ABSTRACT
The current model of accretion disks, that of Shakura and Sunyaev (1973) asserts that such disks are geometrically thin, and are prone to a number of instabilities. One such instability is that of fragmentation due to self-gravity at radii larger than a few hundred black hole gravitational radii, which would limit the mass accretion rate to be well below that observed to occur. Recent GIZMO (Lagrangian-based) cosmological zoom simulations have revealed a new, geometrically thick disk flow that is magnetically dominated and gravitationally stable. The simulations use particle splitting to achieve particle masses of 0.001 solar masses, and resolution scales of order 1 AU (3.e12 cm). Magnetic fields generated in the galactic disk on kiloparsec scales are accreted inward, and hence enhanced. At distances of order the black hole sphere of influence, the magnetic energy density dominates the dynamics of the accretion flow, halting star formation (since the Toomre Q parameter grows much larger than 1) and producing a disk with thickness H of order 0.1 the radius R from the black hole. The disks have temperatures of order 100,000K at a few gravitational radii, and fall roughly as (1/R)^(3/4), as in the classical disk model. They can sustain mass accretion rates up to 10 or even 100 the Eddington accretion rate. I will describe the simulations, and to the extent that we understand it, the physics driving the accretion, as well as some observational predictions.
BIO
Norman Murray obtained his B.Sc. at Caltech, followed by a Ph.D. at Berkeley. After postdoctoral positions at Queen Mary College London, and then at Caltech, he joined the faculty at the Canadian Institute for Theoretical Astrophysics in 1993. Murray works on a broad range of problems in astrophysics, including planetary dynamics, solar oscillations and magnetic fields, black holes and their effects on their host galaxies, as well as planet, star, and galaxy formation. He has made significant contributions to the theory of outflows from supermassive black hole accretion disks (broad absorption line quasars), the theory of stellar feedback on star and galaxy formation, and on photo-evaporative atmospheric loss from planets. He explained why the giant planets in our Solar System are chaotic, and why some hot Jupiters orbit their host stars in highly inclined or even retrograde orbits. He has also worked on the theory of thermal tides on Earth and on exoplanets. Murray was involved in the construction of Bob Leighton's 10 meter sub-millimeter dishes, one of which is currently in use by the COMap collaboration (of which he is a member) as well as the ongoing construction of the Fred Young Submillimeter Telescope to be installed at the CCAT-prime observatory near the summit of Cerro Chajnantor, in 2025.
Host: Wei Zhu
