Less Common Common Dust Features in Evolved Stars

Less Common Dust Features in Evolved Stars

Although the SiC and silicate features shown in the previous page are the most common types of features observed in dusty objects, they are not the only ones. This page shows a few other cases, three of which are closely related to those shown before and one of which shows an entirely new type of feature. The two spectra on the left are from M-type stars, while the two on the right are from carbon-rich objects. Then at the end I show a very unusual dust feature seen only in supernova remnants, just for comparison.

R Hya: silicate and AlO dust?

In the upper left panel is shown another type of mid-infrared spectrum observed for M-type giants. This star, R Hya, is a typical variable star of spectral type M7IIIe, a Mira variable that has a period of 388 days. It varies between V magnitude of 3.0, visible to the unaided eye, to a V magnitude of 11.0 which is about 100 times too faint to be seen with the unaided eye.

The R Hya infrared spectrum shows emission from the photosphere of the star at shorter wavelengths, and then some dust features at longer wavelengths. There appears to be weak silicate-like features at 9.7 and 19 microns plus another broad feature around 12 microns--attributed to some form of AlO dust--and another feature at 13 microns which has not been identified. In the inset plot the features are shown better than in the full spectrum plot. There is a stellar molecular band at shorter wavelengths also marked.

This group of features is generally seen together in sources that seem to have less dust than those that show the strong "classical" silicate features. There are wide variations in detail in the shapes and strengths smaller features, which may indicate changes in grain chemistry or in the degree of crystalline grain materials. The 13 micron feature is also seen in objects with much stronger "classical" amorphous silicate features.

There are cases of stars with these sets of features and with significant amounts of crystalline silicates (see next section).

IRAS 18095+2704: different types of silicate dust?

At lower left I show a spectrum with a different type of less usual silicate features, in an object that has only cool dust, causing the amorphous silicate features to look different than what is seen in most M-type star spectra. In this object IRAS 18095+2704 it seems that the 10 micron silicate feature is broader than normal, extending 1 micron further to short wavelengths than for the usual silicates (after one allows for the low temperature of the dust grains and the effect this has on the feature shape...its not obvious at a glance at the raw spectrum). The 18 micron feature is present, but a bit harder to see because of the shape of the dust continuum which peaks at about 20 microns. This means that the maximum dust temperature is something like 150 K. The star seems to have stopped producing dust some hundreds of years ago, and the dust it had around it has kept expanding and cooling to produce the observed spectrum. The object is thought to be a post-AGB star or proto-planetary nebula, as the star has a spectral type of F3Ib so its too hot to normally have produced dust.

There are also some weak features at longer wavelengths attributed to crystalline--rather than amorphous--forms of silicate. In fact one would expect the silicates around stars to form as crystalline materials since they probably cool relatively slowly during the formation process. In this context "fast" cooling means in a fraction of a second. If one melts silicate material and then cools it past the crystallization temperature over a period of a few seconds the material crystallizes. Since the dust shell models suggest that this transition in temperatures takes some period of order weeks in a star as the stellar wind moves outwards, one would think that the dust forms in the crystalline state. There are processes to transform the grains to amorphous form, such as bombardment by cosmic rays, but these processes should act over longer periods of time than the simple expansion time for the stellar material to move out from the star to 10 to 50 times the stellar radius, which is where the hotter dust seems to be in ordinary AGB stars. The question seems rather to be why there is so much amorphous silicate dust around rather than why we observe crystalline forms of silicate dust. The crystalline features are interesting because they tend to be specific to certain forms of the silicates, allowing the detailed mineralogy to be studied. For example features at 23.6 and 33.6 microns are attributed to crystalline olivines and features at 10.7, 15.2, and 17.5 microns are attributed to crystalline pyroxines. In both cases the crystalline silicates are thought to be Mg rich and Fe poor.

The crystalline silicate features in this particular spectrum appear to be primarily due to olivines.

IRAS 21489+5301: a highly obscured carbon star

At upper left is shown a typical spectrum of a highly dust enshrouded carbon star, similar in general properties to IRC+10216 but with an even thicker dust shell. That spectrum shows a broad dust feature at about 30 microns observed only in carbon-rich objects (including IRC+10216). There are also molecular bands of carbon-based molecules in the spectra of such "extreme carbon stars". The relatively few such stars known in our Galaxy may show the SiC feature at 11.3 microns either in emission or in weak absorption, as well as the 30 micron feature. Others like this one do not show any SiC feature. The 30 micron feature is also seen in a number of less heavily obscured carbon stars, with the normal 11.3 micron SiC emission feature and with the stellar continuum detected at the shorter wavelengths. It seems that the thicker the dust shell is the more likely the 30 micron feature is to be present. The 30 micron feature can vary widely in strength from object to object with otherwise similar properties. The object IRAS 05133-6934 whose spectrum is shown on the normal dust features page is another extreme carbon star, and it has a rather weak 30 micron feature in its spectrum.

