Showing papers by "Scott Tremaine published in 1982"

The Dynamics of Planetary Rings

Abstract: The discovery of ring systems around Uranus and Jupiter, and the Pioneer and Voyager spacecraft observations of Saturn, have shown that planetary rings are both more common and more complex than previously suspected. These ring systems, interesting in their own right, also serve as prototypes for more massive disk systems such as accretion disks and spiral galaxies occurring elsewhere in astronomy. Disks and rings are a natural consequence of dissipation in rotating systems. A cloud of debris surrounding a spherical planet settles into a flat circular ring because interparticle collisions dissipate energy but conserve total angular momentum. Since planets are oblate, only the component of angular momentum along the spin axis is conserved, and the flat ring lies in the equatorial plane. Collisions redistribute angular momentum among the particles and the ring spreads, transferring mass inward and angular momentum outward (Lynden-Bell & Pringle 1974). However, the spreading process occurs on a much longer timescale than the flattening process since the collision speeds in a flat ring are much lower than the orbital speeds (see Sections 2.2 and 5. 3). Spreading can be slowed by gravitational interactions with satellites; nevertheless, a ring cannot live forever and an important constraint on possible ring models is that they yield survival times at least comparable to the age of the solar system (cf. Sections 5, 6). This review was written in October 1981, shortly after the Voyager 2 encounter with Saturn. Analysis of the data from the Voyager encounters is not yet complete. Therefore the emphasis in this review is on the basic physical processes that occur in planetary rings, rather than on a detailed confrontation of theoretical predictions with observation. Table 1 lists some of the important properties of the planets with known ring systems. For the sake of brevity we shall refer to a series of papers we have written on rings (Goldreich & Tremaine 1978a, b, c, 1979a, b, c, 1980, 1981) as GT 1, . . . , GT 8.

Sharp edges of planetary rings

Abstract: The ring systems of Saturn and Uranus exhibit several sharp edges across which the optical depth drops from order unity to essentially zero. At least two and perhaps all of these features are associated with the location of orbital resonances between a satellite and the ring particles. It is remarkable that the optical depth varies on a distance scale which is much finer than that over which angular momentum can be transferred between a satellite and the ring material. The important features of this phenomenon are: (1) A perturbed band of width Δa/a ≃ (M_s/M_p)^(½) adjacent to the edge within which the angular momentum transfer occurs. (2) Streamlines perturbed such that the angular momentum luminosity decreases smoothly across the band to zero at the edge even though the optical depth remains constant. (3) Dynamical equilibrium requires a relation between the random velocity, the rate of deformation and the optical depth.

The dynamics of the nucleus of M31

Abstract: Self-consistent dynamic models of the M31 nucleus were constructed which satisfy Stratoscope II and ground-based photometry, as well as ground-based velocity dispersion measurements. Two models designed to yield the highest and lowest mass-to-light ratios (M/L) consistent with observations are concentrated on. It is found that self-consistent static models of the M31 nucleus can be constructed with a nuclear mass-to-blue magnitude ratio between 0 and 50, and that it is very difficult to construct high M/L models with plausible evolutionary histories. The low values of M/L that were found suggest that the nucleus must be composed of stars on nearly radial orbits whose apocenters lie well outside the observable part of the nucleus. Such nuclei may be formed either during core collapse of a preexisting dense nucleus or by orbital decay of globular clusters in a triaxial bulge.