Elementary processes in astrophysics (NAST024)

Boundary layer

The standard model of accretion disc is derived under assumption of an inner boundary, where dΩ dR = 0. Note that this condition is not possible for purely Keplerian rotation of the fluid in Newtonian gravity, however, it may fullfilled in realistic situations in (at least) two ways. First, in the regime of strong gravity, i.e., when the central object is very compact, the generalised Newtonian rotation profile does fullfill the above given condition at the last marginally stable circular orbit. Second possibility is presence of an additional force which supports matter against gravity together with the centrifugal force. This section discusses briefly the second option. Section 4.5 of the textbook by Vladimir Karas is the main study material; my hand-notes probably do not provide anythig substantial in addition.

The most important take-off from this lesson is supposed to be the qualitative logical consideration which leads to the conclusion that it is the gradient of pressure that mostly ballances the gravitational pull of the central body (star). Let me briefly sketch the argumentation here: