Other Kinds of Mid-infrared Spectra

In this final page I show a few more types of spectra of dusty objects. Two of these are related to the spectra shown on the last two pages. The other two are new types of spectra.

I will start with the lower right panel. This is a spectrum of a pre-main sequence object, or young stellar object (YSO). There is a fairly wide range of shapes of these spectra. Since I do not work on YSO research these are not a concern for me, but the spectra show a wide variety of features. This particular spectrum shows silicate dust absorption features at 10 and 18 microns, plus narrower absorption bands due to different types of ices (water, CO, CO₂). As far as I know only YSO spectra show the ice bands. The spectra also tend to show silicate emission or absorption features, sometimes the aromatic/UIR features, and the continuum shape can very widely--usually it is either flat or rises to longer wavelengths in this type of plot. In some phases of pre-main-sequence evolution its difficult to distinguish between a YSO and an post-AGB object, especially if only photometry is available.

The lower left panel shows two ISO spectra of Wolf-Rayet stars, one with no dust where the photosphere and the typical broad Wolf-Rayet emission lines are seen and one with a lot of dust which shows a relatively featureless continuum and a broad absorption feature near 10 microns. Such dusty Wolf-Rayet stars like the latter object are quite rare. Its not entirely clear what is causing the broad absorption feature in the dusty object spectrum: it could be a weak silicate absorption feature due to the interstellar medium (as such stars are not expected to have any silicate dust around them) but it is then odd that no 18 micron absorption is present. The shape is also unusual for silicate absorption. Another possibility is that it is some type of unusual UIR/aromatic features, in which case the feature is not an absorption feature. The main dust in this object is probably some type of amorphous carbon dust. There is no trace of the other features commonly seen in carbon-star spectra.

At upper left is a spectrum of a very dusty carbon-star in the Large Magellanic Cloud, or so I assume because the C₂H₂ band-head at 13.71 microns is strong. Oddly, no other dust features are seen and the continuum is quite similar to blackbody emission. This type of spectrum is very unusual. It is possible that the dust is pure graphite or amorphous carbon, without the SiC and 30 micron dust.

Finally at upper right is shown an ISO spectrum of another PPN candidate object, and in this case one sees both crystalline silicate dust and the aromatic/UIR features in the spectrum. Such cases of "mixed chemistry" are very interesting. It is generally postulated that these objects are in binary systems and that the crystalline silicate dust is present in some type of disk around both the stars. The disk is assumed to be old, allowing all the amorphous silicate dust to be made crystalline, while the UIRs are assumed to be due to newer material from the PPN itself. The Red Rectangle is another example of such a system.

I personally find it striking that there is (to my knowledge) no example of a star with SiC dust or the 30 micron feature carrier along with crystalline silicate dust. I am not sure what it means in terms of the physics behind the dual chemistry systems.

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"bare" stellar spectra in the mid-infrared

the most common features: silicates and SiC

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