Laboratory experiments on cosmic dust analogues: the structure of small carbon grains

Abstract

In this paper we present new results of our experiments aimed to study the internal structure of cosmic analogue carbon grains. The samples, produced by arc discharge between two carbon electrodes in an argon atmosphere, were annealed in the temperature range 250–780°C in order to produce modification of the internal grain structure. These changes were monitored by analysing the variations of the extinction profile between 190 and 2600 nm and of the optical gap as a function of the annealing temperature. The shift of the UV peak position towards longer wavelengths. the overall increase of the extinction coefficient and the closing of the gap as the temperature increases are all consistent with the evolution of carbon grains outlined by Mennella et al. (Astrophys. J., 444, 288, 1995 ; Astrophys. J. Suppl. Ser., 100, 149, 1995). It provides a growth in number and size of the sp 2 clusters forming the grains during the annealing. The relevance of the electronic structure of the aromatic clusters in the extinction processes and the dependence of the energy π transitions on their size are confirmed by the present results. These results may be relevant in the context of interstellar bump attribution, as they show that the internal structure of small carbon grains is dominant in extinction processes.