Our knowledge of interstellar dust regarding its composition, size and shape 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. The principal observational keys, both direct and indirect, used to constrain the properties of dust are the following:
Interstellar Extinction. Extinction 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 (the absorbed energy is then re-radiated in the IR and far-IR).
- The wavelength dependence of interstellar extinction - "interstellar extinction curve", most commonly determined from the "pair-method", 4 rise from the near-IR to the near-UV, with a broad absorption feature at about -1 4.6 µm-1 ( 2175 Å), followed by a steep rise into the far-UV -1 10 µm-1. The extinction curve tells us the size (and to a less extent, the composition) of interstellar dust. Since it is generally true that a grain absorbs and scatters light most effectively at wavelengths comparable to its size 2 a (Krügel 2003), 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 = 0.1 µm (see Li 2004a for details).
- The optical/UV extinction curves show considerable regional variations. Dust grains on different sightlines have different size distributions (and/or different compositions).
- The optical/UV extinction curve in the wavelength range of 0.125 3.5 µm can be approximated by an analytical formula involving only one free parameter: RV AV / E(B - V), the total-to-selective extinction ratio (Cardelli, Clayton, & Mathis 1989), whereas the near-IR extinction curve (0.9 µm 3.5 µm) can be fitted reasonably well by a power law A() ~ -1.7, showing little environmental variations.
The Galactic mean extinction curve is characterized by RV 3.1. Values of RV as small as 2.1 (the high latitude translucent molecular cloud HD 210121; Larson, Whittet, & Hough 1996; Li & Greenberg 1998) and as large as 5.6 (the HD 36982 molecular cloud in the Orion nebula) have been observed in the Galactic regions. More extreme extinction curves have been reported for extragalactic objects. 5 The optical/UV extinction curve and the value of RV depend on the environment: lower-density regions have a smaller RV, a stronger 2175 Å hump and a steeper far-UV rise (-1 > 4 µm-1), implying smaller grains in these regions; denser regions have a larger RV, a weaker 2175 Å hump and a flatter far-UV rise, implying larger grains.
- In the Small Magellanic Cloud (SMC), the extinction curves of most sightlines display a nearly linear steep rise with -1 and an extremely weak or absent 2175 Å hump (Lequeux et al. 1982; Prévot et al. 1984). Grains in the SMC are smaller than those in the Milky Way diffuse ISM as a result of either more efficient dust destruction in the SMC due to its harsh environment of the copious star formation associated with the SMC Bar or lack of growth due to the low-metallicity of the SMC, or both. Regional variations also exist in the SMC extinction curves. 6
- The Large Magellanic Cloud (LMC) extinction curve is characterized by a weaker 2175Å hump and a stronger far-UV rise than the average Galactic extinction curve (Nandy et al. 1981; Koornneef & Code 1981). 7
Interstellar Polarization. For a non-spherical grain, the light of distant stars is polarized as a result of differential extinction for different alignments of the electric vector of the radiation.
- The interstellar polarization curve rises from the IR, has a maximum somewhere in the optical and then decreases toward the UV. This tells us that (1) some fraction of interstellar grains are both non-spherical and aligned by some process; (2) the bulk of the aligned grains responsible for the peak polarization (at 0.55 µm) have typical sizes of a / 2 0.1 µm; and (3) the ultrasmall grain component responsible for the far-UV extinction rise is either spherical or not aligned.
- The optical/UV polarization curve P() can be closely approximated by the "Serkowski law", an empirical function:
P() / Pmax = exp[- K ln2( / max)], where the only one free parameter, max, is the wavelength where the maximum polarization Pmax occurs, and K is the width parameter: K 1.66 max+0.01; max is indicative of grain size and correlated with RV: RV (5.6 ± 0.3) max (max is in micron; see Whittet 2003). The sightlines with larger max are rich in larger grains and have larger RV for their extinction curves. A close correlation between max and the size of the aligned grains [e.g., max 2 a(n - 1) for dielectric cylinders of radius a and refractive index n] is predicted by interstellar extinction calculations.
- The near-IR (1.64 µm < < 5 µm) polarization is better approximated by a power law P() -, with 1.8 ± 0.2, independent of max (Martin & Whittet 1990, Martin et al. 1992).
Interstellar Scattering. Scattering of starlight by interstellar dust is revealed by reflection nebulae (dense clouds illuminated by embedded or nearby bright stars), dark clouds (illuminated by the general interstellar radiation field [ISRF]), and the diffuse Galactic light ("DGL"; starlight scattered off the general diffuse ISM of the Milky Way Galaxy illuminated by the general ISRF). The scattering properties of dust grains (albedo = ratio of scattering to extinction, and phase function) provide a means of constraining the optical properties of the grains and are therefore indicators of their size and composition and provide diagnostic tests for dust models.
