Annu. Rev. Astron. Astrophys. 1990. 28: 37-70
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6.2 Sites of Dust Formation

Probably most dust is injected into the ISM from stars on the asymptotic giant branch, either C rich or O rich, although supernovae, in spite of a low rate of mass injection, might be important because of their large heavy-element composition. Carbon stars show an 11.15-µm feature of SiC with a quite uniform profile (26, 99). There is also a featureless optical/NIR continuum that can be modelled by amorphous carbon but not graphite (92, 106), unless the graphite is in small particles with a loose fractal structure (183). Oxygen-rich stars show the silicate feature with a profile that varies appreciably from star to star (99, 126). Differing physical conditions in the atmosphere affect the nature of the grains, and the silicate band in one star (2) shows clear signs of annealing. A few circumstellar shells show the 3.08-µm ice band as well.

The UV bump is found in C stars at 0.24 µm, not 0.2175 µm (67), and in one H-poor, C-rich planetary nebula (Abell 30) at 0.25 µm (63). These shifted wavelengths are consistent with amorphous C or fractal graphite grains. There is no bump in the circumstellar dust of oxygen-rich alpha Sco (153). The bump is seen in circumstellar dust surrounding a few hot stars (152) at the normal lambdamax, but the dust might be interstellar, remaining from the epoch of the star's formation.

Novae and Wolf-Rayet stars (late-type WCs) are minor sources of dust because of their low mass input into the ISM. Both presumably inject carbon-rich dust. Planetary nebulae represent a considerable source of the return of gas to the ISM (102), but are not a large source of dust because they have a low dust/gas ratio (131).

Isotopic anomalies in meteorites (see Section 6.3) prove that some dust forms in expanding supernova shells. Dust is also found within hot supernova remnants (48, 49), but it might have been produced by a presupernova red supergiant. How much dust is actually formed in supernovae is not known.