Non-LTE dust nucleation in sub-saturated vapors

TL;DR

This paper investigates dust nucleation in sub-saturated vapors using kinetic theory, revealing conditions under which stable grain formation occurs despite sub-saturation, with implications for early universe dust production.

Contribution

It introduces a new understanding of non-LTE dust nucleation in sub-saturated vapors, highlighting the role of radiation losses and providing conditions for stable nucleation in the n-T phase diagram.

Findings

Stable nucleation can occur in sub-saturated vapors due to temperature differences caused by radiation losses.

Nucleation rates are slower in non-LTE regions compared to super-saturated vapors.

Conditions for nucleation depend mainly on properties of large stable grains, not poorly known molecular properties.

Abstract

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.