Dust grains and circumstellar motion: The problem of the hen and the egg

In the last ten years, the number of models of circumstellar envelopes has exponentially increased, both concerning evolved stars and forming stars wi th their planetary systems. The main progress was intoduction of chemistry and coupling with dynamics. However, paradoxically, the nucleation theories did not evolve at the same rate; moreover, dynamics and nucleation are rar ely analysed at the same degree of accuracy: one is of them is always mostl y drawing attention. At the same time observations showed the presence of d ust grains at a lot of sites where it could not be expected from usual quas i equilibrium theories. Self consistent systems are omnipresent. Thus, it i s important to distinguish causes and effects, even if the phenomena are ar e strongly coupled.