Relativistic simulations of the phase-transition-induced collapse of neutron stars

An increase in the central density of a neutron star may trigger a phase transition from hadronic matter to deconfined quark matter in the core, causing it to collapse to a more compact hybrid star configuration. We present a study of this, building on previous work by Lin et al.. We follow them in considering a supersonic phase transition and using a simplified equation of state, but our calculations are general relativistic (using 2D simulations in the conformally flat approximation) as compared with their 3D Newtonian treatment. We also improved the treatment of the initial phase transformation, avoiding the introduction of artificial convection. As before, we find that the emitted gravitational wave spectrum is dominated by the fundamental quasi-radial and quadrupolar pulsation modes but the strain amplitudes are much smaller than suggested previously, which is disappointing for the detection prospects. However, we see significantly smaller damping and observe a non-linear mode resonance which substantially enhances the emission in some cases. We explain the damping mechanisms operating, giving a different view from the previous work. Finally, we discuss the detectability of the gravitational waves, showing that the signal-to-noise ratio for current or second generation interferometers could be high enough to detect such events in our Galaxy, although third generation detectors would be needed to observe them out to the Virgo cluster, which would be necessary for having a reasonable event rate.     

Magnetically tilted accretion disks

Accretion disks around magnetized, compact stars are expected to be tilted near their inner edges, due to the stresses exerted by the corotating magnetosphere of the inclined central rotator. We reassess numerically the results obtained analytically by Lipunovet al. (1981). Four qualitatively different situations occur, depending on the relative orientations of the outer accretion disk, the spin of the central rotator, and its magnetic dipole axis. In at least two of them, the inner part of the disk is expected to be decomposed into massive, magnetically confined clumps.     

Microphytobenthos: The ecological role of the “secret garden” of unvegetated,shallow-water marine habitats. I. Distribution,abundance and primary production

The microphytobenthos consists of unicellular eukaryotic algae and cyanobacteria that grow within the upper several millimeters of illuminated sediments, typically appearing only as a subtle brownish or greenish shading. The surficial layer of the sediment is a zone of intense microbial and geochemical activity and of considerable physical reworking. In many shallow ecosystems, the biomass of benthic microalgae often exceeds that of the phytoplankton in the overlying waters. Direct comparison of the abundance of benthic and suspended microalgae is complicated by the means used to measure biomass and by the vertical and horizontal distribution of the microphytobenthos in the sediment. Where biomass has been estimated as chlorophyll a, there may be negligible to large (40%) error due to interference by degradation products, except where chlorophyll is measured by high-performance liquid chromatography. The vertical distribution of microphytobenthos, aside from mat-forming species, is determined by the opposing effects of their vertical migration, which tends to concentrate them near the surface, and physical mixing by overlying currents, which tends to cause an even vertical distribution through the mixed layer of sediment. Uncertainties in vertical distribution are compounded by frequently patchy horizontal distribution. Under-sampling on small (<1 m) scales can lead to errors in the estimate that are comparable to the ranges of seasonal and geographic variation. These uncertainties are compounded by biases in the techniques used to estimate production by the microphytobenthos. In most environments studied, biomass (as chlorophyll a) and light availability appear to be the principal determinants of benthic primary production. The effect of variable light intensities on integral production can be described by a functional response curve. When normalized to the chlorophyll content of the surficial sediment, the residual variation in the data described by the functional response curve is due to changes in the chlorophyll-specific response to irradiance. Production by the benthos is often a significant fraction of production in the water column and microphytobenthos may contribute directly to water column production when they are resuspended. Thus on both the basis of biomass and biogeochemical reactivity, benthic microalgae play significant roles in system productivity and trophic dynamics, as well as such habitat characteristics as sediment stability. *** DIRECT SUPPORT *** A01BY074 00003     

Heat and moisture budgets from airborne measurements and high-resolution model simulations

High-resolution simulations with a mesoscale model are performed to estimate heat and moisture budgets of a well-mixed boundary layer. The model budgets are validated against energy budgets obtained from airborne measurements over heterogeneous terrain in Western Germany. Time rate of change, vertical divergence, and horizontal advection for an atmospheric column of air are estimated. Results show that the time trend of specific humidity exhibits some deficiencies, while the potential temperature trend is matched accurately. Furthermore, the simulated turbulent surface fluxes of sensible and latent heat are comparable to the measured fluxes, leading to similar values of the vertical divergence. The analysis of different horizontal model resolutions exhibits improved surface fluxes with increased resolution, a fact attributed to a reduced aggregation effect. Scale-interaction effects could be identified: while time trends and advection are strongly influenced by mesoscale forcing, the turbulent surface fluxes are mainly controlled by microscale processes.     

