SPECTRAL FEATURES OF PRESOLAR DIAMONDS IN THE LABORATORY AND IN CARBON STAR ATMOSPHERES

Laboratory analyses on fine-grained diamond residues from primitive me teorites have shown that nano-diamonds represent the most abundant for m of presolar dust preserved in meteoritic samples. The presolar diamo nds carry isotopic anomalies which indicate a very complex formation h istory. Several groups of diamonds may exist with origin in different types of stars. In order to identify the sites of formation observatio nally, we have extracted presolar diamonds from the Allende meteorite and measured the monochromatic absorption coefficient in a form which is useful for stellar atmosphere calculations. The monochromatic absor ption coefficient was measured in the wavelength ranges 400-4000 cm(-1 ) (2.5-25 mu m) and 12200-52600 cm(-1) (190-820 nm). We have made iden tical laboratory measurements on CVD diamonds as on the meteoritic dia monds, in order to get a more solid basis for the interpretation of th e diamond spectrum. The monochromatic absorption coefficient for the p resolar diamonds was incorporated in self-consistent carbon star photo spheric models. The main influence of the diamond dust in our photosph eric models is a heating of the upper photospheric layers and a reduct ion of the C2H2 abundance. Due to the relatively small absorption coef ficient of the diamonds compared to other stellar dust grains, their s pectral appearance is weak. However, the weak interaction of the diamo nds with the radiation field may give them an important role in the du st nucleation process. The gas pressure will stay high and the gas wil l be much closer to hydrostatic equilibrium during possible diamond nu cleation than is normally the case in dust forming stellar regions, an d therefore allow ample time for the nucleation process.