Prof Patrick Roche

Abstract:

An 8-13 micron spectrum of the planetary nebula IC 2165 is presented. In addition to a bright S IV forbidden emission line, weak emission from the A III and Cl IV fine structure forbidden lines and the H7-6 hydrogen recombination line is identified. These lines are used to derive ionic abundances which are in agreement with those expected from the known excitation of the nebula. The rather weak continuum is attributed to free-free emission near 8 microns, but with increasing contributions from dust emission with increasing wavelength. From the presence of emission from silicon carbide dust, it is inferred that the emitting dust grains were formed in a carbon-rich environment, in accord with abundance determinations from optical and ultraviolet line emission.

Abstract:

High spatial resolution spectral observations of the planetary nebula NGC 7027 in the 10-micron region are reported. These show that the unidentified infrared features at 8.65 and 11.25 microns originate from a more extensive region than the shell of ionized gas; the data are consistent with these features arising entirely from the contiguous neutral region. This situation places further constraints upon a fluorescent excitation mechanism (Allamandola, Greenberg and Norman, 1979) and it is argued that the features are due to thermal emission from surface bonds on graphite grains in an H I region (Barlow, 1982, in preparation; Dwek et al., 1980).

Abstract:

Infrared spectral observations of the Wolf-Rayet system Gamma Velorum in the atmospheric windows between 1.45 and 12 microns are presented. Results show an extremely rich emission line spectrum in which permitted lines of helium and carbon and forbidden lines of neon and sulfur are present, some of which have not previously been observed in any object. The relative abundances of He I/He II and C/He are estimated, and lower limits to the Ne/He and S/He abundance ratios are presented.

Abstract:

8-13 micron spectra are presented of 24 planetary nebulae and other emission line objects, 19 of which are published here for the first time. This brings the total of planetary nebulae for which such spectra are available to 23 including four classed as very low excitation (VLE) objects and three as WC-11 objects. On the basis of these spectra, roughly one-third of the planetaries are identified as having oxygen-rich (silicate) and rather less than one-third carbon-rich (silicon carbide) dust grains. Of the oxygen-rich nebulae, three are VLEs while the rest are very compact and, on various grounds, considered to be young. The remainder of the planetary nebulae exhibit strongly the unidentified feature at 11.25 microns and frequently that at 8.65 microns and their dust chemistry is uncertain. All of the 23 planetaries exhibit one or more of the fine structure forbidden lines of Ar III, S IV and Ne II roughly in accordance with their optical excitation class. The line intensities are consistent with approximately solar abundance of these elements.