Ungaged inflow and loss patterns in urban and agricultural sub-reaches of the Logan River Observatory

Streams in semi-arid urban and agricultural environments are often heavily diverted for anthropogenic purposes. However, they simultaneously receive substantial inflows from a variety of ungaged sources including stormwater returns, tile drainage, and irrigation runoff that help sustain flow during dry periods. Due to the inability to identify sources or directly gage many of these inflows, there is a clear need for methods to understand source origination while quantifying potential gains and losses over highly impacted reaches. In the context of the Logan River Observatory, historical gage data illustrate the importance of ungaged and unidentified inflows on maintaining or enhancing flows in both urban and agricultural reaches containing large diversions. To understand the inflows in this portion of the Logan River, we first analysed water samples for ions collected from a subset of representative inflow sources and applied clustering analyses to establish inflow source classifications and associated ion concentration ranges. These representative concentration ranges, combined with mainstem flow and river ion samples taken at sub-reach scales, allow for the application of flow and mass balances to quantify inflow rates from different sources as well as any losses. These calculations demonstrate significant gains and losses occurring in many sub-reaches during three sampling events. The dominant land use (urban or agriculture) and flow regime at the time of sampling were the primary drivers of gains and losses. These exchanges were found to be most important below large diversions during low flow conditions. This highlights the need to classify inflow sources (urban or agriculture, surface or groundwater) and estimate their contributions to anticipate instream consequences of land use and water management decisions. As irrigation and water conveyance practices become more efficient, a portion of these ungaged inflows could be diminished or eliminated, thus further depleting streamflow during dry periods.     

The Pedagogical Benefits of Participatory GIS for Geographic Education

In this article, participatory GIS (PGIS) is explored and established as a powerful platform for geographic education. PGIS pedagogy can help educators meet diverse learning objectives pertaining to: (1) local knowledge and place-based thinking; (2) community engagement; (3) field mapping with geospatial technologies; (4) mixed-methods research; and (5) fostering of critical reflexivity in students. The discussion is supported with insights from multiple student-facilitated PGIS projects organized in rural Tanzania. There also is a thorough discussion of the challenges and caveats associated with involving students in PGIS projects, and a call for new research for assessing and advancing PGIS pedagogy.     

Mineralogy and possible origin of an unusual Cr-rich inclusion in the Los Martinez (L6) chondrite

Abstract— During a petrological study of the previously unclassified ordinary chondrite Los Martínez, we discovered a highly unusual Cr-rich inclusion which we believe is unique in both extraterrestrial and terrestrial mineralogy. The inclusion is highly zoned both compositionally and optically, with a Ca-Al rich, cloudy core and an opaque, Cr-Na-rich rim (up to 24 wt.% Cr₂O₃). Detailed SEM and TEM studies show that the inclusion now consists of a highly zoned, single crystal of plagioclase intergrown with chromium-rich spinel. The spinel has a well-developed crystallographic orientation relationship with the host plagioclase, which indicates that it is the product of exsolution. Although superficially similar to a plagioclase feldspar in composition, in detail the inclusion is Si-deficient and Al-enriched relative to a stoichiometric feldspar. We have not been able to identify a viable precursor mineral phase to the plagioclase-chromite intergrowth and suggest that it may be an unknown metastable phase. The Cr-rich precursors of the inclusion probably have close affinities to the chromite-plagioclase chondrules observed by Ramdohr (1967) in several ordinary chondrites. Based on the zoning in the inclusion, we suggest that it is the product of fractional crystallization from a melt, which may have formed as a liquid condensate, or by melting of solid condensates, in the solar nebula. Subsequent cooling of this melt condensate resulted in crystallization of the, as yet, unidentified phase. After crystallization, the inclusion was probably incorporated into a parent body where it underwent metamorphism and was probably shocked to some degree. During this period of parent body metamorphism, exsolution and decomposition of the unknown precursor occurred to produce the observed intergrowth of plagioclase and chromite. Finally, we have classified Los Martínez as an L6 ordinary chondrite breccia.