We show that most of hot, optically thin accretion disk models which ignore advective cooling are not self-consistent. We have found new types of optically thin disk solutions where cooling is dominated by radial advection of heat. These new solutions are thermally and viscously stable.

We present the results of numerical simulations of protostellar accretion disks that are perturbed by a protoplanetary companion that has a much smaller mass than the central object. We consider the limiting cases where the companion is in a coplanar, circular orbit and is initially embedded in the disk. Three independent numerical schemes are employed, and generic features of the Ñow are found in each case. In the Ðrst series of idealized models, the secondary companion is modeled as a massless, orbiting sink hole able to absorb all matter incident upon it without exerting any gravitational torque on the disk. In these simulations, accretion onto the companion induces surface density depression and gap formation centered on its orbital radius. After an initial transitory adjustment, the accretion rate onto the sink hole becomes regulated by the rate at which viscous evolution of the disk can cause matter to di †use into the vicinity of the sink hole orbit, and thus the sink hole grows on a disk viscous timescale. In the second series of comprehensive models, the companionÏs gravity is included. When the tidal torque exerted by the companion on the disk becomes important, the angular momentum exchange rate between the companion and the disk induces the protoplanetary accretion to reduce markedly below that in the idealized sink hole models. Whether this process is e †ective in inhibiting protoplanetary accretion depends on the equation of state and disk model parameters. For polytropic or isothermal equations of state, we Ðnd, in basic agreement with earlier work, that when the mean Roche lobe radius of the companion exceeds the disk thickness and when the mass ratio, q, between the companion and the central object exceeds D40/R, where R is the e †ective Reynolds number, a clean deep gap forms. Although precise estimation is rendered difficult due to the limitation of numerical schemes in dealing with large density contrasts, the generic results of three series of simulations indicate that accretion onto sufficiently massive protoplanets can become ine †ective over the expected disk lifetimes in their neighborhood. We suggest that such a process operates during planetary formation and is important in determining the Ðnal mass of giant planets.

The evolution of binary systems consisting of an asymptotic giant branch star of mass equal to 3M ⊙ or 5M ⊙ , and a main sequence star of mass equal to 0.4M ⊙ or 0.6M ⊙ with orbital periods ∼ > 200 days has been followed from the onset through the late stages of the common envelope phase. Using a nested grid technique, the three-dimensional hydrodynamical simulations of an asymptotic giant branch star with radii ∼ 1 A.U. indicate that a significant fraction of the envelope gas is unbound (∼ 31% and 23% for binaries of 3M ⊙ and 0.4M ⊙ , and 5M ⊙ and 0.6M ⊙ respectively) by the ends of the simulations, and that the efficiency of the mass ejection process ∼ 40%. During an intermediate phase, a differentially rotating structure resembling a thick disk surrounds the remnant binary briefly before energy input from the orbits of the companion and remnant core drive the mass away. While the original volume of the giant is virtually evacuated in the late stages, most of the envelope gas remains marginally bound on the grid. At the ends of our simulations, when the orbital decay timescale exceeds about 5 years, the giant core and companion orbit one another with a period of ∼ 1 day (2.4 days for a binary involving a more evolved giant), although this is an upper limit to the final orbital period. For a binary of 5M ⊙ and 0.4M ⊙ , the common envelope may not be completely ejected. The results are not found to be sensitive to the degree to which the initial binary system departs from the synchronous state.

We provide a uni ed description of thermal equilibria of black hole accretion disks, including the newly-discovered advection-dominated solutions. We classify the solutions on the basis of optical depth and importance of advection cooling. We demonstrate that only four physically distinct topological types of equilibria exist. Two of the types correspond to optically thin and optically thick equilibria, while the other two types are distinguished by whether advection is negligible or dominant. A stable Shakura-Sunyaev disk exists only for accretion rates _ M below a certain maximum. However, there is a critical viscosity parameter crit , which is a function of radius, such that for > crit advection-dominated solutions exist for all _ M. Even when < crit , the advection-dominated solutions are available for a wide range of _ M except for a gap around the Eddington rate. We therefore suggest that advection-dominated ows may be more common than standard thin disks in black hole systems. For certain ranges of radii and _ M, no stable steady state solution is possible. In these cases, we suggest that limit cycle behavior may occur, leading to variability.

We obtained global transonic solutions representing optically thin advectiondominated accretion flow by solving the full set of differential equations describing such systems. We found that far from the sonic point self-similar solutions are an excellent approximation of the global flow structure if the accretion rate is well below the maximum value above which no optically thin solutions exist.