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Grains in Diffuse Clouds: Carbon-Coated Silicate Cores

Published online by Cambridge University Press:  23 September 2016

David A. Williams
David A. Williams*
Affiliation:
Department of Mathematics UMIST Manchester M60 1QD United Kingdom

Abstract

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A new model of interstellar grains is proposed in which grains in diffuse clouds consist of small (radius ≲ 10nm) and large asymmetric (0.05μm – 0.25μm) silicate cores thinly coated with mantles of amorphous carbon (thickness ≲ 5nm). This model can account successfully for many of the observed properties of interstellar dust and gives a simple interpretation of the interstellar extinction curve. The extinction bump at 220nm is attributed to absorption by finely divided silicates, as indicated by laboratory data. The large silicates provide a “grey” background extinction through the visual and UV, but the bulk of the extinction in this region is attributed, on the basis of laboratory data, to the amorphous carbon coatings. The average interstellar carbon depletion required in this model is about 50%.

Wide variations in the observed interstellar extinction along different lines of sight are well known. These variations have a natural explanation in this model in terms of two parameters: the thickness of the carbon coatings, and the proportion of graphitic to diamond-like structure within the amorphous carbon. The underlying silicate cores are generally unchanged in these variations. The average interstellar extinction curve requires roughly equal proportions of graphitic and diamond-like forms of amorphous carbon. A higher graphitic fraction produces more visual extinction and a higher diamond-like fraction produces more far UV extinction. Varying both the proportions, and the total carbon content encompasses a wide range of extinction curve shapes.

Amorphous carbon deposited at low temperature is generally diamond-like. Temperature excursions in the material reduce the hydrogen content and enhance the graphitic nature of the material. In the interstellar medium, this process has a time scale ≳ 106 yr. Carbon coatings are therefore expected to be deposited in the interstellar medium in diamond-like form and to be slowly converted to graphitelike. The growth of carbon mantles will be reversed by intermittent shocks Thus, interstellar space should contain grains with a range of thicknesses of carbon coatings, and compositions between graphitic and diamond-like.

The chemical and physical properties of amorphous carbon have been the subject of intensive laboratory study. These properties enable an understanding of a variety of observations of dust (especially in the IR) and lead to a number of predictions which are described in this paper.

Type
Section VI: Interstellar Dust Models
Information
Symposium - International Astronomical Union , Volume 135: Interstellar Dust , 1989 , pp. 367 - 373
Copyright
Copyright © Kluwer 1989 

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