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Astron. Astrophys. 330, 1080-1090 (1998)

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1. Introduction

Until 1987, dust particles around stars had only been recognised by their appearance in stellar spectra, but the possibility of studying unprocessed stellar condensates directly in the laboratory has given important information not only about the Solar System formation, but has also provided precise data for testing astrophysical stellar models.

Presolar diamonds were the first grains to be isolated from meteorites of the carbonaceous chondrite type, which could be shown to have originated from outside the Solar System (Lewis et al. 1987). The identification of their presolar origin has been possible on the basis of isotopic anomalies that appear inconsistent with any known solar system process (Anders & Zinner 1993). Primitive meteorites show the presence of two different components. One component consist of chondrules and other inclusions which have experienced various melting processes during the formation of the solar system. The other component, called the matrix, is fine grained and has experienced little or no heating. It is in the matrix that the presolar grains are found. The presolar diamond content is very similar for all chondrite classes, around 500-1000 ppm of the matrix (Alexander et al. 1990; Huss 1990). Their median grain size is about 2 nm (Fraundorf et al. 1989), which means that each diamond contain only about one thousand carbon atoms. A large fraction of the atoms are therefore on the surface of the crystal, and the meteoritic diamonds therefore consist of a mixture of diamond and hydrogenate amorphous carbon (a-C:H). The amorphous part has been estimated to account for 0.46 the volume fraction of the presolar diamonds (Bernatowicz et al. 1990).

The size distribution of the presolar diamonds is log-normal rather than power-law, reflecting growth rather than fragmentation and suggesting a short interstellar residence time (Lewis et al. 1989). This size distribution is surprising, as interstellar dust normally shows a power-law distribution, which is the steady state form for fragmentation. A log-normal distribution reflects either size-sorting or growth followed by partial conversion of small grains to large ones (Lewis et al. 1989). The good fit for the size distribution obtained by Lewis et al. (1989) down to small sizes suggests minimal contributions from processes that affect small grains preferentially, such as fragmentation, sputtering and erosion. This suggests that the distribution is young and unevolved.

Various mechanisms have been proposed to account for the production of diamond grains in space, but the most likely scenario appears to be that the nano-diamonds have condensed directly from stellar outflows (Lewis et al. 1987; Jorgensen 1988; Clayton 1989; Clayton et al. 1995). The conditions in cool stellar outflows are remarkably similar to those employed in industry to produce diamonds by Chemical Vapour Deposition (CVD) (Angus & Hayman 1988). The CVD mechanism makes use of the fact the the free energy difference between diamond and graphite is only around 1 kcal/mol. Almost any chemical reaction yielding graphite as the thermodynamically stable product can, in principle, yield diamond as a metastable product. The trick is to steer the kinetics so as to favour diamond over graphite. It has been suggested by Krüger et al. (1996), that in circumstellar envelopes the surface growth processes on carbonaceous seed particles will take place at sp3 bonded carbon atoms rather than at sp2 bonded carbon atoms, which suggest that the grain material formed in circumstellar envelopes will be amorphous diamond-like carbon.

For studies of radiative processes in stellar environments, knowledge of opacities of the relevant atoms, molecules and grains are essential. In order for the grains to be taken into account in stellar atmosphere computations, knowledge of the spectral properties of the relevant grains are needed. Transmission spectroscopic measurements of presolar diamonds from the Allende meteorite in the ultra-violet, the optical and the infrared region are presented in this paper. The measurements were designed such that it has been possible to determine the monochromatic absorption coefficient. This coefficient is necessary in order to include the nano-diamonds in model atmosphere calculations and in synthetic spectrum calculations. By use of the derived absorption coefficient, synthetic spectra of carbon stars with the nano-diamonds included have been calculated, under the assumption that the diamonds we have extracted have the same optical properties as they had when they formed in a stellar atmosphere. Finally, we have performed similar laboratory measurements on CVD diamonds, in order to get a more solid basis for understanding the spectral features of the presolar diamonds.

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© European Southern Observatory (ESO) 1998

Online publication: January 27, 1998