4. Summary and conclusions
The discovery of meteoritic dust grains with an origin outside the solar system has opened the possibility of studying presolar material directly in the laboratory. A large fraction of this material is likely to be dust from the envelopes of asymptotic giant branch (AGB) stars. We have previously measured the absorption coefficients of presolar diamonds (Mutschke et al. 1995; Andersen et al. 1998). We here report the results of mid-IR measurements of meteoritic SiC grains.
Measurements were performed on two different extractions of presolar SiC from the Murchison meteorite. The two samples show very different spectral appearances which we interpret as being due to different grain size distributions in the two extractions. The spectral feature of the smaller meteoritic SiC grains is at 11.3 µm, whereas the large ( 5 µm) SiC grains have no extinction maximum at 11.3 µm, but instead are characterized by an extinction minimum around 10 µm. It is a common interpretation that the 11.3 µm band observed in carbon stars is due to SiC dust. It is also a common interpretation (based on comparison of isotopic ratios in the meteoritic SiC and nucleosynthesis models of AGB stars) that the majority of the presolar SiC grains come from carbon stars. If both of these common interpretations are correct, we conclude from our measurements that the 11.3 µm feature in carbon stars can be understood as being caused by the smaller end of the size distribution of SiC of the type identified in primitive meteorites, and the larger grains must correspond to a grain distribution not yet identified in carbon stars.
In the observational data by Speck et al. (1997) the (interpreted) SiC feature in carbon star spectra peak around 11.3 µm for about 40% of cases (13 out of the sample of 30 stars) with nearly symmetric profiles and a FWHM around 1.8 µm. These features, which are relatively broad just as the feature of the small meteoritic SiC (sample I ), may be interpreted as an indication that the circumstellar SiC is of this small grain size. If the grain size distribution evolves towards larger grains, for example during later stages of the carbon star evolution, it would result in a weakening (and possible disappearance) of the 11.3 µm feature, which could explain the remaining 60% of stellar spectra. However, there are several strong molecular features in the 10-14 µm area in carbon stars (Hron et al. 1998), and a unique interpretation of the observational data still awaits a self-consistent simulation taking both the molecular absorption and the dust emission into account for a wide range of types of carbon stars.
The fact that large ( 5 µm) SiC grains have a different spectral appearance than smaller ( 2 µm) SiC grains, will make large SiC grains difficult to observe in the interstellar medium because of the presence of silicate-related absorption around 10 µm. If the majority of the cosmic SiC grains have the size distribution found by Amari et al. (1994), this will probably not significantly change the abundance limit of less than a few percent SiC compared to silicates in the ISM (Whittet et al. 1990),
The results of spectral measurements on commercially available SiC grain samples of different polytypes and the meteoritic SiC grain samples show that the variations among polytypes of SiC grains are smaller than the variations due to different grain size. It is therefore not possible to distinguish, by IR spectroscopy, between - and -SiC of dusty material as also discussed by Papoular et al. (1998) and Mutschke et al. (1999).
© European Southern Observatory (ESO) 1999
Online publication: March 1, 1999