Abstract
Context. Nano-diamonds have been observed in only a handful of circumstellar regions 10–100 au from moderately bright stars (Teff ~ 8000–10 000 K). They have also been extracted from primitive meteorites; some of these are clearly pre-solar, that is to say that they formed far from the solar system and therefore traversed the interstellar medium, where they must exist but, because we see no evidence of them, must be extremely well hidden. Aims. Our goal is to understand if it is possible to constrain the sizes and shapes of nano-diamonds in circumstellar media using the observed ratio, [CH]/[CH2], of their surface CH2 and CH infrared bands at ≃3.43 μm and ≃3.53 μm, respectively. Methods. We calculated the CH and CH2 abundances on nano-diamonds using two approaches. The first assumes regular and semi-regular polyhedra (tetrahedra, octahedra, and cubes and their truncated forms). The second uses a diamond bonding network to derive the structures of tetrahedral and octahedral particles, and their truncated variants, and also of spherical nano-diamonds. Results. As a function of the particle size and shape, and for the two different calculation methods, we derived the relative abundance ratio [CH]/[CH2], which can then be weighted by their laboratory-measured infrared band intensities. The two methods give good agreement and indicate that the spread in values, over the different particle forms, is more that an order of magnitude for any size. Conclusions. We conclude that the ratio [CH]/[CH2], and their infrared band ratio, strongly depend upon particle size and shape. For a given shape or size, the ratio can vary by more than an order of magnitude. It may therefore be difficult to constrain nano-diamond sizes using the observed 3–4 μm spectra alone. James Webb Space Telescope mid-infrared spectra may help, but only if bands are size-specific.
Highlights
Nano-diamonds with median radii of 1.3–1.5 nm (Daulton et al 1996) have been extracted from meteorites in abundances of up to 1400 ppm (Huss & Lewis 1995)
We developed several different approaches to the calculation of the CH and CH2 group abundances on nano-diamonds: regular and semi-regular polyhedral shapes, and diamond bonding
20 Nano-diamonds in the interstellar medium (ISM) would have to express their presence in multiple, but indirect, ways in order to most efficiently use the rather limited supply of carbon available (e.g. Jones & d’Hendecourt 2000), i.e. they would have to contribute to the FUV extinction, the 3–15 μm IR emission bands, the mid-IR emission
Summary
Nano-diamonds with median radii of 1.3–1.5 nm (Daulton et al 1996) have been extracted from meteorites in abundances of up to 1400 ppm (Huss & Lewis 1995). Diamond-like materials and nanodiamonds have long been synthesised and studied and there exists a vast literature on this subject. Diamond-like materials and nanodiamonds have long been synthesised and studied and there exists a vast literature on this subject1 In most of these laboratory studies, we have the luxury of being able to directly analyse the textures, particle sizes, and shapes through a myriad of sophisticated techniques (e.g. scanning and tunnelling electron microscopy and atomic force microscopy). In astronomical observations this option is not available to us and so we must progress using solid-state material models. Our understanding is critically constrained by the limitations of our knowledge and our models
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