Physical conditions of protosolar matter

I review a few astronomical constraints concerning physical conditions in and evolutionary time–scales of protoplanetary discs. Some revisions are suggested to the scenario by which short–lived radioactive species would enter the protosolar cloud. The increasing evidence for substantial grain growth in discs at ages of 1 Myr is also outlined. Protoplanetary discs are accretion discs; when (inner) dust emission decreases strongly, accretion stops, demonstrating a relationship between the disappearance of infrared excess emission and accretable gas. The time–scale for disc ‘clearing’ is ca. 3–10 Myr, with a large range for individual systems. If disc masses estimated from dust emission are at all accurate, then the amount of material accreted onto the central star during the T Tauri phase is a substantial fraction of the total disc mass available at 1 Myr; and this in turn implies substantial radial motion of the accreting material during disc evolution. It may be that the formation of planets is the primary mechanism resulting in the cessation of accretion; certainly it is difficult to see how gas can be removed from T Tauri discs on 10 Myr time–scales.