The density distribution of the Galactic stellar halo as traced by blue horizontal branch stars

Data of blue horizontal branch (BHB) stars and RR Lyrae variable stars from the literature are combined with unpublished observations of BHB stars in five fields. A flattened power law is used to model the spatial distribution of the horizontal branch stars. Completeness of the data sample and contamination by blue stragglers and metal-rich main-sequence A stars are considered, and taken into account. Using a maximum-likelihood method, the following best-fitting parameters are obtained: a power-law index α=−3.2±0.3 and an axial ratio of q =0.52±0.11 for the isodensity surfaces. From the fit a value for the local density of BHB stars of ρ₀=26⁺²⁰₋₁₁ kpc⁻³ is found. The values of the three parameters are in complete agreement with recent determinations by other authors.     

Wave propagation along freely propagating surface gravity current fronts

This work deals with the propagation and evolution of disturbances which move along freely propagating two-dimensional gravity current fronts. Examples of evolving perturbations on fronts are displayed in real-aperture radar images of gravity currents in the coastal zone. The theory of Cooper et al. (2001), which is based upon the ray tube formulation of Whitham (1974), is employed to simulate disturbances of the sort seen in this imagery and in the larger body of literature. Initial anomalies in both shape and velocity are introduced and allowed to evolve, and several types of new and interesting behaviors emerge. Shape perturbations of the form x=a sech δy evolve into two anomalies, which separate in time as they propagate in opposite directions along the front. When the value of a is increased, the disturbances, which propagate along the gravity current, can break, forming breaking frontal waves (BFWs). These manifest themselves as sharp angular features to either side of the main bulge. Two types of velocity disturbances are employed. The first has the form U=U₀(1+â sech δy), and evolves to preserve a single frontal bulge that increases in amplitude and width as it propagates. Here again, large values of â result in BFWs. In this case, they replicate the general behavior present in the imagery. The second type of velocity perturbation used is U=U₀(1+â cos δy). The smallest values of a generate no BFWs, but yield fronts which oscillate in space and time. Larger values produce a string of BFWs which are qualitatively similar to the cusp-and-trough morphology observed so frequently in nature. The largest values of a allow the gravity current to form a string of large, bulbous structures which intersect one another as they propagate forward and spread laterally. And finally, we make an effort to correlate the results of the simulations with the shapes seen in radar and visible imagery in the literature.     

The extent of lunar regolith mixing

The activity of solar cosmic-ray-produced⁵³Mn has been measured as a function of depth in the upper 100 g/cm² (～55 cm) of lunar cores 60009–60010 and 12025–12028. Additional samples which supplement our earlier work were analyzed from the Apollo 15 and 16 drill stems. These data, taken in conjunction with our previously published results and the²²Na and²⁶Al data of the Battelle Northwest group, indicate that in at least three of the four cases studied the regolith has been measureably disturbed within the last 10 m.y. In one case gardening to 19 g/cm² is required. Activities measured in the uppermost 2 g/cm² indicate frequent mixing within this depth range. No undisturbed profiles were observed nor were any major discontinuities observed in the profiles. The Monte Carlo gardening model of Arnold has been used to derive profiles for the gardened moon-wide average of⁵³Mn and²⁶Al as a function of depth. The⁵³Mn and²⁶Al experimental results are compared with these theoretical predictions. Agreement is good in several respects, but the calculated depths of disturbance appear to be too low.     

A Monte Carlo model for the gardening of the lunar regolith

The processes of movement and turnover of the lunar regolith are described by a Monte Carlo model, which includes the effects of collisions by particles from 10^?7 g to 10¹⁰ g. The movement of material by the direct cratering process is the dominant mode, but slumping is also included for angles exceeding the static angle of repose. Using a group of interrelated computer programs a large number of properties are calculated, including topography, formation of layers, depth of the disturbed layer, nuclear track distributions, cosmogenic nuclides and others. In the most complex program, the history of a 36 point square array is followed for times up to 4 × 10⁸ yr. As expected the histories generated are complex and exhibit great variety. Because a crater covers much less area than its ejecta blanket, there is a tendency for the height change at a test point to exhibit the ‘gambler's ruin’ phenomenon: periods of slow accumulation followed by sudden excavation. In general the agreement with experiment and observation seems good. Two areas of disagreement stand out. First, the calculated surface is rougher than that observed. This problem is understood, and will not occur in a newer version of the model. Second, the observed bombardment ages, of the order of 4 × 10⁸ yr, are shorter than expected (by perhaps a factor of 5). We cannot accept Fireman's (1974) explanation; this remains an important puzzle.     

Treatment of pulsating white dwarfs including general relativistic effects

In this paper, pulsating white dwarfs are treated via general relativity. Numerical integration of Einstein's equations was used to find equilibrium white dwarfs models and the fundamental periods of small oscillations about these equilibrium models. In these calculations account was taken of coulomb, Thomas-Fermi, and exchange interactions as well as ion zero point energies. It is shown that general relativity makes not just a quantitative difference in the results but a qualitative differences; pure C¹² models which are stable in Newtonian mechanics can be unstable against collapse (at a central density of 3×10¹⁰ g/cm³) when general relativity is taken into account. The collapsing model may become a neutron star or may continue towards the Schwarzschild radius.More realistic white dwarf models with carbon burning products at the center, also were studied. For these models, the density at which the star becomes unstable against collapse due to electron capture (3×10⁹ g/cm³) was found to be lower than the density at which general relativistic instability occurs.     

A pendulum inclinometer for use with small deep-submersibles

The authors developed a pendulum inclinometer suitable for use with small deep-submersibles or surface craft. The instrument uses a relatively short heavy pendulum and a viscous damping system for minimizing the effects of unwanted oscillatory motion. The pendulum relative motion is transmitted to the dial pointer through a flexible cord. The mechanism is designed so that the inclinometer gives a direct reading of the tangent of the angle of inclination. Adjustments are provided for levelling the instrument and for setting the dial to zero. Upper and lower clamping devices protect the pendulum suspension from damage during transit. The inclinometer has been used successfully in recent inclining experiments for the small research submarine Alvin.     

Mitteilungen

Ohne Zusammenfassung     

岩芯饼裂机制及其对工程地质特性影响的分析

提出了岩芯饼裂主要受地应力平均值控制的观点,说明了岩饼表面圆周状对称拉裂坎以及垂直拉裂坎方向擦痕的形成原因,结合工程地质讨论了岩体的弹性模量和内聚力。