Discovery and Analysis of 21 micrometer Feature Sources in the Magellanic Clouds (Postprint)

摘要

Spitzer Space Telescope mid-infrared spectroscopy has been obtained for 15 carbon-rich protoplanetary nebulae (PPNe) in the Large Magellanic Cloud (LMC) and for two other such stars in the Small Magellanic Cloud (SMC). Of these 17 PPNe, the unidentified 21 micrometer feature is strong in 7 spectra, weak in 2 spectra, and very weak or questionable in 4 spectra. Two of the four spectra without the 21 micrometer feature have a very strong feature near 11 micrometer, similar to a feature observed in some carbon-rich planetary nebulae (PNe) in the LMC. We attribute this feature to unusual SiC dust, although the feature-to-continuum ratio is much larger than for SiC features in Galactic or Magellanic Cloud carbon star spectra. The remaining two objects show typical carbon-rich PPNe spectra with no 21 micrometer features. One of the LMC objects that lacks the 21 micrometer feature and one SMC object with a questionable 21 micrometer detection may have mixed dust chemistries based upon their spectral similarity to Galactic (WC) PNe. The 13 objects that either definitely or may show the 21 micrometer feature have distinct dust shell properties compared to the Galactic 21 micrometer objects--the 21 micrometer features are weaker, the estimated dust temperatures are significantly higher, the unidentified infrared (UIR) bands are stronger, and the UIRs show more structure. Four of the 21 micrometer objects appear to show normal SiC emission features in their spectra. Many of the PPNe show strong 30 micrometer features, although this feature carries less of the total mid-infrared emission than is normally the case for the Galactic 21 micrometer PPNe. The LMC objects are in the LMC halo rather than in the LMC bar. The estimated luminosities of these PPNe vary from 4700 to 12,500L.