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

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8. SOME THEORIES AND PROBLEMS WITH EACH

Space does not allow even a superficial discussion of many individual theories interpreting the observational evidence summarized above. We summarize a few.

1. Bare silicate/graphite grains: DL84, in a very careful discussion of the optical constants of both graphite and silicates, greatly extended in wavelength an older theory of Mathis, Rumpl, and Nordsieck (114; often referred to as ``MRN''). The features of the theory are that (a) individual grains are bare and homogeneous, composed of either silicate or graphite; (b) the size distribution is a power-law, where n(a), the number of grains of radius a, is proportional to a-3.5; and (c) the size distribution is truncated at the upper end at 0.25 µm, with the lower end of sizes extended downward to a few angstroms to fit the IRAS data (41, 133), and very likely all the way to PAHs in order to produce the UIBs.

DL84 subjected their theory to a much more exacting comparison with observations than did most other authors. The fit to the extinction law for diffuse dust, over the entire observed wavelength range from 0.1 µm to 1000 µm, is impressively good. On the other had, DL84 requires that large graphite particles be produced from the amorphous carbon late-type stars inject into the ISM. It is very difficult to understand how the necessary annealing can take place under interstellar conditions. Furthermore, the materials in the two types of grains (silicates and graphite) must be kept separate, in spite of the many cycles of coagulation and rearrangement of the size distributions which take place as the grains cycle into and out of clouds.

2. Core/mantle grains: Greenberg and his collaborators (see references in ref. 62) believe that the bulk of interstellar grains have refractory silicate cores covered with an organic refractory mantle. This mantle is produced from the processing by both UV photons and cosmic rays deep inside molecular clouds, after the icy mantles observed in such clouds are deposited upon the grain surfaces. Laboratory studies show that molecules in such mantles can be partially converted into free radicals that are chemically active enough to react violently when warmed, producing complex molecules (31). Such runaway reactions could be triggered on inner-cloud grains by cosmic rays. After such an event, a organic polymeric substance known as ``organic refractory'' material, stable at room temperature, remains. In this theory, organic refractory mantles on silicate cores produce the optical/NIR extinction, while the bump is produced by small graphite particles, PAHs produce the UIBs, and the shortest-wavelength extinction is from small particles, probably silicates. Chlewicki and Laureijs (23) suggest that an additional component of iron will produce most of the 60-µm emission observed by IRAS.

These ideas have a great deal of appeal as regards events deep within clouds. The Greenberg scenario explains why interstellar molecules are found in the gas phase within dense clouds, where they should freeze onto grains in a short time - the runaway reactions drive off the molecules, and gas-phase chemistry takes place before refreezing onto grain surfaces. An alternative explanation of gas-phase molecules inside dense clouds is a very rapid circulation of grains between the surface and the center of the cloud (e.g., 21).

There are four problems as regards extending these ideas into diffuse dust: (a) Grains are larger in outer-cloud dust because of coagulation, not accretion of mantles, as shown by the reduced extinction per H atom in some cases. (b) Organic refractory mantles, which are less refractory than silicates and solid carbon, would be more readily destroyed by shocks in the diffuse ISM. The destruction rate of materials depends sensitively upon the binding energy (43, 44). (c) Solar system dust particles suggest that the silicate and carbon materials coagulate into large structures before icy mantles envelop them. (d) The 3.4-µm C-H absorption band, seen in organic refractory material in the laboratory along with the 3.08-µm ice band, is locally weak or absent. The object with the strongest 3.4-µm band, IRS 7 near the galactic center (17), is not typical of local dust. The 3.08- and 3.4-µm bands are not yet seen towards the local star VI Cyg 12 (61) [for which A(V) approx 10 mag], limiting these bands to less than 0.3 the strength, per A(V), of those for IRS 7. The 3.4-µm band is seen in Lynga 8/IRS 3 [154; A(V) = 17], about 0.4 times as strong as for IRS 7. The 3.4-µm band is always accompanied by a stronger 3.08-µm ice feature, and there are some lines of sight (64) with no ice band for A(V) < 20 mag.

It is possible (156) that the organic refractory mantles are so heavily processed that they lose almost all of their N and O, becoming essentially amorphous carbon. This material would be difficult to distinguish from amorphous C injected directly into the ISM from carbon stars. In this case, the Greenberg theory is very similar to composite-grain theories (see below).

3. Silicate cores with amorphous carbon mantles: Duley et al. (47) suggest that grains are silicates with mantles of hydrogenated amorphous carbon (HAC). One population is very small and produces the bump by (OH)- ion absorption in the presence of Si atoms. (All other theories produce the bump from well-ordered carbon). The UIBs are caused by absorption of UV photons by ``islands'' of HAC on the silicate core surfaces, so thermally isolated that they can radiate like free particles [for about 1 second! (133)]. The rapid increase of extinction with wavenumber for lambda-1 > 6 µm-1 is produced by diamond-like bonding in the ``amorphous'' C.

This theory makes several predictions that can be tested. It explains the differences between diffuse dust and outer-cloud dust rather naturally, as arising from different depletions of carbon onto the silicate cores. However, it requires a very large fraction of the Si atoms to have OH- ions nearby, near the surfaces of small grains, even if the bump transition in OH- has an oscillator strength of unity. The thermal isolation of the ``islands'' of HAC is difficult to achieve.

4. Composite grains: Mathis and Whiffen (113) and Tielens (156) suggest that interstellar grains consist of an assembly of small particles of carbon and silicates, jumbled together loosely. These grains are the natural result of coagulation and disruption of grains as they cycle into clouds. The particles inside the porous structure are protected from shocks and might well be covered with highly processed organic material. The bump is provided by small graphitic particles; PAHs can produce the UIBs. The rise in extinction for lambda < 0.16 µm is provided by the diamond bonding in ``amorphous'' C. The composite grains are mostly open, in analogy with interplanetary dust particles. However, too much porosity provides too large an opacity in the FIR, making the grains too cold because they radiate efficiently. It is difficult to calculate the extinction of composite grains, so the calculated fit should be taken as provisional.

5. Fractal grains: Wright (182) suggested that interstellar grains are the product of coagulation into very large fractal structures resembling twisted branches (65). If one defines the fractal dimension, alpha, by M propto Ralpha, then alpha depends upon the sequence of coagulation (and the probability of the fractal grains' breaking up, neglected in the calculations). In general alpha < 3, and in some cases alpha < 2. One of the major features of fractal grains is an FIR absorption per unit mass larger by an order of magnitude or more over solid grains.

Fractal grains can explain very large radar backscattering in comets (183) without large masses of dust. They also explain the very shallow (lambda-1) dependence of the opacity submillimeter opacity observed in some very dense nebulae (Section 3.2.3). However, the FIR opacity of fractal grains is so large that the grains would be too cold to explain the observed FIR spectrum of galactic dust (T approx 20 K).

6. Biological grains: Hoyle, N.C. Wickramasinghe, and others (76, 161, and references therein) have suggested that the grains producing visual extinction have a biological origin, with the bump provided by graphite. The extinction and polarization laws are fitted reasonably well. However, there are two problems with the model: (a) There is not enough cosmic phosphorous to accommodate the amount found in organisms (45, 164; but see 74). The cosmic abundance of P is low, and most of it is in the gas phase for low-density lines of sight, so this criticism seems valid. (b) Organisms, even when dried, show strong O-H and C-H stretch absorptions (75), which are not seen except deep within molecular clouds.

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