B. W. Carroll

Oscillation spectra of neutron stars with strong magnetic fields

The effects of strong frozen-in vertical magnetic fields on nonradial oscillation spectra in neutron stars are investigated theoretically, focusing on the surface layers near the polar cap of a cylindrically symmetric neutron-star model with shear-supporting crust and molten-crust oceans. The pulsation equations are derived; analytical estimates are obtained; and the results of numerical experiments are presented in tables and graphs. Significant modifications in the frequencies and displacements of the modes are found when a magnetic field is present: Alfven-like g modes (designated magneto-gravity), pseudotoroidal a modes with periods less than 100 ns for a 1-TG field, p-mode displacements almost totally parallel to the field, and a mode spectrum for periods of 100 microsec or more comprising only t, s, and p modes at 1 TG. 42 references.

Modeling vertebrate dispersal distances: alternatives to the geometric distribution

The geometric probability distribution is a common way by which to model frequency distributions of vertebrate dispersal distances. This model has two weaknesses: (1) it requires that the probability of stopping, p, is constant across the range of habitat units, and (2) it provides no mechanism for modeling the maximum dispersal distance. The weaknesses of this model are discussed and a family of alternative models, which allows the stopping probability to change and which takes into account the maximum dispersal distance (dispersal scale) is presented. The new models in their simplest form reduce to the goemetric probability distribution model. The new models are applied to five sets of published field dispersal data to determine the general properties of their fit as compared to the geometric and to assess their potential as better models of dispersal patterns. See full-text article at JSTOR

The quasi-adiabatic analysis of nonradial modes of stellar oscillation in the presence of slow rotation

We have computed the vibrational stability of nonradial modes of oscillation in the quasi-adiabatic approximation for slowly, uniformly rotating stars, in the Cowling approximation. Only first-order effects are taken into account in the ratio of rotation angular frequency to oscillation angular frequency. We have applied the theory to iron white dwarf models and to zero-age main-sequence modesl. We find that slow rotation renders prograde traveling waves of nonradial oscillation (m<0) slightly more unstable than retrograde modes, essentially independently of the stability of the model in a given mode in the nonrotating state.

The Four-Ball Gyro and Motorcycle Countersteering

Most two-wheel motorcycle riders know that, at highway speeds, if you want to turn left you push on the left handlebar and pull on the right handlebar. This is called countersteering. Countersteering is counterintuitive since pushing left and pulling right when the front wheel is not spinning would turn the wheel to the right. All good motorcycle instructors teach countersteering but few understand the physics of why it works, even though there is considerable discussion about it among motorcycle riders. This paper gives a simplified explanation of gyroscopic precession and then applies this to the front wheel of a motorcycle using two steps: 1) explaining how the wheel’s lean is initiated, and 2) explaining how the lean will cause the wheel to turn. To assist with this discussion and to demonstrate the conclusions, a “wheel” was constructed using copper pipe, a bicycle wheel hub, and one pound of lead in each of four “balls” at the end of the spokes (see Fig. 1).

White Dwarf/Accretion Disk Interactions: Instabilities of Infinite, Differentially Rotating Cylinders

We have initiated an investigation of the coherent and quasiperiodic oscillations observed in some dwarf novae. The periods and relatively high Q associated with the coherent oscillations suggest a nonradial pulsation of the white dwarf primary for these variations, while the longer periods and lower Q of the quasi-periodic oscillations indicate a pulsation of the accretion disk. Guided by a terrestrial analogy, we have made a preliminary exploration of the shear layer instability which may drive the observed oscillations.