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

3. EMISSION FROM DUST

The realization (150, 151) that diffuse dust produces strong unidentified infrared emission bands (UIBs) in the 3.3-11.3 µm range, as well as an associated continuum, has stimulated much research within the last five years. The carriers of the UIBs are surely important components of the ISM. The UIBs have been discussed extensively 3, 94, 133, 157). Some of the main features of the UIBs are:

1. The strongest bands are at 3.3, 6.2, 7.7, 8.6, and 11.3 µm. These wavelengths all closely correspond to the C-H or C-C bond vibrations in aromatic (benzene-ring) structures. The simplest substances that can produce these bands are simple, planar molecules called polycyclic aromatic hydrocarbons (PAHs), but other, less well ordered configurations of carbon and hydrogen can also produce them (10, 142). A suggestive fit to the bands is provided by absorption from vitrinite (125), partially ordered graphite from coal. A mixture of PAHs can reproduce all of the UIBs, both weak and strong (57, 180).

2. Diffuse UIB emission, found throughout the Galaxy (59), is responsible for 10-20% of the total radiation from dust. UIBs and the associated continuum dominate the Infrared Astronomical Satellite (IRAS) filter responses at 12 and 25 µm (141), and are presumably responsible for the galactic ``cirrus'' emission in these filters (12).

3. The bands are also found in planetary nebulae, ``reflection'' nebulae, H II regions, extragalactic objects (27, 29, 171, and references therein), and carbon-rich or interstellar-dust environments, but not in dust produced by oxygen-rich objects. There is a direct relationship between the C/O ratio in planetary nebulae and the strength of the UIBs (29).

4. The wavelength of the 11.3-µm UIB shows that the hydrocarbons are not saturated with H. This band is due to the out-of-plane C-H bending, and occurs at 11.6-12.5 µm if there are two C-H bonds on the same aromatic ring, and 12.4-13.3 µm for three (94). The indicated amount of H coverage on the outer rings is 20-30%. Observational selection of relatively intense emission regions has meant that rather high radiation fields and subsequent dehydrogenation, are favored; perhaps the 11-13 µm emission from low-radiation environments will indicate more than one C-H bond on the same ring.

5. The bands are excited by the absorption of a single UV photon by the carrier. This is easy to understand (133) if the carriers (planar PAHs or three-dimensional carbon structures no larger than about 5 Å) float freely in space, so that a single photon can provide the energy required to emit the UIBs. The degree of excitation suggests that roughly 50 carbon atoms are required, with an upwards size range. If the carriers are attached to larger grains, the absorbed energy must be localized within a 5-Å region for the time required for the emission (of the order of a second). This process requires an exceedingly small thermal coupling.

6. The carriers of the UIBs are modified significantly by environment and history. The IRAS 12-µm response shows that the UIBs are not present in regions of very high radiation fields (13, 141), demonstrating that the carrier can be modified or destroyed by intense radiation. The wavelength of at least the 7.7-µm UIB is significantly different in planetary nebulae (where the carriers are newly produced by the carbon-rich material from the star) than in H II regions and reflection nebulae (where the carrier was presumably in the ISM before any interactions with the star presently causing the excitation).

7. PAHs would be mostly ionized in the diffuse ISM, since their first ionization potential is < 13.6 eV. Up to now, many laboratory studies of PAHs have, necessarily, involved only neutral molecules.

8. An individual PAH has strong discrete absorption bands in the visual through the UV, and there are no such features observed in interstellar extinction. A mixture of PAHs of varying sizes and structural arrangements produces continuous absorption.