2. INTERSTELLAR GRAINS, WHAT DO WE KNOW?

Our knowledge of interstellar dust regarding its size, shape and composition is mainly derived from its interaction with electromagnetic radiation: attenuation (absorption and scattering) and polarization of starlight, and emission of IR and far-IR radiation. Presolar grains identified in meteorites and interplanetary dust particles (IDPs) of cometary origin also contain useful information regarding the nature of interstellar grains. The principal observational keys, both direct and indirect, used to constrain the properties of dust were summarized in recent reviews of Draine (2003) and Li (2004b).

(1) Grain Sizes. From the wavelength-dependent interstellar extinction and polarization curves as well as the near, mid and far IR emission, we know that there must exist a distribution of grains sizes, ranging from a few angstroms to a few micrometers.

- The interstellar extinction curve contains important information regarding the grain sizes since generally speaking, a grain absorbs and scatters light most effectively at wavelengths comparable to its size 2a. The extinction curve rises from the near-IR to the near-UV, with a broad absorption feature at about ^-1 4.6 µm^-1 ( 2175Å), followed by a steep rise into the far-UV ^-1 10 µm^-1. There must exist in the ISM a population of large grains with a > / 2 0.1 µm to account for the extinction at visible wavelengths, and a population of ultrasmall grains with a < / 2 0.016 µm to account for the far-UV extinction at = 0.1µm (see Li 2004a for details).

- The interstellar polarization curve rises from the IR, has a maximum somewhere in the optical and then decreases toward the UV. There must exist a population of aligned, nonspherical grains with typical sizes of a / 2 0.1 µm responsible for the peak polarization at 0.55 µm.

- Interstellar grains absorb starlight in the UV/visible and re-radiate in the IR. The IR emission spectrum of the Milky Way diffuse ISM, estimated using the IRAS 12, 25, 60 and 100 µm broadband photometry, the DIRBE-COBE 2.2, 3.5, 4.9, 12, 25, 60, 100, 140 and 240 µm broadband photometry, and the FIRAS-COBE 110 µm < < 3000 µm spectrophotometry, is characterized by a modified black-body of ^-1.7 B(T = 19.5 K) peaking at ~ 130 µm in the wavelength range of 80 µm < < 1000 µm, and a substantial amount of emission at < 60 µm which far exceeds what would be expected from dust at T 20 K. In addition, spectrometers aboard the IRTS (Onaka et al. 1996; Tanaka et al. 1996) and 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. There must exist a population of "cold dust" in the size range of a > 250Å, heated by starlight to equilibrium temperatures of 15 K < T < 25 K and cooled by far-IR emission to produce the emission at > 60µm which accounts for ~ 65% of the total emitted power (see Li & Draine 2001b); there must also exist a population of "warm dust" in the size range of a < 250Å, stochastically heated by single starlight photons to temperatures T >> 20 K and cooled by near- and mid-IR emission to produce the emission at < 60 µm which accounts for ~ 35% of the total emitted power (see Li & Draine 2001b; Li 2004a).

- The scattering properties of dust grains (albedo and phase function) provide a means of constraining the optical properties of the grains and are therefore indicators of their size and composition. The albedo in the near-IR and optical is quite high (~ 0.6), with a clear dip to ~ 0.4 around the 2175Å hump, a rise to ~ 0.8 around ^-1 6.6 µm^-1, and a drop to ~ 0.3 by ^-1 10 µm^-1; the scattering asymmetry factor almost monotonically rises from ~ 0.6 to ~ 0.8 from ^-1 1 µm^-1 to ^-1 10 µm^-1 (see Gordon 2004). An appreciable fraction of the extinction in the near-IR and optical must arise from scattering; the 2175Å hump is an absorption feature with no scattered component; and ultrasmall grains are predominantly absorptive.

- The "anomalous" Galactic foreground microwave emission in the 10-100GHz region (Draine & Lazarian 1998a, b), the photoelectric heating of the diffuse ISM (Bakes & Tielens 1994, Weingartner & Draine 2001b), and (probably) the ERE (Witt & Vijh 2004) also provide direct or indirect proof for the existence of nanometer-sized grains in the ISM (see Section 2 in Li 2004a for details).

- Both micrometer-sized presolar grains (such as graphite, SiC, corundum Al₂O₃, and silicon nitride Si₃N₄) and nanometer-sized presolar grains (such as nanodiamonds and titanium carbide nanocrystals) ¹⁸ of interstellar origin as indicated by their anomalous isotopic composition have been identified in primitive meteorites (see Clayton & Nittler 2004 for a recent review). Presolar silicate grains have recently been identified in IDPs (Messenger et al. 2003). Submicron-sized GEMS (Glass with Embedded Metals and Sulfides) of presolar origin have also been identified in IDPs and their 8-13µm absorption spectrum were similar to those observed in interstellar molecular clouds and young stellar objects (see Bradley 2003 for a recent review).

