The Evolution of Viscous Discs and the Origin of the Nebular Variables

The evolution of discs under the action of viscosity is studied by both similarity solutions and Green's functions. The angular momentum is steadily concentrated onto a small fraction of the mass which orbits at greater and greater radii while the rest is accreted onto the central body. We assume that the angular momentum excess of a proto-star is initially concentrated onto one-third of the total mass which forms a disc orbiting the new-born star. Viscous dissipation in this disc will cause it to shine with a luminosity greater than the final main sequence star for a period of 105 yr or so. Most of the properties of T Tauri stars can be explained as a consequence of disc evolution. Flares in Flare stars are interpreted as the entry of blobs of an old disc into the late type stellar atmospheres. On this hypothesis flaring activity could be observed in M stars of up to 5 × 108 yr old, and planetary systems will be common. Disc solutions appropriate to dwarf novae and X-ray sources are also given.