Viscothermal instability in Keplerian disc and formation of overdense regions

Viscosity have a significant effect in evolution of accretion disc. In this paper, we investigate the thermal effect of viscosity in the accretion disc that may cause instability to produce overdense regions through it. For this purpose, the linear perturbation method is used to investigate instability on this so-called viscothermal effect. The results show that instability can occur in accretion disc so that larger overdense regions are formed at far greater distance of protostar. This mechanism may explain formation of larger protoplanets farther from protostars.