- The albedo in the near-IR and optical is quite high (~ 0.6), with a clear dip to ~ 0.4 around the 2175 Å 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 2175 Å hump is an absorption feature with no scattered component (see Section 2.4); and ultrasmall grains are predominantly absorptive.
- Surprisingly high near-IR albedo has been reported for several regions: ~ 0.86 at the K' band ( 2.1 µm) for the prominent dust lane in the evil eye galaxy NGC4826 (Witt et al. 1994), ~ 0.9 at the K' band for the dust in the M51 arm (Block 1996), and ~ 0.7 at the J ( 1.26 µm) and H ( 1.66 µm) bands and ~ 0.6 at the K ( 2.16 µm) band for the Thumbprint Nebula (Lehtinen & Mattila 1996), in comparison with ~ 0.2 at = 2.2 µm predicted by the conventional dust models (Draine & Lee 1984; Li & Greenberg 1997). This implies that for NGC4826 and M51 (1) a population of grains at least ~ 0.5 µm in radii, twice as large as assumed by standard models, may exist in these environments and are responsible for the near-IR scattering; and/or (2) the measured high K' surface-brightness and the deduced high albedo may in part be caused by the thermal continuum emission from stochastically heated ultrasmall grains (Witt et al. 1994; Block 1996). For the Thumbprint Nebula, the high near-IR albedo is readily explained by grain growth (larger-than-average grain sizes) and the accretion of an ice mantle (see Pendleton, Tielens, & Werner 1990; Section 8 in Li & Greenberg 1997).
- Scatterings of X-rays by interstellar dust have also been observed as evidenced by "X-ray halos" formed around an X-ray point source by small-angle scattering. The intensity and radial profile of the halo depends on the composition, size and morphology and the spatial distribution of the scattering dust particles (see Dwek et al. 2004 for a review). The total and differential cross sections for X-ray scattering approximately vary as a4 and a6, respectively, the shape and intensity of X-ray halos surrounding X-ray point sources therefore provide one of the most sensitive constraints on the largest grains along the sightline, while these grains are gray at optical wavelengths and therefore the near-IR to far-UV extinction modeling is unable to constrain their existence.
A recent study of the X-ray halo around Nova Cygni 1992 by Witt, Smith, & Dwek (2001) pointed to the requirement of large interstellar grains (a ~ 0.25-2 µm), consistent with the recent Ulysses and Galileo detections of interstellar dust entering our solar system (Grün et al. 1994; Frisch et al. 1999; Landgraf et al. 2000). But Draine & Tan (2003) found that the silicate-graphite-PAH model with the dust size distributions derived from the near-IR to far-UV extinction modeling (Weingartner & Draine 2001a) and IR emission modeling (Li & Draine 2001b) is able to explain the observed X-ray halo.
Spectroscopic Extinction and Polarization
Features: The 2175 Å
Extinction Hump - the strongest spectroscopic extinction feature.
- Its strength and width vary with environment while its peak position is quite invariant: the central wavelength of this feature varies by only ± 0.46% (2) around 2175 Å (4.6 µm-1), while its FWHM varies by ± 12% (2) around 469 Å ( 1 µm-1).
- Its carrier remains unidentified 39 years after its first detection (Stecher 1965). 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; Draine 2003a).
- For most sightlines, this feature is unpolarized. So far only two lines of sight toward HD147933 and HD197770 have a weak 2175 Å 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 Li & Greenberg 2003).
- Except for the detection of scattering in the 2175 Å hump in two reflection nebulae (Witt, Bohlin, & Stecher 1986), the 2175 Å hump is thought to be predominantly due to absorption, suggesting its carrier is sufficiently small to be in the Rayleigh limit.
- The detections of this feature in distant objects have been reported by Malhotra (1997) in the composite absorption spectrum of 96 intervening MgII absorption systems at 0.2 < z < 2.2; by Cohen et al. (1999) in a damped Ly absorber (DLA) at z = 0.94; by Toft, Hjorth & Burud (2000) in a lensing galaxy at z = 0.44; by Motta et al. (2002) in a lensing galaxy at z = 0.83; and very recently by Wang et al. (2004) in 3 intervening quasar absorption systems at 1.4 z 1.5.
Spectroscopic Extinction and Polarization Features: The 9.7 µm and 18 µm (Silicate) Absorption Features - the strongest IR Absorption features.
- 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 Mg2xFe2-2xSiO4).
