Thermonuclear detonations in collapsing white dwarf stars

Collapsing white dwarf stars (or degenerate cores) may occur in binary systems, in the formation of Type I supernovae or in the formation of pulsars. These collapsing configurations may explode their nuclear fuel (¹²C or¹⁶O) by the detonation wave mechanism. A combination of analytical and numerical models is used to investigate the formation of detonation waves. The tentative conclusion is that a detonation wave will form which will lead to the ignition of esentially all the fuel in such a collapsing star. This potentially explosive configuration will be strongly affected, however, by rapid beta processes which occur in the detonated matter and which should cause a fraction of the stellar mass to collapse toward a neutron star state. The nature and effect of such beta processes, which have not yet been incorporated in the dynamical calculations, are discussed.An appendix gives approximate expressions for the pressureP(,T) and the internal energy densityU(,T) for a degenerate relativistic electron gas and an analysis of the errors expected in making such approximations to the standard parametric form of the equation of state. These expressions are useful in analyzing shock waves in a degenerate electron gas.Supported in part by the National Science Foundation [GP-15911, GP-9114, GP-19887] and the Office of Naval Research [Nonr-220 (47)] at the California Institute of Technology, and National Science Foundation Grant GP-12455 at the University of Colorado.     

Lightcurve Analysis of Four New Monolithic Fast-Rotating Asteroids

We present lightcurves and analysis for four new monolithic fast-rotating asteroids: 2000 AG₆, 2000 DO₈, 2000 EB₁₄, and 2000 HB₂₄. Their rotation periods of 4.60, 1.30, 107.47, and 13.05 min place them well below the critical threshold for the rotation rate of strengthless prolate ellipsoids, as we demonstrate. These four objects join the five previously identified fast-rotating asteroids. The sharp segregation in spin rates between these nine objects and asteroids with more typical spin rates is somewhat puzzling. No observed objects larger than about 200 m spin with rates faster than the critical rate for strengthless prolate ellipsoids, while no objects smaller than 200 m have shown spin rates slower than this critical limit. We hypothesize that these small, fast-rotating objects are representative of the building blocks of the “rubble pile” asteroids and are in fact derived from impacts into already existing “rubble piles.”     

A focal plane detector design for a wide-band Laue-lens telescope

The energy range above 60 keV is important for the study of many open problems in high energy astrophysics such as the role of Inverse Compton with respect to synchrotron or thermal processes in GRBs, non thermal mechanisms in SNR, the study of the high energy cut-offs in AGN spectra, and the detection of nuclear and annihilation lines. Recently the development of high energy Laue lenses with broad energy bandpasses from 60 to 600keV have been proposed for a Hard X ray focusing Telescope (HAXTEL) in order to study the X-ray continuum of celestial sources. The required focal plane detector should have high detection efficiency over the entire operative range, a spatial resolution of about 1mm, an energy resolution of a few keV at 500keV and a sensitivity to linear polarization. We describe a possible configuration of the focal plane detector based on several CdTe/CZT pixelated layers stacked together to achieve the required detection efficiency at high energy. Each layer can operate both as a separate position sensitive detector and polarimeter or work with other layers to increase the overall photopeak efficiency. Each layer has a hexagonal shape in order to minimize the detector surface required to cover the lens field of view. The pixels would have the same geometry so as to provide the best coupling with the lens point spread function and to increase the symmetry for polarimetric studies.     

Near‐infrared spectra of clinopyroxenes: Effects of calcium content and crystal structure

Abstract– Pyroxenes are among the most common minerals in the solar system and are ideally suited for remote geochemical analysis because of the sensitivity of their distinctive spectra to mineral composition. Fe²⁺ is responsible for the dominant pyroxene absorptions in the visible and near‐infrared, but substitutions of other cations such as Ca²⁺ change the crystal structure and site geometries and thus the crystal field splitting energies of the Fe cations. To define spectral systematics resulting from major pyroxene cations (Ca²⁺, Mg²⁺, and Fe²⁺), we focus on a suite of pyroxenes synthesized with only Ca²⁺, Mg²⁺, and Fe²⁺ in the two octahedral sites, specifically examining the effect of Ca²⁺ on pyroxene absorption bands. The modified Gaussian model is used to deconvolve pyroxene spectra into component bands that can then be linked directly to crystal field absorptions. In orthopyroxenes and low‐Ca clinopyroxenes, Ca²⁺‐content has a strong and predictable effect on the positions of the absorption bands. At a threshold of Wo₃₀, the crystal field environment stagnates and the M2 bands cease to change significantly as more Ca²⁺ is added. At Wo₅₀, when most of the M2 sites are filled by Ca²⁺, band positions do not change drastically, although the presence and strengths of the 1 and 2 μm bands are affected by even trace amounts of Fe²⁺ in the M2 site. It is thus apparent that next‐nearest neighbors and the distortions they impose on the pyroxene lattice affect the electronic states around the Fe²⁺ cations and control absorption band properties.     

