The population of propellers in Saturn's A Ring

We present an extensive data set of ~150 localized features from Cassini images of Saturn's Ring A, a third of which are demonstrated to be persistent by their appearance in multiple images, and half of which are resolved well enough to reveal a characteristic "propeller" shape. We interpret these features as the signatures of small moonlets embedded within the ring, with diameters between 40 and 500 meters. The lack of significant brightening at high phase angle indicates that they are likely composed primarily of macroscopic particles, rather than dust. With the exception of two features found exterior to the Encke Gap, these objects are concentrated entirely within three narrow (~1000 km) bands in the mid-A Ring that happen to be free from local disturbances from strong density waves. However, other nearby regions are similarly free of major disturbances but contain no propellers. It is unclear whether these bands are due to specific events in which a parent body or bodies broke up into the current moonlets, or whether a larger initial moonlet population has been sculpted into bands by other ring processes.Comment: 31 pages, 10 figures; Accepted at A

Crustal Accretion in the Gulf of California: An Intermediaterate Spreading Axis

An important objective of Deep Sea Drilling Project (DSDP) Leg 65 was to study crustal accretion at an ocean ridge axis with an intermediate-spreading rate for comparison with previously studied sections displaying slowand fast-spreading rates. The southern Gulf of California was selected for this purpose because the basement displays high seismic velocities (comparable to those observed for Cretaceous basement in the western North Atlantic) and high ambient sedimentation rates, which facilitated penetration of zero-age basement. Four sites were drilled, forming an axial transect immediately south of the Tamayo Fracture Zone (Figs. 1 and 2) and providing a series of characteristic sections into the crust. This chapter attempts to provide a brief synthesis of the results from Leg 65, focusing particularly on the lithology, geochemistry, and paleomagnetic properties of the cored basement material. From these data, we present an interpretation of the processes of magmatic evolution and crustal accretion occurring at the Gulf of California spreading axis

Development of the X-ray camera for the OGRE sub-orbital rocket

Current theories regarding the matter composition of the universe suggest that half of the expected baryonic matter is missing. One region this could be residing in is intergalactic filaments which absorb strongly in the X-ray regime. Present space based technology is limited when it comes to imaging at these wavelengths and so new techniques are required. The Off-Plane Grating Rocket Experiment (OGRE) aims to produce the highest resolution spectrum of the binary star system Capella, a well-known X-ray source, in the soft X-ray range (0.2keV to 2keV). This will be achieved using a specialised payload combining three low technology readiness level components placed on-board a sub-orbital rocket. These three components consist of an array of large format off-plane X-ray diffraction gratings, a Wolter Type 1 mirror made using single crystal silicon, and the use of EM-CCDs to capture soft X-rays. Each of these components have been previously reviewed with OGRE being the first project to utilise them in a space observation mission. This paper focuses on the EM-CCDs (CCD207-40 by e2v) that will be used and their optimisation with a camera purposely designed for OGRE. Electron Multiplying gain curves were produced for the back-illuminated devices at -80 degrees Celsius. Further tests which will need to be carried out are discussed and the impact of the OGRE mission on future projects mentioned

Family history of Alzheimer's disease alters cognition and is modified by medical and genetic factors

In humans, a first-degree family history of dementia (FH) is a well-documented risk factor for Alzheimer's disease (AD); however, the influence of FH on cognition across the lifespan is poorly understood. To address this issue, we developed an internet-based paired-associates learning (PAL) task and tested 59,571 participants between the ages of 18-85. FH was associated with lower PAL performance in both sexes under 65 years old. Modifiers of this effect of FH on PAL performance included age, sex, education, and diabetes. The Apolipoprotein E epsilon 4 allele was also associated with lower PAL scores in FH positive individuals. Here we show, FH is associated with reduced PAL performance four decades before the typical onset of AD; additionally, several heritable and non-heritable modifiers of this effect were identified.Mueller Family Charitable Trust; Arizona Department of Health Services; National Institutes of Health [R01-AG041232, R01-AG049465-05]; Flinn FoundationOpen access journalThis item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]

Modeling and Inferring Cleavage Patterns in Proliferating Epithelia

The regulation of cleavage plane orientation is one of the key mechanisms driving epithelial morphogenesis. Still, many aspects of the relationship between local cleavage patterns and tissue-level properties remain poorly understood. Here we develop a topological model that simulates the dynamics of a 2D proliferating epithelium from generation to generation, enabling the exploration of a wide variety of biologically plausible cleavage patterns. We investigate a spectrum of models that incorporate the spatial impact of neighboring cells and the temporal influence of parent cells on the choice of cleavage plane. Our findings show that cleavage patterns generate “signature” equilibrium distributions of polygonal cell shapes. These signatures enable the inference of local cleavage parameters such as neighbor impact, maternal influence, and division symmetry from global observations of the distribution of cell shape. Applying these insights to the proliferating epithelia of five diverse organisms, we find that strong division symmetry and moderate neighbor/maternal influence are required to reproduce the predominance of hexagonal cells and low variability in cell shape seen empirically. Furthermore, we present two distinct cleavage pattern models, one stochastic and one deterministic, that can reproduce the empirical distribution of cell shapes. Although the proliferating epithelia of the five diverse organisms show a highly conserved cell shape distribution, there are multiple plausible cleavage patterns that can generate this distribution, and experimental evidence suggests that indeed plants and fruitflies use distinct division mechanisms

Optical Spectroscopy of Supernova 1993J During Its First 2500 Days

We present 42 low-resolution spectra of Supernova (SN) 1993J, our complete collection from the Lick and Keck Observatories, from day 3 after explosion to day 2454, as well as one Keck high-dispersion spectrum from day 383. SN 1993J began as an apparent SN II, albeit an unusual one. After a few weeks, a dramatic transition took place, as prominent helium lines emerged in the spectrum. SN 1993J had metamorphosed from a SN II to a SN IIb. Nebular spectra of SN 1993J closely resemble those of SNe Ib and Ic, but with a persistent H_alpha line. At very late times, the H_alpha emission line dominated the spectrum, but with an unusual, box-like profile. This is interpreted as an indication of circumstellar interaction.Comment: 19 pages plus 13 figures, AASTeX V5.0. One external table in AASTeX V4.0, in landscape format. Accepted for publication in A

Enhanced Stability and Activity for Water Oxidation in Alkaline Media with Bismuth Vanadate Photoelectrodes Modified with a Cobalt Oxide Catalytic Layer Produced by Atomic Layer Deposition

Atomic-layer deposition (ALD) of thin layers of cobalt oxide on n-type BiVO_4 produced photoanodes capable of water oxidation with essentially 100% faradaic efficiency in alkaline, pH = 13 electrolytes. By contrast, under the same operating conditions, BiVO_4 photoanodes without the Co oxide catalytic layer exhibited lower faradaic yields, of ca. 70%, for O_2 evolution and were unstable, becoming rapidly photopassivated. High numbers (>25) of ALD cycles of Co oxide deposition gave electrodes that displayed poor photoelectrochemical behavior, but 15–20 ALD cycles produced Co oxide overlayers ~1 nm in thickness, with the resulting photoelectrodes exhibiting a stable photocurrent density of 1.49 mA cm^(–2) at the oxygen-evolution potential and an open-circuit potential of 0.404 V versus the reversible hydrogen electrode, under 100 mW cm^(–2) of simulated air mass 1.5 illumination