  We use the kinetic theory of nucleation to explore the properties of dust
nucleation in sub-saturated vapors. Due to radiation losses, the sub-critical
clusters have a smaller temperature compared to their vapor. This alters the
dynamical balance between attachment and detachment of monomers, allowing for
stable nucleation of grains in vapors that are sub-saturated for their
temperature. We find this effect particularly important at low densities and in
the absence of a strong background radiation field. We find new conditions for
stable nucleation in the n-T phase diagram. The nucleation in the non-LTE
regions is likely to be at much slower rate than in the super-saturated vapors.
We evaluate the nucleation rate, warning the reader that it does depend on
poorly substantiated properties of the macro-molecules assumed in the
computation. On the other hand, the conditions for nucleation depend only on
the properties of the large stable grains and are more robust. We finally point
out that this mechanism may be relevant in the early universe as an initial
dust pollution mechanism, since once the interstellar medium is polluted with
dust, mantle growth is likely to be dominant over non-LTE nucleation in the
diffuse medium.
