Dynamos and the Structure of Ionized Accretion Disks

Johns Hopkins University, Baltimore MD

Investigators

Abstract

AST 0098615 Vishniac This project deals with the intersection of two important issues in the field of astrophysics, the generation of large scale magnetic fields, and the accretion of gas onto collapsed objects, such as black holes, neutron stars, or white dwarfs. There has been continuing work on the `dynamo problem' for more than 80 years. Very recently, numerical simulations have indicated that there are problems with the dynamo equations. Some simulations designed to produce strong magnetic fields have failed to do so, while others that fail to meet the basic conditions for dynamo activity show strong magnetic field generation. Meanwhile, the last thirty years has seen steady progress in our understanding of how gas collects together to form stars, or to fall into black holes. When the gas is able to radiate its energy, it tends to form a disk of ionized material that slowly spirals into the center, eventually falling onto the central object. This process is believed to lie at the heart of such diverse objects as quasars, where the central object is a supermassive black hole, and proto-stars, where the central object is a newly formed star. Crude models of this process have been successful in providing qualitative models of kinds of radiation emitted from such objects, and some of the time-varying behavior seen in dwarf novae (where the central object is a white dwarf) and soft X-ray transients (where the central object is usually a black hole, with a mass several times the mass of the Sun). Within the last ten years there has been a growing consensus that these two problems are actually closely related, and that the `friction' in accretion disks is actually due to magnetic fields that spontaneously wiggle inside the accretion disk. The behavior of accretion disks is actually closely connected to the way that they generate magnetic fields. Computer simulations indicate that the turbulent motions generated by the magnetic field actually helps generate the magnetic field. This project will study the generation of large scale magnetic fields in hot accretion disks using a novel approach to dynamo theory recently developed by the PI and collaborators. The project will have three main components. First, the new dynamo theory will be extended and tested quantitatively to ensure that it provides a viable explanation for the generation of large scale magnetic fields. Second, models of accretion disks in which the detailed evolution of the magnetic field can be replaced with large scale approximate equations will be constructed. These will describe the evolution of the large scale magnetic field, and its ability to provide a source of `friction' and therefore heating within the disk. Finally, this approach will be used to study dwarf novae disks. These disks have a number of features which make them ideal for comparison with the models. In particular, they are largely unaffected by the radiation coming from the inner parts of the disk and the central object, and they show outbursts in which the properties of the disks change dramatically. They are also well-observed, so that the success, or failure, of the models will be immediately apparent. Funding for this project was provided by the NSF program for Extragalactic Astronomy & Cosmology (AST/EXC). ***