Next Contents

1. INTRODUCTION: Historical Perspectives of Studies of Interstellar Grain Compositions and Sizes

When the existence of solid particles in interstellar space was first convincingly demonstrated by Trumpler (1930) through the discovery of color excess between the photographic (with an effective wavelength lambdaeff approx 4300 Å) and visual (lambdaeff approx 5500 Å) magnitudes, the grains were estimated to be gtapprox 10-19 g, corresponding to radii of gtapprox 20 Å (Trumpler 1930). By the end of the 1930s, a lambda-1 extinction law (i.e., the interstellar extinction varied approximately inversely with wavelength lambda) in the wavelength range 1-3 µm-1 had been well established (Hall 1937; Stebbins, Huffer, & Whitford 1939). At that time, metallic grains with a dominant size of ~ 0.05 µm (Schalén 1936) or a power-law size distribution dn(a) / da ~ a-3.6 in the size range 80 Å ltapprox a ltapprox 1 cm (Greenstein 1938) were proposed to explain the lambda-1 extinction law, partly because meteoritic particles (predominantly metallic) and interstellar grains were then thought to have the same origin.

In view of the spatial correlation between gas concentration and dust extinction, Lindblad (1935) suggested that interstellar grains were formed by condensation from the interstellar gas through random accretion of gas atoms. However, it was found later that in typical interstellar conditions, the Lindblad condensation theory would result in a complete disappearance of all condensable gases and the grains would grow to sizes (~ 10 µm) well beyond those which would account for the interstellar extinction (see Oort & van de Hulst 1946 and references therein). By introducing a grain destruction process caused by grain-grain collisions as a consequence of interstellar cloud encounters, Oort & van de Hulst (1946) further developed the interstellar condensation idea and led to the "dirty ice" model consisting of saturated molecules such as H2O, CH4, and NH3 with an equilibrium size distribution which could be roughly approximated by a functional form dn(a) / da ~ exp[ -5 (a / 0.5 µm)3] and an average size of ~ 0.15 µm.

The discovery of interstellar polarization (Hall 1949; Hiltner 1949) cast doubts on the "dirty ice" model since ice grains are very inefficient polarizers. This stimulated Cayrel & Schatzman (1954) to consider graphite grains as an interstellar dust component since aligned graphite particles would be a very efficient interstellar polarizer because of their strong optical anisotropy. The "dirty ice" model was also challenged by nondetection of the 3.1 µm feature of H2O ice outside of molecular clouds (Danielson, Woolf, & Gaustad 1965; Knacke, Cudaback, & Gaustad 1969). This gave the late J. Mayo Greenberg (1922-2001) the incentive to perform the early experiments on the ultraviolet (UV) photoprocessing of low temperature mixtures of volatile molecules simulating the "original" dirty ice grains (Greenberg et al. 1972) to understand how and why the predicted H2O was not clearly present. From such experiments was predicted a new component of interstellar dust in the form of complex organics known as "organic refractories", containing a mixture of aliphatic and aromatic carbonaceous molecules (Greenberg et al. 2000).

As an alternative to the interstellar condensation process, Hoyle & Wickramasinghe (1962) proposed that graphite particles with radii a few times 0.01 µm could form in the atmospheres of cool N-type carbon stars, and subsequently be ejected into interstellar space by the stellar radiation pressure, and be responsible for (part of) the interstellar extinction and polarization. The graphite proposal further gained its strength from the prominent 2175 Å interstellar extinction hump detected by the Aerobee rocket observation (Stecher 1965), which was generally attributed to small graphitic grains (Stecher & Donn 1965), although its exact nature still remains unidentified (Draine 1989, 2003a).

Similarly, Kamijo (1963) first proposed that SiO2 whose size is of the order of ~ 20 Å, condensed in the atmospheres of cool M-type stars and expelled into interstellar space, could serve as condensation nuclei for the formation of "dirty ices". It was later shown by Gilman (1969) that grains around oxygen-rich cool giants are mainly silicates such as Al2SiO3 and Mg2SiO4. Interstellar silicates were first detected in emission in the Trapezium region of the Orion Nebula (Stein & Gillett 1969); in absorption toward the Galactic Center (Hackwell, Gehrz, & Woolf 1970), and toward the Becklin-Neugebauer object and the Kleinmann-Low Nebula (Gillett & Forrest 1973). Silicates are now known to be an ubiquitous interstellar dust component (see Section 1 in Li & Draine 2001a for a review).

