An Etymological Dictionary of Astronomy and Astrophysics

A simple → accretion disk model in which the → angular momentum is transported outward by action of some kind of → viscosity. In this model, first proposed by Shakura & Sunyaev (1973), the turbulent kinematic viscosity is given by ν = α c_s H, where α is a parameter, c_s the sound speed in the medium, and H → scale height. The α parameter controls the amount of → turbulence in the medium whose H and c_s are upper limits for
→ mixing length and turbulent speed, respectively. Values of α = 10^-3 to 10^-2 yield evolution → time scales
that are broadly consistent with the ages inferred for → T Tauri stars. A weak point of this model is the arbitrariness of the choice of the parameter α, which reflects the lack of a rigorous theory of turbulence.

See also: → alpha, the parameter name; → disk; → model.