Next Contents


In 1930 - exactly 75 years ago, the existence of solid dust particles in interstellar space was by the first time firmly established, based on the discovery of color excesses (Trumpler 1930). But the history of the interstellar dust-related studies is a much longer and complex subject, and can be dated back to the late 18th century when Herschel (1785) described the dark markings and patches in the sky as "holes in the heavens". Below is a summary of the highlights of this history. For a more detailed record of the historical development of dust astronomy, I refer the interested readers to Aiello & Cecchi-Pestellini (2000), Dorschner (2003), Li & Greenberg (2003), and Verschuur (2003).

Early Chronology: From "Holes in the Heavens" to the Firm Establishment of Interstellar Dust

Interstellar Absorption and Scattering

From Metallic Grains to Dirty Ices:
Meteoritic Origin or Interstellar Condensation?

Interstellar Polarization

From Dirty Ices to Graphite: Interstellar Condensation or Stellar Origin?

Interstellar Silicate Dust of Stellar Origin

Interstellar Iron, SiC, and Diamond Grains of Stellar Origin?

Grain Mixtures with Multi-modal Size Distributions

The Infrared Era: Ices, Silicates, PAHs and Aliphatic Hydrocarbons

Interstellar Depletion: Where Have All Those Atoms Gone?

Dust Luminescence: The "Extended Red Emission"

Stochastically Heated Ultrasmall Grains or "Platt" Particles

Interstellar Grain Models: Modern Era

1 To be precise, this should be called "extinction" which is a combined effect of absorption and scattering: a grain in the line of sight between a distant star and the observer reduces the starlight by a combination of scattering and absorption. Back.

2 The photometric distances were obtained by comparing apparent and absolute magnitudes, with the latter determined from the spectral types of the stars in the clusters. The geometrical distances were determined from the angular diameters of the clusters, assuming that all their diameters were the same. Back.

3 As mentioned earlier in this review, general star counts did suggest the existence of interstellar extinction which increases with distance. However, this evidence is not decisive because interpretation of the star-count data rests on assumptions (generally unproved at the time) as to the true spatial distribution of the stars. Back.

4 Trumpler (1930) derived a color-excess of ~ 0.3 mag kpc-1 between the photographic (with an effective wavelength lambdaB approx 4300Å) and visual (lambdaV approx 5500Å) bands, and a general (visual) absorption of ~ 1.0 mag kpc-1. Back.

5 The exact nature of the carrier of this bump remains unknown. It is generally believed to be caused by aromatic carbonaceous (graphitic) materials, very likely a cosmic mixture of polycyclic aromatic hydrocarbon (PAH) molecules (Joblin, Léger & Martin 1992; Li & Draine 2001b). Back.

6 Very recently, on the basis of the FUSE observations of 9 Galactic sightlines at 1050Å < lambda < 1200Å, Sofia et al. (2005) found that the CCM prediction for short-wavelengths (lambda-1 > 8 µm-1) is not valid for all sightlines. Back.

7 Van de Hulst (1949) pointed out that this is not the case for H, He and Ne since they will evaporate rapidly at grain temperatures exceeding ~ 5 K. Back.

8 The "Serkowski law" P(lambda) / Pmax = exp [- K ln2(lambda / lambdamax)] is determined by only one parameter: lambdamax - the wavelength where the maximum polarization Pmax occurs; the width parameter K is related to lambdamax through K approx 1.66 lambdamax + 0.01. The peak wavelength lambdamax is indicative of grain size and correlated with RV: RV approx (5.6 ± 0.3) lambdamax (lambdamax is in micron; see Whittet 2003). Back.

9 Many years later, the idea of metallic iron grains as an interstellar dust component was reconsidered by Chlewicki & Laureijs (1988) who attributed the 60 µm emission measured by IRAS for the Galactic diffuse ISM to small iron particles with a typical size of a approx 70Å (which would obtain an equilibrium temperature of ~ 53 K in the diffuse ISM). But their model required almost all cosmic iron to be contained in metallic grains: ~ 34.5ppm (parts per million) relative to H. Exceedingly elongated metallic needles with a length (l) over radius (a) ratio l /a approx 105, presumably present in the intergalactic medium, have been suggested by Wright (1982), Hoyle & Wickramasinghe (1988), and Aguirre (2000) as a source of starlight opacity to thermalize starlight to generate the microwave background. Very recently, elongated needle-like metallic grains were suggested by Dwek (2004) as an explanation for the flat 3-8 µm extinction observed by Lutz et al. (1996) toward the Galactic Center and by Indebetouw et al. (2005) toward the l = 42° and 284° lines of sight in the Galactic plane. But these results heavily rely on the optical properties of iron needles (see Li 2003a, 2005b). Back.

