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Astron. Astrophys. 329, 1087-1096 (1998)


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Identification of carbon forms in soot materials
of astrophysical interest

A. Rotundi 1, F.J.M. Rietmeijer 2, L. Colangeli 3, V. Mennella 3, P. Palumbo 1 and E. Bussoletti 1

1 Istituto di Fisica Sperimentale, Istituto Universitario Navale, via A. De Gasperi 5, I-80133 Napoli, Italy
2 Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131, USA
3 Osservatorio Astronomico di Capodimonte, via Moiariello 16, I-80131 Napoli, Italy

Received 24 February 1997 / Accepted 19 August 1997

Abstract

We determined the carbon structures in condensed soot samples of variable C/H ratio by using both scanning and transmission electron microscopy. We identified several types of carbon structures. All the samples are formed mainly by "chain-like aggregates" of amorphous grains. A small amount of more ordered forms of carbon is also detected. A "poorly graphitized carbon" is formed on a pre-existing amorphous carbon substrate due to auto-annealing. "Bucky-carbons" are "proper" products of condensation. Occasionally, rare graphitic carbon ribbons and single-crystal carbon platelets are observed. The "chain-like aggregates" are the only form sensitive to hydrogen content variations and, probably, the main form responsible for the UV spectral response. In agreement with previous analyses on similar soot samples, we conclude that the internal structure of the amorphous grains drives the overall spectroscopic properties of hydrogenated soots. The relations between structural and spectral trends, common to several kinds of soot samples, suggest that carbon ordered on a micro-scale only, rather than graphitic carbons, is more appropriate to interpret the interstellar UV extinction bump position. The carbon forms, "proper" products of soot condensation processes, are similar to those detected in chondritic interplanetary dust particles. This could imply that they are of primary origin, rather than derived from parent body processing.

Key words: stars: carbon – stars: circumstellar matter – dust, extinction – methods: laboratory

Send offprint requests to: A. Rotundi

© European Southern Observatory (ESO) 1998

Online publication: December 16, 1997
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