In the 1960s and early 1970s, the extension of the extinction curve toward the middle and far UV (lambda gtapprox 3 µm-1) was made possible by rocket and satellite observations, including the rocket-based photoelectric photometry at lambda = 2600 Å and 2200 Å (Boggess & Borgman 1964); the Aerobee Rocket spectrophotometry at 1200 Å ltapprox lambda ltapprox 3000 Å (Stecher 1965); the Orbiting Astronomical Satellite (OAO-2) spectrophotometry at 1100 Å ltapprox lambda ltapprox 3600 Å (Bless & Savage 1972); and the Copernicus Satellite spectrophotometry at 1000 Å ltapprox lambda ltapprox 1200 Å (York et al. 1973). By 1973, the interstellar extinction curve had been determined over the whole wavelength range from 0.2 µm-1 to 10 µm-1. The fact that the extinction continues to increase in the far UV (e.g., see York et al. 1973) implies that no single grain type with either a single size or a continuous size distribution could account for the observed optical to far-UV interstellar extinction (Greenberg 1973). This led to the abandonment of any one-component grain models and stimulated the emergence of various kinds of models consisting of multiple dust constituents, including silicate, silicon carbide, iron, iron oxide, graphite, dirty ice, solid H2, etc.

In 1956 Platt first suggested that very small grains or large molecules of less than 10 Å in radius grown by random accretion from the interstellar gas could be responsible for the observed interstellar extinction and polarization. Platt (1956) postulated these "Platt" particles as quantum-mechanical particles containing many ions and free radicals with unfilled electronic energy bands. Donn (1968) further proposed that polycyclic aromatic hydrocarbon-like "Platt particles" may be responsible for the UV interstellar extinction.

Greenberg (1968) first pointed out that very small grains with a heat content smaller than or comparable to the energy of a single stellar photon, cannot be characterized by a steady-state temperature but rather are subject to substantial temporal fluctuations in temperature. Under interstellar conditions, grains larger than a few hundred angstroms in radius would attain an equilibrium temperature in the range of 15-25 K (Greenberg 1968, 1971; Mathis, Mezger, & Panagia 1983; Draine & Lee 1984; Li & Draine 2001b). This temperature was long thought to be ~ 3 K before van de Hulst (1949) made the first realistic estimation of 10-20 K for both metallic and dielectric grains.

In a study of the scattering properties of interstellar dust (albedo and phase function) determined from the OAO-2 observations at 1500 Å ltapprox lambda ltapprox 4250 Å of the diffuse Galactic light (Witt & Lillie 1973), Witt (1973) first explicitly suggested a bi-modal size distribution for interstellar grains: large grains with radii gtapprox 2500 Å would provide extinction in the visible region including scattering which is strongly forward directed, and small particles with radii ltapprox 250 Å would dominate the UV region and contribute nearly isotopic scattering.

The presence of "Platt" particles in a dust cloud near M17 was first suggested by Andriesse (1978) based on an analysis of its IR spectral energy distribution which was shown to be characterized by a combination of widely different color temperatures (e.g., the far-IR and near-IR color temperatures are ~ 38 K and ~ 150 K, respectively) and that the 8-20 µm emission spectrum is similar over a distance of ~ 2' through the source. Andriesse (1978) argued that the color temperature differences cannot be easily ascribed to a spatial variation of the dust temperature in this cloud, because the cloud profile appears to be independent of the wavelength; instead, "Platt" particles with sizes of ~ 10 Å exhibiting temperature fluctuations up to ~ 150 K plus large grains with an equilibrium temperature of ~ 36 K can explain the observed IR spectrum of M17.

