Bertram Donn

Extinction by interstellar grains, Mie particles and polycyclic aromatic molecules

Interstellar extinction models of classical Mie particles and quantum mechanical polycyclic aromatic molecules

On the question of interstellar silicate absorption

The observations of the absorption feature in the infrared spectrum by Knacke, Gaustad, Gillett, and Stein are discussed; and doubts concerning their identifying silicate minerals in interstellar grains are raised.

Stability of Solid Hydrogen Coated Graphite Particles

RECENTLY, Wickramasinghe and K. S. K. S.1 have shown that the extinction curves for graphite particles coated with solid hydrogen reproduce well the observed reddening curve over the observed spectral region. The albedo and phase function for the particles were also shown to be consistent with observations. It was suggested in a qualitative manner that the solid hydrogen mantles could be stable in an interstellar radiation field due to the presence of optically active transitions at frequencies ∼ 10 cm−1. It is quite possible to get such transitions due to the presence of ortho molecules of hydrogen in the mantle. In this paper we make some detailed calculations on the temperature of grains coated with solid hydrogen and their evaporation rates in interstellar space. It turns out that the temperature of the grains is considerably higher than the temperature required to maintain them.

On the question of interstellar diamonds

THE evidence to support the recent proposal1 that interstellar extinction may be caused by small diamond crystals is open to substantial criticism. Wickramasinghe2 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 Wickramasinghes results. It may be possible to modify the form of the size distribution until the interstellar extinction curve is reproduced as Greenberg and Shah3 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.