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Astron. Astrophys. 328, 419-425 (1997)

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5. Summary

We have presented the spectra of a variety of [FORMULA] samples for comparison with spectra of astronomical sources displaying an emission feature near 21 microns and have demonstrated that the spectrum of [FORMULA] is only marginally consistent with the astronomical observations. We have shown that solid [FORMULA] has strong absorption bands in the 7-13 micron region but that such bands may be masked in astronomical spectra due to uncertainties in the underlying emission spectra of the sources. Specifically, small differences in the temperature of the shell shifts features in the 8- 13 micron region from emission to absorption and may result in complete elimination of the feature for the "proper" combination of shell temperature and stellar surface temperature. Nevertheless, the peak absorbance for the "bulk-mode" [FORMULA] occurs at longer wavelength than is observed in the astronomical sources and so is not a good match to the observations. Similarly, although the peak absorption of "surface-mode" [FORMULA] does match the observed peak in the astronomical sources, the [FORMULA] feature is too narrow to account for all of the observed emission and is always accompanied by a relatively strong companion absorption at 17 microns that has never been observed in astronomical sources. Again this would appear to rule out [FORMULA] as the source of the 21 micron feature in carbon-rich stellar shells.

We have noted that the spectrum of solid sulfur is consistent with several minor features observed in the spectrum of 07134 + 1005. Specifically, sulfur might account for the minor features observed in emission near 21 and 17 microns, and in absorption at 12 microns in this source. Of course it would seem prudent to firmly verify the composition and spectrum of the major dust component around this star before trying to identify minor species.

We have recently seen the spectrum of nanodiamonds (Hill et al. 1996) and were amazed to learn that this material sometimes displays an absorption near 475 [FORMULA] that is not accompanied by any nearby features. Arguments put forward to advance the PAH hypothesis are equally applicable to the formation of nanodiamonds. In fact, there should be no reason to suspect that nanodiamonds can not co-exist with PAHs in some carbon-rich sources. Given the difficulty of fitting the observations using [FORMULA], more detailed spectra of nanodiamonds are needed to adequately consider this alternative.

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© European Southern Observatory (ESO) 1997

Online publication: March 24, 1998

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