- Very large interstellar grains (with radii a > 1µm) entering the solar system have been detected by the interplanetary spacecraft Ulysses and Galileo (Grün et al. 1993, 1994). Huge grains of radii of a ~ 10 µm whose interstellar origin was indicated by their hyperbolic velocities have been detected by radar methods (Taylor et al. 1996). But Frisch et al. (1999) and Weingartner & Draine (2001a) argued that the amount of very large grains inferred from these detections were difficult to reconcile with the interstellar extinction and interstellar elemental abundances.

(2) Grain Shape. The detection of interstellar polarization clearly indicates that some fraction of the interstellar grains must be nonspherical and aligned. The fact that the wavelength dependence of the interstellar polarization exhibits a steep decrease toward the UV suggests that the ultrasmall grain component responsible for the far-UV extinction rise is either spherical or not aligned.

- The 9.7 and 18 µm silicate absorption features are polarized in some interstellar regions, most of which are featureless. ¹⁹ Polarization has also been detected in the 3.1 µm H₂O, 4.67 µm CO and 4.62 µm OCN^- absorption features (e.g. see Chrysostomou et al. 1996). Hough et al. (1996) reported the detection of a weak 3.47 µm polarization feature in the Becklin-Neugebauer object in the OMC-1 Orion dense molecular cloud, attributed to carbonaceous materials with diamond-like structure. The detection of polarization in both silicate and ice absorption features is consistent with the assumption of a core-mantle grain morphology (e.g. see Lee & Draine 1985).

- So far only two lines of sight toward HD147933 and HD197770 have a weak 2175Å polarization feature detected (Clayton et al. 1992; Anderson et al. 1996; Wolff et al. 1997; Martin, Clayton, & Wolff 1999). Even for these sightlines, the degree of alignment and/or polarizing ability of the carrier should be very small (see Section 2.1.2.1 in Li & Greenberg 2003 for details). The 2175Å bump carrier is a very inefficient polarizer (i.e. it is either nearly spherical or poorly aligned).

- So far, no polarization has been detected for the DIBs (see Somerville 1996 for a review), the 3.4 µm absorption feature (Adamson et al. 1999), ²⁰ and the "UIR" emission bands (Sellgren, Rouan, & Léger 1988). Their carriers do not align or lack optical anisotropy.

(3) Grain Composition. It is now generally accepted that interstellar grains consist of amorphous silicates and some form of carbonaceous materials; the former is inferred from the 9.7 µm Si-O stretching mode and 18 µm O-Si-O bending mode absorption features in interstellar regions as well as the fact that the cosmically abundant heavy elements such as Si, Fe, Mg are highly depleted; the latter is mainly inferred from the 2175Å extinction hump (and the ubiquitous 3.4 µm C-H stretching vibrational band) and the fact that silicates alone are not able to provide enough extinction (see Footnote-14 of Li 2004b).

- The 9.7 µm and 18 µm absorption features are ubiquitously seen in a wide range of astrophysical environments. These features are almost certainly due to silicate minerals: they are respectively ascribed to the Si-O stretching and O-Si-O bending modes in some form of silicate material (e.g. olivine Mg_2xFe_2-2xSiO₄). In the ISM, these features are broad and relatively featureless. Interstellar silicates are largely amorphous rather than crystalline. ²¹

- The strength of the 9.7 µm feature is approximately _{9.7 µm} / A_V 1/18.5 in the local diffuse ISM. Almost all Si atoms have been locked up in silicate dust, if assuming solar abundance for the ISM (see Footnote-9 of Li 2004b). ²²

- The 3.4 µm absorption feature is also ubiquitously seen in the diffuse ISM (but never in dense regions) of the Milky Way and other galaxies (e.g. Seyfert galaxies and ultraluminous infrared galaxies, see Pendleton 2004 for a recent review). This feature is generally attributed to the C-H stretching mode in aliphatic hydrocarbon dust, although its exact nature remains uncertain. ²³

- In principle, we could estimate the volume ratio of the silicate component to the aliphatic hydrocarbon component (1) if we know the band strength of the carrier of the 3.4 µm absorption feature (see Li 2004b), or (2) if we know the total abundances of interstellar elements (see Li 2005a). However, neither is precisely known.

(4) Distribution of Dust and its Association with Gas. Interstellar grains are unevenly distributed but primarily confined to the galactic plane with an effective thickness of ~ 200 pc. On average, the "rate of extinction" (the amount of visual extinction per unit distance) <A_V / L> is about ~ 1.8 mag kpc^-1 for the sightlines close to the galactic plane and for distances up to a few kiloparsecs from the Sun (Whittet 2003). Assuming a mean grain size of ~ 0.1 µm and a typical mass density of ~ 2.5 g cm^-3 for the interstellar grain material, we can estimate the mean dust number density and mass density in the solar neighbourhood ISM respectively to be n_dust 1.1 × 10^-12 cm^-3 and _dust 1.2 × 10^-26 g cm^-3 from the "rate of extinction". ²⁴

