Protoplanetary accretion disks with coagulation and evaporation

Abstract

Recently, three new developments have influenced the concept, purpose, and action of accretion disk theory in the context of star and planet formation. These are: (1) The strength of convectively driven turbulence may be as much as three orders of magnitude lower than was believed hitherto. (2) Coagulation of dust could significantly change the opacity of the nebula, in fact it probably has to, if planet formation is a general feature. It has been suggested that this could inhibit turbulence and possibly make it intermittent. (3) The role of grain evaporation in the inner disk region has been analyzed and it was concluded that the structure of accretion disks could be profoundly influenced by the nonlinear coupling introduced via associated opacity changes. In this paper we investigate steady disk models which incorporate these new features. In particular we show that accretion disks can adjust to remain optically thick even if substantial coagulation of dust gives very much smaller (than the canonical) opacities. Also, we demonstrate that evaporation of grains and the associated lowering of opacities leads to a protoplanetary disk which has a broad radial regime with central temperature ∼1500°K. For inefficient turbulence and high mass accretion rates this regime may extend beyond the orbit of Earth. Cosmochemical consequences are briefly discussed.