2010MNRAS.401.2722R

The study of the scattering and extinction properties of possible nanodiamond grains in the interstellar medium (ISM) are reported. Calculations using the discrete dipole approximation (DDA) for varying ellipsoidal shapes and sizes from 2.5 to 10nm are considered. Nanodiamonds show negligible extinction from the infrared to the near-ultraviolet, and a very sharp far-ultraviolet rise. Comparison with observations rules out the possibility of independent nanodiamond dust but points towards the possibility of nanodiamonds as a component in the ISM. Radiation-induced transformations may lead to carbonaceous grains with different cores and mantles. Calculations are therefore also performed for a core-mantle target model with nanodiamond cores in graphite mantles. The graphite extinction features become modified, with the peak at 2175Å being lowered, broadened, blueshifted and accompanied by enhanced extinction in the far-ultraviolet. Such variations in the 2175-Å band and a simultaneous far-ultraviolet rise are observed along some sources. A three-component dust model incorporating silicate, graphite and graphite with a nanodiamond core is also considered. The model extinction compares very well with the average Galactic extinction in the complete range from 0.2 to 10µ/m. The best fit requires a small size and a small number of nanodiamonds.