10 Whittet, Duley, & Martin (1990) estimated from the 7.7-13.5 µm spectra (with a spectral resolution of ~ 0.23 µm) of 10 sightlines toward the Galactic Center the abundance of Si in SiC dust to be no more than ~ 5% of that in silicates. Since about half of the dust in the ISM is injected by carbon stars in which an appreciable fraction of the stardust is SiC, it is unclear how SiC is converted to gas-phase and recondense to form silicates in the ISM. Back.

11 Nanodiamonds were identified in the dust disks or envelopes surrounding two Herbig Ae/Be stars HD 97048 and Elias 1 and one post-asymptotic giant branch (AGB) star HR 4049, based on the 3.43 µm and 3.53 µm C-H stretching emission features expected for surface-hydrogenated nanodiamonds (Guillois, Ledoux, & Reynaud 1999; van Kerckhoven, Tielens, & Waelkens 2002). Back.

12 The reason why so many different materials with such a wide range of optical properties could be used to explain the observed interstellar extinction was that the number of free parameters defining the size distribution was sufficiently large. Back.

13 The reason why Wickramasinghe (1970a) considered ice-coated silicate grains was that he thought that graphite grains of a typical size ~ 0.06 µm would attain an equilibrium temperature of ~ 40 K in the ISM and would be too warm to possess an ice mantle, while silicate grains would tend to take up lower temperatures because of their lower optical and UV absorptivity and therefore the condensation of ice mantles could occur on their surfaces. Back.

14 Since the "UIR" emission bands were initially found to be associated with UV-rich objects, it had been thought that they were pumped primarily by UV photons. Li & Draine (2002b) demonstrated that the excitation of PAHs does not require UV photons - since the PAH electronic absorption edge shifts to longer wavelengths upon ionization and/or as the PAH size increases (see Mattioda, Allamandola, & Hudgins 2005 for their recent measurements of the near-IR absorption spectra of PAH ions), therefore long wavelength (red and far-red) photons are also able to heat PAHs to high temperatures so that they emit efficiently at the "UIR" bands (also see Smith, Clayton, & Valencic 2004). Li & Draine (2002b) have modeled the excitation of PAH molecules in UV-poor regions. It was shown that the astronomical PAH model provides a satisfactory fit to the UIR spectrum of vdB133, a reflection nebulae with the lowest ratio of UV to total radiation among reflection nebulae with detected UIR band emission (Uchida, Sellgren, & Werner 1998). Back.

15 The most recent estimates of the solar C ([C/H]odot approx 245 ppm; Allende Prieto, Lambert, & Asplund 2002) and O abundances ([O/H]odot approx 457 ppm; Asplund et al. 2004) are also "subsolar", just ~ 50%-70% of the commonly-adopted solar values (e.g. those of Anders & Grevesse 1989) and close to the "subsolar" interstellar abundances originally recommended by Snow & Witt (1996). If the interstellar abundances are indeed "subsolar", there might be a lack of raw material to form the dust to account for the interstellar extinction. Mathis (1996) argued that this problem could be solved if interstellar grains have a fluffy, porous structure since fluffy grains are more effective in absorbing and scattering optical and UV starlight than compact grains (on a per unit mass basis). However, using the Kramers-Kronig relation, Li (2005a) demonstrated that fluffy dust is not able to overcome the abundance shortage problem. The abundances of refractory elements in stellar photospheres may under-represent the composition of the interstellar material from which stars are formed, resulting either from the possible underestimation of the degree of heavy-element settling in stellar atmospheres, or from the incomplete incorporation of heavy elements in stars during the star formation process. Back.

16 Very recently, Vijh, Witt, & Gordon (2004) reported the discovery of blue luminescence at lambda < 5000Å in the Red Rectangle and identified it as fluorescence by small three- to four-ringed PAH molecules. Nayfeh, Habbal, & Rao (2005) argued that this blue luminescence could be due to hydrogen-terminated crystalline silicon nanoparticles. Back.

17 Such a power-law size distribution is a natural product of shattering following grain-grain collisions (e.g. see Hellyer 1970, Biermann & Harwit 1980, Dorschner 1982, Henning, Dorschner, & Gürtler 1989). Back.

Next Contents