- The observed interstellar silicate bands are broad and relatively featureless. Interstellar silicates are largely amorphous rather than crystalline. 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. 8
- The strength of the 9.7 µm feature is approximately 9.7 µm / AV 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. 9
- The 9.7 and 18 µm silicate absorption features are polarized in some interstellar regions, most of which are featureless. 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).
Spectroscopic Extinction and Polarization Features: The 3.4 µm (Aliphatic Hydrocarbon) Absorption Feature.
- Widely seen in the diffuse ISM of the Milky Way Galaxy and other galaxies (e.g. Seyfert galaxies and ultraluminous infrared galaxies, see Pendleton 2004 for a recent review), this strong absorption band is attributed to the C-H stretching mode in aliphatic hydrocarbon dust. Its exact nature remains uncertain, despite 23 years' extensive investigation with over 20 different candidates 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.
- The 3.4 µm band strength for interstellar aliphatic hydrocarbon dust is unknown. If we adopt a mass absorption coefficient of abs(3.4 µm) ~ 1500 cm2 g-1 (Li & Greenberg 2002), we would require ~ 68 ppm C to be locked up in this dust component to account for the local ISM 3.4 µm feature ( 3.4 µm / AV 1/250; Pendleton et al. 1994).
- This feature is ubiquitously seen in the diffuse ISM while never detected in molecular clouds. Mennella et al. (2001) and Muñoz Caro et al. (2001) argue that this can be explained by the competition between dehydrogenation (destruction of C-H bonds by UV photons) and rehydrogenation (formation of C-H bonds by H atoms interacting with the carbon dust): in diffuse clouds, rehydrogenation prevails over dehydrogenation; in dense molecular clouds, dehydrogenation prevails over rehydrogenation as a result of the reduced amount of H atoms and the presence of ice mantles which inhibits the hydrogenation of the underlying carbon dust by H atoms while dehydrogenation can still proceed since the UV radiation can penetrate the ice layers.
- Whether the origin of the interstellar aliphatic hydrocarbon dust occurs in the outflow of carbon stars or in the ISM itself is a subject of debate. The former gains strength from the close similarity between the 3.4 µm interstellar feature and that of a carbon-rich protoplanetary nebula CRL 618 (Lequeux & Jourdain de Muizon 1990; Chiar et al. 1998). However, the survival of the stellar-origin dust in the diffuse ISM is questionable (see Draine 1990).
- So far, no polarization has been detected for this feature (Adamson et al. 1999). 10 Spectropolarimetric measurements for both the 9.7 µm silicate and the 3.4 µm hydrocarbon features for 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).
Spectroscopic Extinction and Polarization Features: The Ice Absorption Features.
- Grains in dark molecular clouds (usually with AV > 3 mag) obtain ice mantles consisting of H2O, NH3, CO, CH3OH, CO2, CH4, H2CO and other molecules (with H2O as the dominant species), as revealed by the detection of various ice absorption features (e.g., H2O: 3.1, 6.0 µm; CO: 4.67 µm; CO2: 4.27, 15.2 µm; CH3OH: 3.54, 9.75 µm; NH3: 2.97 µm; CH4: 7.68 µm; H2CO: 5.81 µm; OCN-: 4.62 µm; see Boogert & Ehrenfreund 2004 for a review).
- Polarization has been detected in the 3.1 µm H2O, the 4.67 µm CO and 4.62 µm OCN- absorption features (e.g. see Chrysostomou et al. 1996).
The Extended Red Emission: Dust Photoluminescence. The "Extended Red Emission" (ERE), widely seen in reflection nebulae, planetary nebulae, HII regions, the Milky Way diffuse ISM, and other galaxies, is a far-red continuum emission in excess of what is expected from simple scattering of starlight by interstellar dust (see Witt & Vijh 2004). The ERE is characterized by a broad, featureless band between ~ 5400 Å and 9500 Å, with a width 600 Å FWHM 1000 Å and a peak of maximum emission at 6100 Å p 8200 Å, depending on the physical conditions of the environment where the ERE is produced.
- The ERE is generally attributed to photoluminescence (PL) by some component of interstellar dust, powered by UV/visible photons with a photon conversion efficiency PL >> 10% (Gordon, Witt, & Friedmann 1998). The ERE carriers are very likely in the nanometer size range because nanoparticles are expected to luminesce efficiently through the recombination of the electron-hole pair created upon absorption of an energetic photon, since in such small systems the excited electron is spatially confined and the radiationless transitions that are facilitated by Auger and defect related recombination are reduced (see Li 2004a).
