for their discovery and study of the magnetorotational instability, and for demonstrating that this instability leads to turbulence and is a viable mechanism for angular momentum transport in astrophysical accretion disks.
The Shaw Prize in Astronomy for 2013 is awarded to Steven A Balbus, Savilian Professor of Astronomy, University of Oxford, UK and John F Hawley, Associate Dean for the Sciences and VITA Professor and Chair of Department of Astronomy, University of Virginia, USA for their discovery and study of the magnetorotational instability, and for demonstrating that this instability leads to turbulence and is a viable mechanism for angular momentum transport in astrophysical accretion disks.
Accretion is a widespread phenomenon in astrophysics. It plays a key role in star formation, mass transfer in stellar binaries, and the growth of supermassive black holes in the centres of galaxies. Sources powered by accretion can even outshine those of similar mass powered by nuclear fusion.
The attractive force of gravity is responsible for the formation of bound structures over a wide range of scales, from planets to clusters of galaxies. Unbalanced, gravity would cause matter to collapse into black holes. Fortunately, the concentration of mass by gravity is impeded, at least temporarily, by the requirement that the contracting material rid itself of excess energy and angular momentum. Bulk kinetic energy can be converted into heat and radiated away but angular momentum is less readily disposed of. Consequently, contracting material often assumes the form of a differentially rotating disk. Familiar examples include Saturn’s rings and spiral galaxies. Nascent stars grow by accreting mass from disks that live several million years. The coplanar orbits of the solar system planets and multi-planet systems around other stars are vestiges of these disks.
More exotic accretion disks are found around compact objects such as white dwarfs, neutron stars and black holes. These systems shine by radiating gravitational potential energy released as mass spirals inward. For fluid to spiral in, its angular momentum must be transported out, but how this happens was for long a mystery. For decades, it was speculated that accretion disks were unstable and that the accretion torque arises from turbulent stresses. However, analytic analyses and numerical simulations consistently failed to identify any appropriate instability. In 1991 Balbus & Hawley announced an elegant solution to this longstanding problem. They demonstrated that even a weak seed magnetic field is sufficient to unleash a powerful instability, the magnetorotational instability (MRI), that both creates and sustains turbulence while also amplifying the magnetic field.
Steven A Balbus was born in 1953 in Philadelphia, USA and is currently Savilian Professor of Astronomy at the University of Oxford, UK. He received BS degrees in both Mathematics and Physics from the Massachusetts Institute of Technology in 1975, obtained his PhD in Theoretical Astrophysics from the University of California, Berkeley in 1981, and held postdoctoral positions at the Massachusetts Institute of Technology and Princeton University, USA. He joined the faculty of the University of Virginia in 1985, moved to the École Normale Supérieure de Paris in 2004 and joined the University of Oxford in September 2012.
John F Hawley was born in 1958 in Annapolis, Maryland, USA and is currently Associate Dean for the Sciences, and John D Hamilton Professor of Astronomy at the University of Virginia, USA. He obtained his PhD from the University of Illinois in 1984 and was Bantrell Fellow at the California Institute of Technology from 1984 to 1987. He has been associated with the Department of Astronomy at the University of Virginia since 1987, first as Assistant Professor (1987–1993), then as Associate Professor (1993–1999) and Professor of Astronomy (1999–2014), and the John D Hamilton Professor of Astronomy since 2014. He was chair of the department from 2006–2012, and became the Associate Dean for the Sciences in the College of Arts and Sciences in 2012. His research interests include computational simulation and black hole astrophysics. He received the 1993 Helen B Warner Prize from the American Astronomical Society for his contributions in astrophysical theory.
