Aerodynamic properties of fractal grains: Implications for the primordial solar nebula

Under conditions in the primordial solar nebula and dense interstellar clouds, small grains have low relative velocities. This is the condition for efficient sticking and formation of fractal aggregates. A calculation of the ratio of cross section, sigma, to number of primary particles, N, for fractal clusters yielded 1n sigma/N = 0.2635 + 0.5189N sup (-0.1748). This ratio decreases slowly with N and approaches a constant for large N. Under the usual assumption of collisions producing spherical compact, uniform density aggregates, sigma/N varies as N sup -1/3 and decreases rapidly. Fractal grains are therefore much more closely coupled to the gas than are compact aggregates. This has a significant effect on the aerodynamic behavior of aggregates and consequently on their evolution and that of the nebula

Laboratory studies of refractory metal oxide smokes

Studies of the properties of refractory metal oxide smokes condensed from a gas containing various combinations of SiH4, Fe(CO)5, Al(CH3)3, TiCl4, O2 and N2O in a hydrogen carrier stream at 500 K greater than T greater than 1500 K were performed. Ultraviolet, visible and infrared spectra of pure, amorphous SiO(x), FeO(x), AlO(x) and TiO(x) smokes are discussed, as well as the spectra of various co-condensed amorphous oxides, such as FE(x)SiO(y) or Fe(x)AlO(y). Preliminary studies of the changes induced in the infrared spectra of iron-containing oxide smokes by vacuum thermal annealing suggest that such materials become increasingly opaque in the near infrared with increased processing: hydration may have the opposite effect. More work on the processing of these materials is required to confirm such a trend: this work is currently in progress. Preliminary studies of the ultraviolet spectra of amorphous Si2O3 and MgSiO(x) smokes revealed no interesting features in the region from 200 to 300 nm. Studies of the ultraviolet spectra of both amorphous, hydrated and annealed SiO(x), TiO(x), AlO(x) and FeO(x) smokes are currently in progress. Finally, data on the oxygen isotopic composition of the smokes produced in the experiments are presented, which indicate that the oxygen becomes isotopically fractionated during grain condensation. Oxygen in the grains is as much as 3 percent per amu lighter than the oxygen in the original gas stream. The authors are currently conducting experiments to understand the mechanism by which fractionation occurs

On Ion Clusters in the Interstellar Gas

In a recent paper V.I. Krassovsky (1958) predicts the occurrence of clusters of large numbers of atoms and molecules around ions in the interstellar gas. He then proposes a number of physicochemical processes that would be considerably enhanced by the high particle density in such clusters. In particular, he suggests that absorption by negative ions formed in the clusters would account for the interstellar extinction without any necessity for the presence of grains. Because of the important consequences that ion clusters could have, it is necessary to examine their occurrence more fully. This note re-examines the formation of ion clusters in space and shows that even ion-molecule pairs are essentially non-existent. Ion clusters have been considered by Bloom and Margenau (1952) from the same point of view as that used by Krassovsky, whose basic reference (Joffe and Semenov 1933) unfortunately is not available. A different approach has been used by Eyring, Hirschfelder, and Taylor (1936) following the methods of chemical equilibrium. Both the references cited here enable one to conclude that clustering is negligible. Therefore, the treatment of Eyring et al. is more appropriate than the method of Bloom and Margenau, which depends on the statistical equilibrium of an atmosphere in a force field

On the question of interstellar diamonds

The evidence to support the recent proposal that interstellar extinction may be caused by small diamond crystals is open to substantial criticism. Wickramasinghe has shown that the optical properties of diamond do not readily provide an explanation of the observed interstellar curve. Similar calculations made by us support and extend Wickramasinghe's results. It may be possible to modify the form of the size distribution until the interstellar extinction curve is reproduced as Greenberg and Shah have done for "dirty ice" grains. For diamond, however, considerable variation in the slope occurs in the infrared and it is not obvious that it will be possible simultaneously to fit the infrared, visible and ultraviolet regions

An analysis of the infrared continuum of comets

Observations of infrared cometary continua which depend on thermal emission from grains are a powerful supplement to deductions concerning the nature of grains from light scattering in the visible. For grains of dirty ice, iron, quartz, and graphite, the absorption cross-section, angular scattering cross-sections, and thermal emission have been calculated. From these the grain temperature as a function of heliocentric distance has been obtained. This determines a theoretical cometary flux distribution which may then be compared with observed data. The analysis is applied to the infrared observations of Comet Ikeya-Seki 1965f of Becklin and Westphal. Reliable evaluations of grain properties do not appear possible from existing measurements

INFRARED AND RAMAN SPECTRA OF MATRIX ISOLATED (Pbo)n(Pbo)_{n} SPECIES

Author Institution: Department of Chemistry, University of Maryland; Goddard Space Flight Center, Greenbelt, Maryland 20771The infrared and Raman spectra of PbO vapor co-condensed with excess $N_{2}$ and Ar on a cold substrate ( $\sim$ 12K) were investigated. The spectral changes accompanying the heat treatment (controlled warming for a specified period) of the sample were utilized to decipher the vibrational frequencies of monomeric and polymeric specied. Normal coordinated treatment of the data enabled us to clucidate the structures of dimeric trimeric and polymeric species. Normal coordinate treatment of the data enabled us to species. A comparison with the results of SiO suggests different mechanisms of condensation in the two cases which will be discussed in this report

Atlas of Comet Halley 1910 II

With the impending return of Halley's Comet in 1986, a major effort began to collect the material obtained at its last appearance in 1910. This material displays the evolving coma and tail phenomena, and is useful for comparison with the present quantitative studies of spectroscopic and structural phenomena. Images in the atlas are arranged in chronological order by day. Days that have multiple images with varying scale are arranged in two sequences. Photographs showing tail phenomena are first, followed by photographs obtained with longer focus instruments showing the head or near-nuclear region. Drawings of Comet Halley, made from visual observations in 1835 and 1910, also are included