Dynamos in Accretion Disks cmso发电机在吸积盘 cmso.info.ppt

Dynamos in Accretion Disks: A general review and some moderately biased comments What is an Accretion Disk? Flattened, rotionally supported, gas accreting onto a central object over the course of many rotation periods. A good fluid, i.e. a high collision rate and a short mean free path. Negligible gravity due to disk. (ignoring galaxies, outer parts of AGN disks) Not necessarily ionized, but here we will discuss only good conductors. (ignoring intermediate radii in protostellar disks) A Theoretical Cartoon: An Observational Perspective: Broad spectral energy distribution. Power law emission at high frequencies (X-rays). Radio emission seen in some cases, associated with jets. Conspicuous emission lines in conjunction with optically thick continuum. Accretion Disks are Ubiquitous Protostars - accretion from environment- winds jets. Cataclysmic variables (white dwarf accretors) - accretion from binary companion - winds. Active Galactic Nuclei (supermassive black holes) - accretion from environment - winds jets. Galactic black holes/neutron stars - accretion from companion - winds jets. Accretion Disks are Important Active Galactic Nuclei (AGN) visible over cosmological distances. Protostellar disks define the environment in which planets form. Some CVs are the precursors of Type Ia supernovae. Astrophysical example par excellence of dynamically important magnetic fields. The Flow of Mass, Energy and Angular Momentum The specific angular momentum of a fluid element determines its radial position in the disk. Fluid elements transfer angular momentum outward, which leads the gas to move inward. Half the gravitational binding energy gained by a fluid element goes into its orbital motion. The other half is either dissipated as heat, either locally or at a larger radius. (The outward flux of angular momentum has an associated energy flux.) Viscosity and Other Fictions Accretion disks rotate differentially, with the angular frequency decreasing outwards. Co