- The ERE carrier remains unidentified. Various candidate materials have been proposed, but most of them appear unable to match the observed ERE spectra and satisfy the high-PL requirement (Draine 2003a; Li & Draine 2002a; Li 2004a; Witt & Vijh 2004). Promising candidates include PAHs (d'Hendecourt et al. 1986) and silicon nanoparticles (Ledoux et al. 1998, Witt et al. 1998, Smith & Witt 2002), but both have their own problems (see Li & Draine 2002a).
Spectroscopic Emission Features: The 3.3, 6.2, 7.7, 8.6, 11.3 µm "Unidentified Infrared (UIR)" Emission features. The distinctive set of "UIR" emission features at 3.3, 6.2, 7.7, 8.6, and 11.3 µm are seen in a wide variety of objects, including planetary nebulae, protoplanetary nebulae, reflection nebulae, HII regions, photodissociation fronts, circumstellar envelopes, and external galaxies.
- These "UIR" emission features are now generally identified as vibrational modes of PAHs (Léger & Puget 1984; Allamandola, Tielens, & Barker 1985): C-H stretching mode (3.3 µm), C-C stretching modes (6.2 and 7.7 µm), C-H in-plane bending mode (8.6 µm), and C-H out-of-plane bending mode (11.3 µm). 11 The relative strengths and precise wavelengths of these features are dependent on the PAH size and its ionization state which is controlled by the starlight intensity, electron density, and gas temperature of the environment (Bakes & Tielens 1994, Weingartner & Draine 2001b, Draine & Li 2001).
- Stochastically heated by the absorption of a single UV/visible photon (Draine & Li 2001; Li 2004a), PAHs, containing ~ 45 ppm C, account for ~ 20% of the total power emitted by interstellar dust in the Milky Way diffuse ISM (Li & Draine 2001b).
- The excitation of PAHs does not require UV photons; 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. This is because the PAH electronic absorption edge shifts to longer wavelengths upon ionization and/or as the PAH size increases. 12
- No polarization has been detected for the PAH emission features (Sellgren, Rouan, & Léger 1988).
- The PAH absorption features at 3.3 µm and 6.2 µm have been detected in both local sources and Galactic Center sources (Schutte et al. 1998; Chiar et al. 2000). The strengths of these features are in good agreement with those predicted from the astronomical PAH model (Li & Draine 2001b).
- PAHs can be rotationally excited by a number of physical processes, including collisions with neutral atoms and ions, "plasma drag", and absorption and emission of photons. It is shown that these processes can drive PAHs to rapidly rotate, with rotation rates reaching tens of GHz. The rotational electric dipole emission from these spinning PAH molecules is capable of accounting for the observed "anomalous" microwave emission (Draine & Lazarian 1998a, b; Draine 1999; Draine & Li 2004).
IR Emission from Interstellar Dust. 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 (see Draine 2003a). 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.
- The emission at 60 µm accounts for ~ 65% of the total emitted power. There must exist a population of "cold dust" in the size range of a > 250 Å, heated by starlight to equilibrium temperatures 15 K T 25 K and cooled by far-IR emission (see Li & Draine 2001b).
- The emission at 60 µm accounts for ~ 35% of the total emitted power. There must exist a population of "warm dust" in the size range of a < 250 Å, stochastically heated by single starlight photons to temperatures T >> 20 K and cooled by near- and mid-IR emission (see Li & Draine 2001b; Li 2004a).
Interstellar Depletions. Atoms locked up in dust grains are "depleted" from the gas phase. The dust depletion can be determined from comparing the gas-phase abundances measured from optical and UV absorption spectroscopic lines with the assumed reference abundances of the ISM (total abundances of atoms both in gas and in dust; also known as "standard abundances", "interstellar abundances", and "cosmic abundances"). The total interstellar abundances are usually taken to be solar, although Snow & Witt (1996) argued that interstellar abundances are appreciably subsolar (~ 70% solar). Interstellar depletions allow us to extract important information about the composition and quantity of interstellar dust:
- In low density clouds, Si, Fe, Mg, C, and O are depleted. Interstellar dust must contain an appreciable amount of Si, Fe, Mg, C and O. Indeed, all contemporary interstellar dust models consist of both silicates and carbonaceous dust.
- From the depletion of the major elements Si, Fe, Mg, C, and O one can estimate the gas-to-dust mass ratio to be ~ 165. 13
- In addition to the silicate dust component, there must exist another dust population, since silicates alone are not able to account for the observed amount of extinction relative to H although Si, Mg, and Fe are highly depleted in the ISM. Even if all Si, Fe, and Mg elements are locked up in submicron-sized silicate grains, they can only account for ~ 60% of the total observed optical extinction. 14