Prof Patrick Roche

Abstract:

Spectrophotometric observations between 8 and 13 microns are presented for six planetary nebulae: SwSt 1, M1-26, Hb 12, NGC 6790, NGC 6543, and NGC 7027. The first three of these show an emission feature typical of the Trapezium region of Orion and of the circumstellar shells of some oxygen-rich stars. This feature, usually attributed to grains of silicate material, has not been previously observed in planetary nebulae. NGC 6790 shows an emission feature seen in some other planetary nebulae and attributed to silicon carbide grains. Emission lines of Ne II, Ar III, or S IV are seen in five of the nebulae, in qualitative agreement with their excitation class. The currently available data on planetary nebulae in this wavelength region are reviewed, and it is concluded that there are significant compositional differences in the dust content of these objects. However, they may be divided into two classes according to whether they contain oxygen-rich or carbon-rich grain materials.

Abstract:

Spectrophotometric observations between 8 and 13 microns of an ionization-front region in Orion show that a large fraction of the radiation arises from the narrow unidentified emission features at 8.7 and 11.3 microns. This strongly featured continuum emission occurs throughout the length of the infrared bar. It is shown that the emission arises in a neutral region subjected to strong UV irradiation longward of the Lyman limit.