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K.-W. Chan, T. Onaka (Dept. of Astronomy, University of Tokyo), T.L. Roellig (NASA Ames Research Center)
A broad 22 micron emission feature has recently been observed in H II regions and starburst galaxies (Chan and Onaka 2000, ApJ, 533, L33). The feature shape is similar to that of the 22 micron emission feature of newly synthesized dust observed in the Cas A supernova remnant (Arendt et al., 1999, ApJ, 521, 234). This finding suggests that both of the features are arising from the same carrier and that supernovae are probably the dominant production sources of this new interstellar grain. Identification of the carrier of this new grain is not definite and several candidates (Mg protosilicate, Fe protosilicate, and FeO) can each provide a good spectral fit to the feature. However, from the recent ISOCAM observations we found that the observed 22 micron emission feature in Cas A is very likely contaminated by a warm silicate component, which is present all over the SNR. After subtraction of this warm silicate component from the observed spectrum of the 22 micron emission feature we found that the resultant feature is better fitted by Ca protosilicate. Furthermore, Ca protosilicate gives a better fit to the observed 9.5 micron feature in Cas A than the other candidates. Based on these results, we propose that Ca protosilicate is the carrier of the 22 micron emission feature.