In a near-IR (1.25-4.8 µm) photometric and spectrophotometric study of three visual reflection nebulae NGC 7023, 2023, and 2068, Sellgren, Werner, & Dinerstein (1983) discovered that each nebula has extended near-IR emission consisting of emission features at 3.3 and 3.4 µm and a smooth continuum which can be described by a greybody with a color temperature ~ 1000 K. They found that the emission spectrum (i.e., the color temperature for the continuum and the 3.3 µm feature) shows very little variation from source to source and within a given source with distance from the central star. Sellgren (1984) argued that this emission could not be explained by thermal emission from dust in radiative equilibrium with the central star since otherwise the color temperature of this emission should fall off rapidly with distance from the illuminating star; instead, she proposed that this emission is emitted by ultrasmall grains, ~ 10 Å in radius, which undergo large excursions in temperature due to stochastic heating by single stellar photons.

The presence of a population of ultrasmall grains in the diffuse ISM was explicitly indicated by the 12 µm and 25 µm "cirrus" emission detected by the Infrared Astronomical Satellite (IRAS) (Boulanger & Pérault 1988), which is far in excess (by several orders of magnitude) of what would be expected from large grains of 15-25 K in thermal equilibrium with the general interstellar radiation field. Subsequent measurements by the Diffuse Infrared Background Experiment (DIRBE) instrument on the Cosmic Background Explorer (COBE) satellite confirmed this and detected additional broadband emission at 3.5 µm and 4.9 µm (Arendt et al. 1998).

More recently, spectrometers aboard the Infrared Telescope in Space (IRTS) (Onaka et al. 1996; Tanaka et al. 1996) and the Infrared Space Observatory (ISO) (Mattila et al. 1996) have shown that the diffuse ISM radiates strongly in emission features at 3.3, 6.2, 7.7, 8.6, and 11.3 µm. These emission features, first seen in the spectra of the planetary nebulae NGC 7027 and BD+30°3639 (Gillett, Forrest, & Merrill 1973), have been observed in a wide range of astronomical environments including planetary nebulae, protoplanetary nebulae, reflection nebulae, HII regions, circumstellar envelopes, and external galaxies (see Tielens et al. 2000 for a review). Often referred to as "unidentified infrared" (UIR) bands, these emission features are now usually attributed to free-flying PAH molecules which are vibrationally excited upon absorption of a single UV/visible photon (Léger & Puget 1984; Allamandola, Tielens, & Barker 1985; Allamandola, Hudgins, & Sandford 1999; Draine & Li 2001; Li & Draine 2001b) although other carriers have also been proposed such as carbonaceous grains with a partly aromatic character (e.g., HAC [Duley & Williams 1981, Duley, Jones, & Williams 1989], quenched carbonaceous composite [QCC; Sakata et al. 1992], coal [Papoular et al. 1996], fullerenes [C60; Webster 1993]), and surface-graphitized nanodiamonds (Jones & d'Hendecourt 2000).

Since the late 1970s, various modern interstellar dust models have been developed (see Li & Greenberg 2003, Draine 2004, Dwek et al. 2004 for recent reviews). In general, these models fall into three broad categories:

It is fair to say that, as can be seen from the above summarizing description of the key contemporary dust models, a consensus on the interstellar dust compositions and sizes is now approaching among various grain models, although the debate in details is still going on and is not expected to disappear in the near future (except that the IR spectropolarimetric observation at 3.4 µm and 9.7 µm may allow a direct test of the core-mantle model for interstellar dust; see Li & Greenberg 2002) --

In the following sections of this review, we will focus on the ultrasmall grain component (a ltapprox 25 nm), with particular emphasis on the photophysical processes including the stochastic heating and the vibrational excitation of PAH molecules, and the excitation of the photoluminescence of silicon nanoparticles. In astrophysical literature, one frequently encounters terms like "ultrasmall grains", "very small grains", "large molecules", "tiny grains", and "nanoparticles"; to avoid confusion, we emphasize here that they are synonymous. In the following we will use the term "nanoparticles" - by "nanoparticles" we mean grains of a few angstroms to a few tens of nanometers in radius.

In Section 2 we summarize the direct and indirect evidence for the existence of nanoparticles in the ISM. The vibrational excitation of nanoparticles is detailed in Section 3. In Section 4 we discuss the physics regarding the excitation of photoluminescence. In Section 5 we present an overview of the nanoparticle species known or proposed to exist in interstellar space. Concluding remarks are given in Section 6.

Next Contents