The "30 micron" feature actually often peaks at more like 27 microns. As discussed on the previous page when the O/C ratio goes below 1 the chemistry shifts from oxygen-dominated to carbon-dominated, and in the carbon-dominated case MgS is expected to form. The 30 micron feature has therefore been identified as due to MgS dust grains, either pure MgS grains or with MgS as a coating on amorphous carbon dust grains. In many circumstances MgS grains are predicted to have two features at around 26 and 39 microns, rather than one feature. This is why I personally am still dubious about the identification of the 30 micron feature with MgS grains. However for a very different view of this question please see the following paper: Hony, Waters, and Tielens (2002).

There was some evidence in the ISO spectra of a 26 micron "sub-feature" in the 30 micron feature, which may vary in shape from object to object. However the more recent Spitzer spectra appear to disprove this idea.

These heavily obscured extreme carbon stars do not always show the 11.3 micron SiC feature, and so the dominant type of dust grains are usually assumed to be either amorphous carbon or graphite. Neither of these forms of dust have features that we can use to definitely identify the type of dust present. The relation between these hypothesized carbon dust grains and other grains such as SiC and possibly MgS is not understood.

IRAS 21282+5050: the UIR or "PAH" features

Finally at lower right is shown an example of a family of narrower features at 3.3, 3.4, 6.2, 7.7, 8.6, 11.3, and 12.4 microns. These were originally called the unidentified infrared bands or UIR bands, and they are still sometimes called that today. In the middle 1980's it was proposed that these features are the result of bending and stretching modes of aromatic ring molecules, essentially clusters of benzene rings. These polycyclic aromatic hydrocarbons (PAHs) do appear to have features that correspond to the observed ones, and these features are prominently seen in many carbon-rich objects. It is thus thought that they are due to some type of carbon-based grain or large molecule, which makes the PAH hypothesis plausible. The current general hypothesis is that the UIR features are excited by the absorption of a single UV photon by one of these PAH molecules, which then radiates via fluorescence. The features are commonly seen throughout the interstellar medium under conditions where the dust and gas are so cold that there should be no radiation at 3.3 or 6.2 or 7.7 microns; this type of non-equilibrium heating would resolve this difficulty.

Associated with the individual UIR features are two broad underlying "plateau" features, which are seen when the UIR emission is very strong, as in the case of IRAS 21282+5050. The object itself is a young carbon-rich planetary nebula so it provides its own UV radiation to excite the features. As well as the UIRs the spectrum shows a cool dust continuum at longer wavelengths, and possibly a weak 30 micron feature.

Its important to note that while the PAHs are what we call "organic molecules" if found here on Earth, the UIRs carriers--which are widely distributed in space--presumably do not have anything to do with living organisms. The evidence we have is consistent with these molecules forming in the atmospheres of stars where life as we know it could not possibly exist.

There has been a suggestion that these UIR features are due to one or two small molecules, Cyclopropenylidene (c-C3H2) and possibly Ethylene Oxyide (c-C2H4O). See the paper Bernstein and Lynch (2009) for details. Its an interesting suggestion; the question is how to either prove or disprove the hypothesis solely from spectroscopy. Life would be much easier if we could just go to these dusty stars and look directly at the material around them. Where is the starship Enterprise when you need it?

The UIR features are widely seen in the interstellar medium (see the next page). They are not as common in evolved stars because UV radiation is generally required to excite the features, and so they are usually not observed around cool M-type stars that have little or no UV emission. The central star of IRAS 21282+5050 is hot enough to excite the features. It seems likely that the UIR carrier is formed around carbon stars but we do not see it because there is no UV radiation to excite the features. Most of the evolved stars where the UIR features are seen appear to be former carbon stars that have evolved to higher temperatures recently.

The small number of evolved stars that show strong UIR bands and no SiC feature or 30 micron feature in their infrared spectra, as is the case for IRAS 21282+5050, mostly seem to have a mixed dust chemistry with both the UIR carrier and crystalline silicates present around the star. IRAS 21282+5050 is unusual in this group of sources as not showing any crystalline silicate features in its spectrum.

The "Silicate" Feature in Cas A

In the past few years a feature at 21 microns has been discovered in the Cas A supernova remnant (see Rho et al. (2008)). This is shown below. The feature is thought to be due to some type of silicate-related dust (SiO2, Mg protosilicates, and FeO grains) formed in the SNR ejectra. I show it here just to illustrate that there are lots of potential "unusual silicates" in astronomical objects.

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"bare" stellar spectra in the mid-infrared

the most common features: silicates and SiC

dusty HII regions, planetary nebulae, and related objects

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