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

2.2 The 9.7- and 18-µm Silicate Features

Spectral absorption features can provide a great deal of information regarding the compositions and nature of the grains. Such features have not been detected in the UV (55, 148, 184), except, of course, for the bump. However, many have been found in the NIR, especially from the icy mantles within molecular clouds. This review will concentrate on diffuse-dust and outer-cloud dust, and will not discuss the bands of molecular ices in general.

There is a broad, smooth absorption feature peaking at 9.7 µm, attributed to the Si-O stretch in silicates, which is always seen in interstellar dust if A(V) is suitably large. The 9.7-µm band is found in emission from warm circumstellar dust surrounding oxygen-rich stars. In these objects the heavy elements (Fe, Mg, etc.) are in silicates in the expanding envelope, while the carbon combines almost completely with oxygen to form CO. The 9.7-µm feature is not seen in circumstellar dust surrounding carbon-rich objects, except for some dusty planetary nebulae in which the grains were expelled by an earlier, and possibly oxygen-rich, phase of stellar evolution. A somewhat weaker and even broader feature peaking at 18 µm, the Si-O-Si bending mode, has been detected in circumstellar dust, in stars near the galactic center, and in molecular clouds (2, 118, 128, 160). The 18-µm band is much less well studied, but is found with the strength relative to that of the 9.7-µ;m feature expected for silicates (about 0.4). The bands are polarized at the same angle (2, 84) with the amplitudes expected for silicates.

The strength and profile of the 9.7-µm band, relative to A(V), has been determined (137) from several Wolf-Rayet type WC stars. These objects have the advantages of being luminous and of having no intrinsic spectral features in the 10-µm region (because they are carbon-rich and therefore contain no circumstellar silicate dust). If _9.7 is the optical depth of the silicate feature above the underlying continuum, the mean value of A(V) / _9.7 in diffuse dust is 18.5 ± 1.0. However, there are substantial variations of A(V) / _9.7 within the Taurus molecular cloud (169). The line of sight towards the galactic center has A(V) / _9.7 9, about half the local value (138), although the strong emission band at 7.7 µm confuses the determination of the continuum underlying the band (28). The dust seems to be diffuse rather than in dense clouds; there are only weak radio lines from molecules commonly seen in molecular clouds. However, conditions in the inner Galaxy (chemical composition, for instance) certainly differ from those in our neighborhood.

The derived shape of the 9.7-µm band is uncertain because the band is only about half as strong at maximum as the continuous extinction at the K band (2.2 µm). In diffuse dust towards WC stars, the shape of the band is similar to the emission seen in dusty oxygen-rich stars such as µ Cep (99, 137). The emission profile near the Trapezium in the Orion Nebula is 40% broader (60) than in µ Cep, and the profile in dense clouds appears to be 10% narrower (128). It is not surprising that the shape of the band is different in various types of objects; very probably the silicates are in different states of order, with different degrees of impurities. I recommend the Roche and Aitken (137) profile as being typical of diffuse dust.

The 9.7-µm profile shows that interstellar silicate is not crystalline (16). Crystalline silicate absorption peaks at about 10.5 µm, rather than 9.7 µm (see spectra in ref. 143). Laboratory measurements of amorphous or radiation-damaged silicates (33, 88) show a satisfactory, but not perfect, fit to the observed profile. Stars embedded in molecular clouds warm and partially anneal nearby grains (2).

The silicate band is weak for observational purposes, but it is difficult to account for its strength even if all of the silicon is in silicates and if a rather large opacity (3000 cm² gm^-1) is assumed for the maximum absorptivity (DL84; 157). This value is larger then most laboratory measurements of amorphous or lunar silicates. The total equivalent width of the band is rather constant from one type of silicate to another, so the broad profile of the interstellar band limits the maximum strength. The fundamental Kramers-Krönig relations (9a) limit the strength of the band (DL84) if astronomical silicates are similar to the minerals studied in the laboratory, but heavy contamination with other materials, and perhaps a porous nature, greatly complicate the issue.

There are suggestions that interstellar silicates are hydrated 68, 85), based on a 6.00-µm H-O-H bending band. However, in general the 6.00-µm feature is not correlated with the 9.7-µm band (172).