5.4. Nano Diamond Grains
Cosmic nanodiamonds were first detected in primitive carbonaceous meteorites and identified as presolar in origin based on their isotopic anomalies (Lewis et al. 1987), although their presence in the ISM was proposed almost two decades earlier by Saslaw & Gaustad (1969) to explain the interstellar UV extinction curve. Five years later, Allamandola et al. (1992) attributed the 3.47 µm absorption band seen toward a large number of protostars to the tertiary C-H stretching mode in diamond-like carbonaceous materials. Jones & d'Hendecourt (2000) further suggested that surface-reconstructed (to sp2-bonded carbon) nanodiamonds could be responsible for the "UIR" emission features, the 2175 Å extinction hump (also see Sandford 1996), and a part of the far-UV extinction at -1 7 µm-1. More recently, circumstellar nanodiamonds were identified in the dust disks or envelopes surrounding two Herbig Ae/Be stars HD 97048 and Elias 1 and one post-asymptotic giant branch (AGB) star HR 4049, based on the 3.43 µm and 3.53 µm C-H stretching emission features expected for surface-hydrogenated nanodiamonds (Guillois, Ledoux, & Reynaud 1999; van Kerckhoven, Tielens, & Waelkens 2002).
Presolar meteoritic nanodiamonds were found to have a log-normal size distribution with a median radius ~ 1.3 nm (Lewis, Anders, & Draine 1989) and an abundance as much as ~ 0.1% of the total mass in some primitive meteorites, more abundant than any other presolar grains by over two orders of magnitude. However, it is important to recognize that this abundance may not be representative of its original ISM proportion but rather its durability (see Draine 2003a). Indeed, Lewis et al. (1989) found that in interstellar space, as much as 10% of the interstellar carbon (~ 36 ppm) could be in the form of nanodiamonds without violating the constraints placed by the interstellar extinction curve. However, a much more stringent upper limit of ~ 0.1 ppm was derived by Tielens et al. (2000) based on nondetection of the characteristic 3.43 µm and 3.53 µm C-H stretching emission features in the ISM. Of course, interstellar nanodiamonds could be more abundant if the bulk of them are not hydrogenated.