  Nano-diamonds remain an intriguing component of the dust in the few sources
where they have been observed in emission. 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. The complex indices of
refraction of nano-diamonds and their optical properties (the efficiency
factors Qext, Qsca, Qabs, and Qpr) 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. 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. 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
there with their seemingly short lifetimes in such regions.
