6. Discussion and conclusion
The obtained spectra of the presolar diamonds show many features which are mostly dominated by O-H, C-H and C-N features. The O-H features does probably not originate from when the nano-diamonds were formed in a carbon/hydrogen rich environment, but is rather an artifact originating from the very rough chemical treatment that is used to extract the diamonds, but the C-H, C-N and N-H (if the nitrogen inclusions are situated at the surface) bonds could very well be responsible for the features we should expect to detect if diamonds are present in a stellar environment.
This mean that the UV/VIS features around 217 and 270 nm (paired N in diamond), the feature around 3200 cm-1 (N-H), the features around 2900 cm-1 (C-H) and 1100 cm-1 (C-N/C-C/interstitial N) and the features around 350 cm-1 (C-N-C) and 125 cm-1 (unidentified) are all promising features when trying to establish the presence of nano-diamonds.
Knowledge of the monochromatic absorption coefficient is necessary in order to include grains in model atmosphere calculations and in synthetic spectrum calculations, which are needed for comparison with high-resolution observed spectra. In the models presented here the diamond spectrum is weak compared to the molecular spectral features, indicating that an observational identification of diamonds not will be straight forward. The effect on the atmospheric structure is however substantial, and nano-diamonds could play an important role in the grain nucleation process.
We have presented here the computed spectrum as it would look like if the diamonds form in carbon stars (under the simplified assumptions described above). Our measured monochromatic absorption coefficient (which is available from the authors) can likewise be used by other authors, doing models of other objects, for predicting the spectrum from such objects.
Beside carbon stars, the other possible candidates for the diamond formation includes novae (Clayton et al. 1995), carbon rich Wolf-Rayet stars (Tielens 1990; Arnould et al. 1993), and young expanding supernova remnants with dust originating from the deeper layers of the star (Clayton 1989; Clayton et al. 1995).
Until now, the observational searches for presolar diamond grains have focused on the interstellar medium. One of the most encouraging results is the present identification by Allamandola et al. (1992) of an absorption band in four dense molecular clouds (proto stars) at 3.47 µm (2882 cm-1), which they attributed to a tertiary C-H stretching mode, and tentatively interpreted as due to hydrogenate nano-diamonds. We notice that this absorption band is in the exact right region to be a mix of the two absorption features we observe at 2927 cm-1 and 2856 cm-1, present in our infrared spectrum of the presolar diamonds. Other searches for diamonds in interstellar space have, however, mainly reached negative results (Sandford et al. 1991 and references therein), and for this there might be at least three reasons; one is that the presence of functional groups attached to the surface of the grains will alter the absorption features. The second reason is that the nano-diamonds might be included in other dust grains, if they work as the nucleation seed, the third reason is that the stellar grains are likely to be covered with ices while in interstellar space. The possible interstellar diamond spectrum is therefore expected to be quite different (or absent if the grain mantle or ice cover is sufficiently thick) from the obtained laboratory spectrum.
Diamonds is the major known presolar component in meteorites, but their origin is still a puzzle. We have presented here data, and preliminary analysis based on these data, which might make it possible to identify their place of origin. An observational identification of the stellar source of the presolar grains would lead to improved understanding of the upper layers of stellar atmospheres, grain formation, the mass loss process, and of the detailed chemical evolution of our Galaxy.
© European Southern Observatory (ESO) 1998
Online publication: January 27, 1998