Dynamical Evolution of Neutrino-cooled Accretion Disks: Detailed Microphysics, Lepton-driven Convection, and Global Energetics

We present a detailed, two-dimensional numerical study of the microphysical conditions and dynamical evolution of accretion disks around black holes when neutrino emission is the main source of cooling. Such structures are likely to form after the gravitational collapse of massive rotating stellar cores or the coalescence of two compact objects in a binary (e.g., the Hulse-Taylor system). The physical composition is determined self-consistently by considering two regimes, neutrino opaque and neutrino transparent, with a detailed equation of state that takes into account neutronization, nuclear statistical equilibrium of a gas of free nucleons and α-particles, blackbody radiation, and a relativistic Fermi gas of arbitrary degeneracy. Various neutrino emission processes are considered, with e± capture onto free nucleons providing the dominant contribution to the cooling rate. We find that important temporal and spatial scales, related to the optically thin/optically thick transition, are present in the disk and manifest themselves clearly in the energy output in neutrinos. This transition produces an inversion of the lepton gradient in the innermost regions of the flow that drives convective motions and affects the density and disk scale height radial profiles. The electron fraction remains low in the region close to the black hole and, if preserved in an outflow, could give rise to heavy-element nucleosynthesis. Our specific initial conditions arise from the binary merger context, and so we explore the implications of our results for the production of gamma-ray bursts.