrared and millimeter region, are followed by HII regions and some “Rosetta Stone” spectra of representative objects. Chapter 5 is devoted to dust and its central role in star formation. Finally, because an understanding of the technology is important in obtaining reliable results, the last chapter covers the basics of infrared instrumentation.

Introduction

The central astronomical role of dust is at its most evident in the infrared. Protostars form from dusty clouds of molecular gas; the cool condensing dust around them emits copiously at sub-millimeter and far-infrared wavelengths. Even fully developed stars such as Vega may be found to be surrounded by remnant dust particles (see Habing et al., 1996), causing excess emission at long wavelengths. Near the end of their existence, a new generation of dust is formed by evolved stars, for example, in the atmospheres of asymptotic giant branch objects and in the ejecta of supernovae.

As in the visible region, dust scatters and absorbs light, giving rise to extinction, though its effects are much smaller in the infrared than in the visible. A dramatic example is the Galactic Center, which suffers 30 mag of extinction at V, so that only about one photon in 1012 comes through, whereas AK (2.2 μm) is about 2.5 mag and ∼10% of the photons can penetrate. Figure 5.1 shows the distribution of several types of objects with cool dust in an IRAS color-color diagram.

Dust also polarizes the light from distant stars and some properties of the polarization are found to be related to the extinction.

Even when the extinction is quite moderate, by observing in the infrared, the effects of interstellar absorption, which bedevil the use of the cepheid and RR Lyrae period-luminosity relations for distance determination, may be greatly reduced (e.g., Laney and Stobie, 1993, in the case of cepheids).

Introduction

We can consider spectroscopy at several different resolution levels.

The crudest resolution amounts to photometry, where the wavelength scale is divided into a small number of regions, or wide bands, to give an indication of a spectral energy distribution (SED). Examples might include the U(0.37), B(0.44), V(0.55), R(0.64) and I(0.80 μm) system in the visible region and the J(1.25), H(1.65), K (2.2), L(3.5), M(4.8), N(10), and Q(20 μm) system of ground-based infrared, as well as the 12, 25, 60 and 100 μm IRAS bands.

Many data with Δλ/λ ∼ 0.01 exist from spectrometers that used circular variable filters (CVFs). This is usually considered to be the least resolution which can really be called spectroscopy.

The next level might be called medium-resolution spectroscopy, where the detailed lines that usually compose molecular bands are not seen, but the bands are treated in an averaged manner. Most infrared spectroscopy falls into this category.

With high-resolution spectroscopy the individual molecular lines can be examined. Sensitivity considerations have limited this type of work hitherto to the study of the Sun, bright stars and planets. With the advent of large infrared array detectors and large telescopes, it is reasonable to predict that fainter objects will soon receive attention. High resolution has in the past been obtained through the use of Fabry–Perot etalons or Michelson interferometers, also called Fourier–transform spectrometers.

Stellar spectra

The theoretical study of stars is usually divided into stellar interiors and stellar atmospheres. The output of the interior can be regarded as something like a blackbody, but the atmosphere modifies the emergent flux according to its temperature structure, pressure structure and composition.

In stars with a convection zone just below the photosphere, the convective motions might create acoustic waves which propagate outwards through the photosphere. These sound-waves produce an extra pressure, i.e. ‘wave pressure’ in the atmosphere. This pressure will depend on the density and on the amplitudes of the waves. The gradient of the wave pressure results in a force that can drive a stellar wind. If a stellar wind is driven by acoustic wave pressure it is called a ‘sound wave driven wind’.

In this chapter we will first explain the concept of wave pressure by studying the motion of a particle in the presence of an oscillating force. This simple case, first developed by Landau and Lifshitz (1959) shows that oscillations may result in a net force in the direction of the oscillations. In § 6.1 we discuss the motions of particles in an oscillatory field, such as in a sound wave, and we show that this produces a ‘wave pressure’. In § 6.2 we introduce the concepts of the ‘wave action density’ and the ‘acoustic wave luminosity’. These are useful concepts for describing sound wave driven winds. The pressure due to acoustic waves is described in § 6.3. Section (6.4) descibes sound wave driven wind assuming no dissipation of acoustic energy. This results in estimates of the both the mass loss rate and the wind velocity. In § 6.5 we discuss sound wave driven winds with dissipation of the acoustic energy.