Accretion flows are ubiquitous in astrophysics: they occur around protostars, accreting compact objects in binary systems, and supermassive black holes at the cores of galaxies. Much of professor James M. Stone's work has concerned studies of the local hydrodynamic and magnetohydrodynamics (MHD) processes that can lead to outward angular momentum transport in accretion disks. As computers become more powerful, previous studies of local patches of an accretion flow are being expanded into global studies that encompass the entire disk.
Accretion flows that cannot cool via emission of radiation become vertically thick and nearly spherical. Thus, they are intrinsically multidimensional. To study the structure and evolution of non-radiative accretion flows, 2D (axisymmetric) hydrodynamical simulations were performed using a non-uniform grid that spanned more than two decades in radius.
The most striking property of the flow is the large fluctuations produced by strong convection. Convective eddies transport a lot of mass both inwards and outwards, but the net mass accretion rate is very small and set by the properties of the flow near the inner boundary. A vanishingly small accretion rate may help to explain the deficit of high energy emission observed from accreting compact sources.
While understanding the properties of hydrodynamical accretion flows is important, it is generally agreed that angular momentum transport is in fact mediated by magnetic stresses. Thus, repeating the global simulations of non-radiative accretion flows with MHD calculations is vital.
Credit: James M. Stone
Rhea, the second largest moon of Saturn, is a dirty snowball of rock and ice. The only moon with an oxygen atmosphere, thin though it may be, Rhea is one of the most heavily cratered satellites in the solar system.
A very faint oxygen atmosphere exists around Rhea, the first direct evidence of an oxygen atmosphere on a body other than Earth. The atmosphere is thin, with oxygen measuring about 5 trillion times less dense than that found on Earth. Oxygen could be released as the surface is irradiated by ions from Saturn’s magnetosphere. The source of the carbon dioxide is less clear, but could be the result of similar irradiation, or from dry ice much like comets.
On March 6, 2008, NASA announced that Rhea may have a tenuous ring system. This would mark the first discovery of rings about a moon. The rings’ existence was inferred by observed changes in the flow of electrons trapped by Saturn’s magnetic field as Cassini passed by Rhea. Dust and debris could extend out to Rhea’s Hill sphere, but were thought to be denser nearer the moon, with three narrow rings of higher density. The case for a ring was strengthened by the subsequent finding of the presence of a set of small ultraviolet-bright spots distributed along Rhea’s equator (interpreted as the impact points of deorbiting ring material).However, when Cassini made targeted observations of the putative ring plane from several angles, no evidence of ring material was found, but there’s still something around Rhea that is causing a strange, symmetrical structure in the charged-particle environment around Saturn’s second-largest moon.
Image credit: NASA/JPL-Caltech/SSI,Gordan Ugarkovic
Revolutionary Telescope Gets Green Light
An 82-foot telescope boasting ten times the resolution of the Hubble Space Telescope has successfully passed design reviews and is ready to be constructed.
The Giant Magellan Telescope will use a light-collecting mirror surface more than six times the area of current instruments to hunt for distant, potentially habitable planets and let astronomers time travel back to a billion years after the Big Bang.