Dust grains in astrophysics

Eleven years ago, it was fairly clear that dust grains were a state of solid state matter involved in a cycle producing and reprocessing the solid bodies in the Universe mainly from evolved stars, and that this cycle progressively enriched the galactic media with heavier and heavier "metallic" components. The main reecent progresses in the field was that chemistry and dynamics are strongly or weakly coupled depending on the surrounding gas density. First approaches of models including this coupling were built, though a lot of problems already known by people investigating separately dynamics and chemistry appeared still more complicated due to the variety of scales to be considered in self consistent models. However, models matched more or less well up to observations. Now due to the exponentially increasing power of computers and the refinement of observations, a lot of new approaches of grain nucleation and evolution processes on the one side and observations on the other side, and also the subsequent derivations of optical properties at many wavelengths suggest that ways of coupling dynamics to chemistry may be still more complex than expected, all the more since grains have been detected in media very far from any thermo-dynamical equilibrium, in which radiation is a phenomenon also strongly involved. The conclusion is that there is no available general solution and that dominant processes and most important scales must be, at the present time, carefully determined before any computation in order to built reasonable models in each case. If the production of dust by evolved stars is not questioned, other sources may be efficient. New species of "hens" and "eggs" should be taken under consideration.