W. W. Duley

The interaction of the spiral density wave and the Sun's galactic orbit

The galactocentric radial motion of the Sun introduces another periodicity to the encounters between the Sun and the spiral density wave. We describe a model simulation of the effect and present the associated periods.

Fluctuations in interstellar grain temperatures

An analysis has been made of temperature fluctuations expected for various grain materials exposed to the interstellar radiation field. It is shown that in all but the densest clouds average grain temperatures in the 5–10 K range are of little statistical significance because of large fluctuations produced by absorbed photons from the interstellar radiation field. At higher average temperatures large fluctuations may still be present for certain grain materials and grain radii. The effect of these fluctuations on the simple problem of H atom recombination on grain surfaces is discussed.

A new model for interstellar dust

Some physical, chemical and optical properties of mixed oxide grain material are discussed and the formation of such dust is examined. Surface chemistry on these grains may be responsible for the selective depletion of elements and for the formation of molecules such as H2CO in diffuse clouds. Spectra of mixed oxide grains yield features at 9.6 and 18 μm together with a broad band at 4.6 μm−1.

Laboratory spectra of protosilicates and the interstellar silicate absorption bands

Infrared spectra of submicrometre-sized particles of several protosilicates have been used to derive mass absorption coefficients. These data have been applied in order to describe the infrared radiation from the source OH 26.5+0.6 in terms of a simple model. A good fit of the 10 μm absorption feature could be achieved, but some discrepancies remain in the 20 μm region. The reasons for the weakness of the 20 μm band in infrared sources are discussed.The laboratory spectra revealed that the samples contained a small admixture of carbonates. Thus, some statements about the infrared spectrum of submicrometre-sized carbonate particles could also be made.

Composition of grain mantles in interstellar clouds

The question of grain mantle composition in dense clouds is examined in the light of new observational and theoretical results on atomic and molecular concentrations in the gas phase. It is shown that if grain temperatures are less than about 20K these mantles will be primarily solid CO. Methods of identifying CO coated grains are discussed.

Sharp emission lines from diamond dust in the Red Rectangle

A surprising similarity exists between sharp emission features observed in luminescence spectra of terrestrial diamonds and those reported in the spectrum of the Red Rectangle. It is argued that diamond or diamond-like inclusions may be important components of the dust in this object. Laboratory data shows that diamond formation is facilitated by the presence of oxygen.

Identification of some diffuse interstellar features

Twenty-eight of the thirty-nine diffuse interstellar bands identified by Herbig (1975) are shown to constitute three vibronic systems with origins at 14 321, 15 153, and 15 343 cm−1 (vac). Structure within these three systems arises from the excitation of vibrational modes withv1=275 cm−1,v2=445.5 cm−1, andv3=793 cm−1. The electronic origins at 14 321 and 15 343 cm−1 correspond to narrow lines observed in the spectrum of Cr3+ ions at cubic sites in MgO solids while the 15 153 cm−1 origin arises in Mn4+ : MgO. Hence, many of the diffuse bands in the visible likely are due to small MgO particles containing these ions. This observation is compatible with recent experimental data showing broad bands at 160 nm and 220 nm from finely divided MgO solids that match features in the interstellar extinction curve.

Elemental depletions and the 217.5 nm interstellar feature

The strength of the 217.5 nm interstellar feature per unit volume of dust is strongly correlated with the presence of silicon and magnesium in dust but not with the presence of carbon.

Silicon monoxide and the 10 μm interstellar feature

Laboratory spectra of SiO particles of 1 μm radius show a broad structureless extinction peak at 9.6μm. The wavelength dependence of extinction from SiO, an amorphous silicon oxide, provides a good match to that of interstellar dust.

Vibronic origin for the diffuse band spectrum

Theorie de Duley sur les bandes interstellaires diffuses observees entre 677 nm et 542 nm et reponse aux arguments contraires donnes par Nuth et Donn