Astronomical instrumentation

Abstract:

Parsamian 13 is found to be a cometary nebula, with a deeply embedded star suffering strong absorption by silicate grains. A feature near 2.7 microns, attributed to absorption by terminal OH groups, may represent the first detection in an astrophysical environment of water ice grains diluted by another molecule, for example by CO. The bolometric luminosity suggests that this star is either an extremely young T Tauri star or an evolved low-mass star, now a red giant, high on its convective track. Most unusual is the presence of cold (50 K) TiO gas in sufficient abundance to show in absorption in the optical and very near-infrared regions; this material probably represents very recently expelled photospheric layers.

Abstract:

8-13-micron spectra of eight moderately extended planetary nebulae have been obtained at a resolution of 0.24 micron using a 20-arcsec circular aperture. More compact planetaries which have been studied in this way generally show a strong continuum due to emission from dust, together with fine structure line emission. In contrast, these more extended objects have very weak continuum emission, and are dominated by fine structure line emission, especially by S IV forbidden lines which in several cases accounts for most of the broad-band 10-micron flux. There appears to be very little warm (equal to or greater than 150 K) dust in some of these objects.

Abstract:

The authors present infrared spectrophotometry of 20 symbiotic stars, mostly of the dust-rich variety. HDE330036 is unique in showing an emission feature at 11.3 μm. The remainder combine a hot grey component and/or optically thin silicate emission. A model in which the grey component is due to optically thick silicate dust is not consistent with the spectra. The authors propose instead that iron-based grains, expected to form in the ejecta of cool stars, are heated by the ultraviolet radiation field of the hot companion.

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.