Evolution of Carbon Dust in Aromatic Infrared Emission Sources: Formation of Nanodiamonds

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Abstract
Particles of crystalline diamond showing extremely narrow (4-6 cm-1) IR absorption features attributable to tertiary CH groups have been prepared in the laboratory by deposition of hydrogenated amorphous carbon (HAC) in the presence of oxygen at 300 K. These tertiary CH features are also observed in emission in type C aromatic infrared (AIR) objects, typified by the post-asymptotic giant branch star HR 4049. We have investigated the connection between the chemistry of HAC and the formation of nanodiamond material and conclude that this may occur via dehydrogenation of nonaromatic precursor molecules in this material. To trace the role of these precursors, we have obtained high-temperature (1200 K) infrared emission spectra of HAC in the region between 3.0 and 3.7 μm (3200 and 2700 cm-1) and find a broad continuum similar to the plateau emission observed in many type A AIR sources, typified by NGC 7027. This continuum is produced primarily from the overlap of two distinct features: a band centered at 3.24 μm (3090 cm-1) assigned to C=CH2 groups in nonaromatic molecules and a broad band at 3.46 μm (2890 cm-1) attributed to the stretching vibration in tertiary CH. Sharp (15-30 cm-1) features associated with CH2 are also observed at 3.425 and 3.51 μm (2920 and 2850 cm-1). We find that the tertiary CH diamond bands are likely formed by dissociation of these groups. It is concluded that hydrogenation of polycyclic aromatic hydrocarbon followed by exposure to ultraviolet radiation in the presence of oxygen is a chemical route to the formation of nanodiamond material in interstellar clouds. We suggest that carbonaceous material in type B AIR sources such as IRAS 05341+0852 can evolve into either nanodiamond dust or aromatic-rich components depending on the time spent under hydrogenating conditions. We also find that the chemical components responsible for plateau emission in AIR sources are present in 3 μm spectra of young stellar objects with ice absorption bands.