A 3‐D study of mineral inclusions in chromite from ordinary chondrites using synchrotron radiation X‐ray tomographic microscopy—Method and applications

Abstract– A method is described for imaging in 3‐D the interiors of meteoritic chromite grains and their inclusions using synchrotron radiation X‐ray tomographic microscopy. In ordinary chondrites, chromite is the only common mineral that survives long‐term weathering on Earth. Information about the silicate matrix of the original meteorite, however, can be derived from mineral inclusions preserved in the protecting chromite. The inclusions are crucial in the classification of fossil meteorites as well as sediment‐dispersed chromite grains from decomposed meteorites and larger impacts, as these are used for characterizing the past influx of material to Earth, but have previously been difficult to locate. The method is non‐destructive and time efficient for locating inclusions. The method allowed quantitative and morphological studies of both host chromite grains and inclusions in three dimensions. The study of 385 chromite grains from eight chondrites (H4–6, L4–6, LL4, LL6) reveals that inclusions are abundant and equally common in all samples. Almost two‐thirds of all chromite grains contain inclusions, regardless of group and type. The study also shows that the size of the inclusions and the host chromite grains, as well as the number of inclusions, within the host chromite grains vary with petrographic type. Thus, the petrographic type of the host of a suite of chromite grains can be determined based solely on inclusion content. The study also revealed that the amount of fractures in the host chromite can be correlated to previously assigned shock stages for the various chondrites. The study has thus shown that the features and inclusions of fossil chromite grains can give similar information about a former host meteorite as do studies of an unweathered whole meteorite, meaning that this technique is essential in the studies of ancient meteorite flux to Earth.     

Isotopic composition of strontium in three basalt-andesite centers along the Lesser Antilles arc

Si⁸⁷/Sr⁸⁶ ratios have been determined for lavas and py lastic rocks from three basalt-andesite centers along the Lesser Antilles arc—Mt. Misery on the island of St. Kitts, Soufriere on the island of St. Vincent, and Carriacou, an island of The Grenadines. The average Si⁸⁷/Sr⁸⁶ content of these rocks is 0.7038 for Mt. Misery, 0.7041 for Soufriere, and 0.7053 for Carriacou. All the Sr⁸⁷/Sr⁸⁶ values from each center are the same within analytical uncertainty (±0.0002). The constancy of strontium isotopic data within each center supports the hypothesis that basalts and andesites for each specific center investigated are generated from the same source — in agreement with petrographic and major- and minor-element data. Strontium isotopic compositions and elemental concentrations, particularly of strontium and nickel, indicate that this source was mantle peridotite and that the relationship between the respective basalts and andesites is probably fractional crystallization.Publication authorized by the Director, U.S. Geological Survey.     

The lunar rock and mineral characterization consortium: Deconstruction and integrated mineralogical,petrologic, and spectroscopic analyses of mare basalts

The lunar rock and mineral characterization consortium (LRMCC) has conducted coordinated mineralogy/petrography/spectroscopy analyses of a suite of pristine lunar basalts. Four basalt slabs (two low‐Ti, two high‐Ti) and paired thin sections were analyzed. Thin sections were analyzed for mineralogy/petrography, while the slabs were used to prepare particulate separates of major mineral phases and bulk samples. Mineral separates and particulate bulk samples were crushed to controlled grain sizes and their reflectance spectra measured in the NASA RELAB at Brown University. The resulting data set provides an essential foundation for spectral mixing models, offers valuable endmember constraints for space weathering analyses, and represents critical new ground truth results for lunar science and exploration efforts.