Annu. Rev. Astron. Astrophys. 1990. 28: 37-70
Copyright © 1990 by . All rights reserved

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1. INTRODUCTION

Interstellar dust is an important constituent of the Galaxy. It obscures all but the relatively nearby regions in visual and ultraviolet wavelengths, and reradiates the absorbed energy in the far-infrared part of the spectrum, thereby providing a major part (~ 30%) of the total luminosity of the Galaxy. The FIR radiation from dust removes the gravitational energy of collapsing clouds, allowing star formation to occur. Dust is crucial for interstellar chemistry by reducing the ultraviolet (UV) radiation which causes molecular dissociations and providing the site of the formation of the most abundant interstellar molecule, H2. Probably grain surfaces are responsible for other chemistry as well. Dust controls the temperature of the interstellar medium (ISM) by accounting for most of the elements which provide cooling, but also providing heating through electrons ejected photoelectrically from grains.

The past decade has seen an increase in interest in interstellar dust because of the discovery of spectroscopic features in both emission and absorption, along with laboratory studies of candidate materials. There have been good observations of the extinction law of dust in many directions. Probably the most important feature to emerge from these studies is that ``interstellar dust'' refers to a variety of materials of widely varying properties.

Many studies of interstellar dust have involved lines of sight through the diffuse, low-density ISM, including some clouds of densities of up to several hundred H atoms per cubic centimeter. This material is reffered to herein as diffuse dust. In the literature, most references to ``interstellar dust'' apply to diffuse dust. Dust in the outer parts of molecular clouds which can be studied by optical and UV observations is called outer-cloud dust. Finally, there have been many studies of sources embedded so deeply within molecular clouds that only the near-infrared or perhaps optical part of the spectrum can be studied. This type of dust is referred to as inner-cloud dust. There is, of course, a continuous gradation of properties from diffuse dust to inner-cloud dust, but these three designations will allow us to emphasize the rather different properties of interstellar dust in the various regions.This review is confined to diffuse dust and outer-cloud dust; for excellent reviews of inner-cloud dust, see (156, 157, 165).

Recent general references regarding interstellar dust are the proceedings of (a) a 1985 workshop held at Wye, Maryland (123); (b) the 1987 conference ``Dust in the Universe,'' held in Manchester, England (7); and (c) IAU Symposium 135 on ``Interstellar Dust'', held in Santa Clara, California, in July, 1988 (3). In general, reviews in these volumes on specialized aspects are not referenced here specifically. In addition, recent references are generally given in preference to older but important papers.

Extinction (absorption plus scattering) is by far the best-studied property of diffuse dust and outer-cloud dust because it can be determined accurately over a wide range of wavelengths and for lines of sight sampling different physical conditions in the ISM. Both continuous extinction and certain spectral features (rather narrow wavelength regions over which the extinction varies appreciably) are discussed in Section 2. Another very important diagnostic is the emission from dust (Section 3), both in spectral features (which provide clues as to specific materials) and in the FIR, representing the emission from grains warmed by incident radiation or particles. Scattering (Section 4), polarization (Section 5), and other diagnostics (Section 6) also provide information. The evolution of dust is outlined in Section 7, and theories are discussed in Section 8. A summary is given in Section 9.

In this article I refer to the wavelength region (0.9 µm < lambda < 10 µm) as the near-infrared (NIR), 10 µm leq lambda leq 30 µm as the mid-infrared (MIR), and lambda > 30 µm as the far-infrared (FIR).

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