The Most Common Dust Features

In early mid-infrared observations (c 1970) two prominent dust emission features were discovered. At about 10 microns there is broad feature that is attributed to silicates, similar to many types of minerals found in the crust of the Earth. A second related feature is seen at about 18 microns. The 10 micron feature is attributed to stretching of the Si-O bonds in silicates, while the 18 micron feature is attributed to O-Si-O bending in the same material. While the features are commonly seen in emission, they are also observed in absorption in a smaller number of objects. The objects with the features in absorption are heavily obscured by the dust and are optically invisible.

Soon after the discovery of the silicate features another feature at about 11.3 microns was discovered in the spectra of some carbon stars, not as wide or as strong as the silicate features but still thought to be due to a solid material. This feature is attributed to SiC dust. The feature is commonly seen in emission for carbon stars. For Galactic sources no really definite SiC absorption spectrum analogous to the case for IRAS 01304+6211 has been found, although there are a small number of objects that seem to have rather weak SiC absorption (this was however a matter of some dispute). Very recently, spectra taken of heavily obscured carbon stars in the Large Magellanic Cloud have shown nice, clear SiC absorption as with the example above. The spectrum is from a recent paper by Robert Gruendl and You-Hua Chu. Its not clear why the situation is different in the LMC than in the Galaxy, as far as producing these absorption features is concerned.

The object CW Leo discussed on the main dust page also shows the SiC emission feature and has a very thick dust shell formed from carbon-based dust grains. Its infrared spectrum shows the C₂H₂ band. Object IRAS 05133-6934 is presumably similar to CW Leo, but it seems to be even more obscured by a thick dust shell. The C₂H₂ bands in the spectra of these two objects must be from their circumstellar shells and not from the stars, since no stellar contributions to the spectra are observed at shorter wavelengths.

The silicate feature is seen with a wide range of optical depths from very thin dust shells to very optically thick dust shells. There are probably other types of dust present beside the silicates, but the other types of dust don't have such prominent features.

In a similar way the 11.3 micron dust feature is attributed to SiC dust grains based upon laboratory measurements of this mineral, also known as carborundum. This substance is not very common in the Earth's crust, presumably because Si and C are both easily oxidized. In space it is thought to form in conditions where there is an excess of carbon over oxygen, on the assumption that C and O combine to form the CO molecule first and then whatever atoms are left over after this form the dust grains. Since the normal cosmic abundances have more oxygen than carbon by a ratio of 1.82 to 1, under normal conditions the dust chemistry is dominated by oxygen and silicates are produced along with the CO molecules and some other molecules such as SO. In carbon stars, where the star produces carbon from nuclear reactions so that the O:C ratio decreases to a value less than 1, it is thought that the oxygen is removed to form CO molecules and then carbon chemistry takes place with the remaining carbon, producing SiC and pure carbon dust. Under such conditions the metals like Fe and Mg may form compounds like MgS and FeS because there is (ideally) no free oxygen to combine with. MgS dust may be detected in carbon-star spectra as is discussed here.

The Figure above shows why we think these silicates are present in space. The ISO spectrum of a typical evolved star, PZ Cas (which has a spectral type of M3Iab, and which is a semi-regular variable of period 900 days, V magnitude range between 9.8 and 12.7), is shown along with a model spectrum for a dust shell of Olivine dust around a cool star. The model spectrum is calculated using the properties of Olivine dust measured in the laboratory. Although the match is not at all perfect, the two big features seen for amorphous Olivine grains appear to match the ISO spectrum features reasonably well. The features are due to oscillation modes in Si-O bonds. The longer wavelength feature is at 17 microns in the Olivine dust model spectrum and at 18 microns in the PZ Cas spectrum, indicating that the dust in this star is slightly different from terrestrial Olivine.

The above Figure shows a schematic of the two modes that are responsible for the two features. The red vectors show the direction of motion of atoms, and the black arrows show the general oscillation modes. Exactly what the frequencies of the oscillations are depends on the details of the mineral structure, and so it can vary from one type of silicate to another.

Further discussion of the silicate features observed in stars can be found here for anyone who is interested.

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"bare" stellar spectra in the mid-infrared

less common features: extreme carbon stars, the AlO/silicate complex, the "unidentified infrared" features, unusual silicate features

dusty HII regions, planetary nebulae, and related objects

unusual features: mixed chemistry sources, Wolf-Rayet stars, ice bands, featureless carbon-star spectra