Showing papers by "Scott Tremaine published in 1985"

Estimating the masses of galaxy groups. Alternatives to the virial theorem

Abstract: The authors present three alternatives to the virial theorem for estimating the masses of groups of galaxies. The projected mass estimator uses the mean value of Vi2|Ri|, where Vi and Riare the radial velocity and projected separation of galaxy i from the group center. The other two methods rely on the average and median of (Vi-Vj)2|Ri-Rj| over all pairs of galaxies in the group. These three estimators and the virial theorem estimator are tested using a series of N-body simulations and Monte Carlo realizations of Michie models. The authors use the mass estimators to calculate the masses and mass-to-light ratios of nearby groups catalogued by Huchra and Geller. The results confirm their work, implying that there is a large amount of dark mass in groups on scales of ≡700 kpc.

The Influence of the Galactic Tidal Field on the Oort Comet Cloud.

Abstract: We examine the effect of the Galactic disk potential on the Oort cloud. The tidal force from the disk produces a slow drift of a comet's orbital angular momentum, J. If J is reduced below a critical value, J∗, then the comet enters the planetary system. Once this occurs the comet is lost from the Oort cloud, either to an escape orbit or a much more tightly bound orbit, within a few orbital period. We determine the rate of orbit evolution due to the Galactic tide by analyzing the orbit averaged Hamiltonian. We calculate the loss rate due to the Galactic tide and compare this with the loss rate due to stellar encounters. We find that when the loss cylinder is empty the loss rate of comets from the Oort cloud due to the Galactic tide is larger than the loss rate due to stellar perturbations by about a factor 1.5–2. The Galactic tide appears to be the dominant cause of orbital 4evolution in the Oort cloud.

Maximum mass of objects that constitute unseen disk material

Abstract: It is suggested that the mass of the individual unseen 'disk things' which furnish unseen mass satisfies an inequality that is based on the demonstrated existence of a large number of wide binary stars in the Galaxy whose projected separations are as large as 0.1 pc. Since these binaries are weakly bound, with orbital velocities of 0.3 km/sec and periods of the order of 1 million years, they can be disrupted by passing stars or other pointlike objects. This implies that the mass of the unseen disk objects, which provide about half the local mass density, must be less than 2 solar masses.