Nano-diamonds in proto-planetary discs

Abstract

Context.Nano-diamonds remain an intriguing component of the dust in the few sources where they have been observed in emission.Aims.This work focusses on the nano-diamonds observed in circumstellar discs and is an attempt to derive critical information about their possible sizes, compositions, and evolution using a recently derived set of optical constants.Methods.The complex indices of refraction of nano-diamonds and their optical properties (the efficiency factorsQext,Qsca,Qabs, andQpr) were used to determine their temperatures, lifetimes, and drift velocities as a function of their radii (0.5–100 nm), composition (surface hydrogenation and irradiated states), and distance from the central stars in circumstellar regions.Results.The nano-diamond temperature profiles were determined for the stars HR 4049, Elias 1, and HD 97048 in the optically thin limit. The results indicate that large nano-diamonds (a= 30–100 nm) are the hottest and therefore the least resistant in the inner disc regions (~10–50 AU), while small (a< 10 nm) fully hydrogenated nano-diamonds remain significantly cooler in these same regions. We discuss these results within the context of nano-diamond formation in circumstellar discs.Conclusions.Large nano-diamonds, being the hottest, are most affected by the stellar radiation field; however, the effects of radiation pressure appear to be insufficient to move them out of harm’s way. The nano-diamonds that best survive and therefore shine in the inner regions of proto-planetary discs are then seemingly small (a< 10 nm), hydrogenated, and close in size to pre-solar nano-diamonds (〈a〉 ≃ 1.4 nm). Nevertheless, it does not yet appear possible to reconcile their existence with their seemingly short lifetimes in such regions.