Accretion discs models with the beta-viscosity prescription derived from laboratory experiments

We examine under which conditions one may apply, to steady state Keplerian accretion discs, the beta -viscosity prescription which has been derived fr om rotating shear flow experiments: (nu = beta OmegaR(2), where Omega is th e angular velocity at radius R and beta is a constant of order 10(-5); Rich ard & Zahn 1999). Using a vertically averaged model, we show that this law may be suitable for all three families of known systems: in young stellar o bjects, evolved binary stars and Active Galactic Nuclei discs (except in th eir outer gas pressure dominated regions where turbulence becomes hypersoni c). According to the standard criterion for for viscous stability, beta -di scs are always stable throughout. Using realistic opacities and equation of state, we demonstrate that these discs are thermally unstable in the tempe rature domain where hydrogen recombines, when they are optically thick, and this could lead to limit cycle behavior. Radiation pressure dominated regi ons are thermally stable, in contrast with cu-discs. This results in a full y stable solution for the innermost parts of AGN discs.