The association of interstellar dust and gas had been demonstrated by Bohlin, Savage, & Drake (1978) who found that the color excess and the total hydrogen column density (determined from the observations of HI Lyman- and H₂ absorption lines with the Copernicus satellite) were well correlated: E(B - V) / N_H 1.7 × 10^-22 mag cm² for the diffuse ISM in the solar neighbourhood. This correlation has recently been confirmed by the observations with the Far Ultraviolet Spectroscopic Explorer (FUSE) up to E(B - V) 1.0 (Rachford et al. 2002), ²⁵ suggesting that the dust and gas are generally well mixed in the ISM. From this ratio of E(B - V) to N_H one can estimate the gas-to-dust mass ratio to be ~210 in the diffuse ISM if we take R_V 3.1 (see Footnote-2 in Li 2004b); together with the "rate of extinction" <A_V / L> 1.8 mag kpc^-1, one can estimate the hydrogen number density to be n_H = R_V <A_V / L> N_H / E(B - V) 1.1 cm^-3 and a gas mass density of _gas 2.6 × 10^-24 g cm^-3.

Acknowledgments I thank the organizers F. Borghese and R. Saija for inviting me to this very exciting and fruitful conference. I thank F. Borghese, C. Cecchi-Pestellini, A. Giusto, M.A. Iatì, M.I. Mishchenko, and R. Saija for helpful discussions.

¹⁸ von Helden et al. (2000) proposed that TiC nanocrystals could be responsible for the prominent 21 µm emission feature detected in over a dozen carbon-rich post-AGB stars which remains unidentified since its first detection (Kwok, Volk, & Hrivnak 1989). Based on the Kramers-Kronig relations (Purcell 1969), Li (2003b) found that the TiC proposal is not feasible because it requires at least 50 times more Ti than available. Back.

¹⁹ The only exception is AFGL 2591, a molecular cloud surrounding a young stellar object, which displays a narrow feature at 11.2 µm superimposed on the broad 9.7 µm polarization band, generally attributed to annealed silicates (Aitken et al. 1988). However, its 3.1 µm ice absorption feature is not polarized (Dyck & Lonsdale 1981, Kobayashi et al. 1980). Back.

²⁰ So far spectropolarimetric measurement of this feature has been performed only for one sightline - the Galactic Center source IRS7 (Adamson et al. 1999). Unfortunately, no such measurements have been carried out for the 9.7 µm silicate absorption feature of this sightline. Spectropolarimetric measurements for both these two bands of the same sightline would allow a direct test of the silicate core-hydrocarbon mantle interstellar dust model (Li & Greenberg 1997), since this model predicts that the 3.4 µm feature would be polarized if the 9.7 µm feature (for the same sightline) is polarized (Li & Greenberg 2002). Back.

²¹ Li & Draine (2001a) estimated that the amount of a < 1 µm crystalline silicate grains in the diffuse ISM is < 5% of the solar Si abundance. Kemper, Vriend & Tielens (2004) placed a much tighter upper limit of ~ 0.2% on the crystalline fraction of the interstellar silicates along the sightline toward the Galactic Center. Back.

²² The silicate absorption feature (relative to the visual extinction) along the path to the Galactic Center is about twice that of the local ISM: _{9.7 µm} / A_V 1/9 (Roche & Aitken 1985). It was originally thought that there were very few carbon stars in the central regions of the Galaxy so that one would expect a much larger fraction of the dust to be silicates than is the case further out in the Galactic disk (Roche & Aitken 1985). However, this explanation was challenged by the fact that the 3.4 µm aliphatic hydrocarbon dust absorption feature for the Galactic Center sources (relative to the visual extinction: _{3.4 µm} / A_V 1/150) is also about twice that of the local ISM ( _{3.4 µm} / A_V 1/250; Pendleton et al. 1994; Sandford, Allamandola, & Pendleton 1995). Back.

²³ Over 20 different candidates have been proposed (see Pendleton & Allamandola 2002 for a summary). So far, the experimental spectra of hydrogenated amorphous carbon (HAC; Schnaiter, Henning & Mutschke 1999, Mennella et al. 1999) and the organic refractory residue, synthesized from UV photoprocessing of interstellar ice mixtures (Greenberg et al. 1995), provide the best fit to both the overall feature and the positions and relative strengths of the 3.42 µm, 3.48 µm, and 3.51 µm subfeatures corresponding to symmetric and asymmetric stretches of C-H bonds in CH₂ and CH₃ groups. Pendleton & Allamandola (2002) attributed this feature to hydrocarbons with a mixed aromatic and aliphatic character. Back.

²⁴ Let interstellar grains be approximated by a single size of a (spherical radius) with a number density of n_d. The visual extinction caused by these grains with a pathlength of L is A_V = 1.086 a² Q_ext(V) n_d L, where Q_ext(V) is the dust extinction efficiency at V-band ( = 5500Å). The dust number density can be derived from

(1)

The dust mass density is approximately _dust = n_dust (4/3) a³ _d 1.2 × 10^-16 g cm^-3 if we take a 0.1 µm, Q_ext(V) = 1.5, and _d = 2.5 g cm^-3. Back.

²⁵ Dark clouds (e.g. the Oph cloud) seem to have lower E(B - V) / N_H values, suggesting grain growth through coagulation (Jura 1980; Vrba, Coyne, & Tapia 1993; Kim & Martin 1996). Back.