Disks around young O-B (proto)stars : observations and theory

Disks are a natural outcome of the star-formation process, in which they play a crucial role. Luminous, massive stars of spectral type earlier than B4 are likely to be those that benefit most from the existence of accretion disks, which may significantly reduce the effect of radiation pressure on the accreting material. The scope of the present contribution is to review the current knowledge about disks in young high-mass (proto)stars and discuss their implications. The issues of disk stability and lifetime are also discussed. We conclude that for protostars of less than ~20 M , disks with mass comparable to that of the central star are common. Above this limit the situation is unclear and there are no good examples of proto O4–O8 stars surrounded by accretion disks: In these objects only huge, massive, toroidal, nonequilibrium rotating structures are seen. It is clear, on the other hand, that the observed disks in stars of 10–20 M are likely to be unstable and with short lifetimes.