Zero Energy Rotating Accretion Flows near a Black Hole 1

We characterize the nature of thin, axisymmetric, inviscid, accretion flows of cold adiabatic gas with zero specific energy in the vicinity of a black hole by the specific angular momentum. Using two-dimensional hydrodynamic simulations in cylindrical geometry, we present various regimes in which the accretion flows behave distinctly differently. When the flow has a small angular momentum (λ<∼λb), most of the material is accreted into the black hole forming a quasi-spherical flow or a simple disk-like structure around it. When the flow has a large angular momentum (typically, larger than the marginally bound value, λ>∼λmb), almost no accretion into the black hole occurs. Instead, the flow produces a stable standing shock with one or more vortices behind it and is deflected away at the shock as a conical outgoing wind of higher entropy. If the flow has an angular momentum somewhat smaller than λmb (λu<∼λ<∼λmb), a fraction (typically, 5− 10%) of the incoming material is accreted into the black hole, but the the flow structure formed is similar to that as for λ>∼λmb. Some of the deflected material is accreted back into the black hole, while the rest is blown away as an outgoing wind. These two cases with λ>∼λu correspond those studied in the previous works by Molteni, Lanzafame, & Chakrabarti (1994) and Ryu et al. (1995). However, the flow with an angular momentum close to the marginally stable value (λms) is found to be unstable. More specifically, if λb<∼λ ∼ λms<∼λu, the flow displays Submitted to the Astrophysical Journal