Near Infrared Surface Properties of the Two Intrinsically Brightest Minor Planets (90377) Sedna and (90482) Orcus

We present low resolution K band spectra taken at the Gemini 8 meter telescope of (90377) Sedna and (90482) Orcus (provisional designations 2003 VB12 and 2004 DW, respectively), currently the two minor planets with the greatest absolute magnitudes (i.e. the two most reflective minor planets). We place crude limits on the surface composition of these two bodies using a Hapke model for a wide variety of assumed albedos. The unusual minor planet (90377) Sedna was discovered on November 14, 2003 UT at roughly 90 AU with 1.6 times the heliocentric distance and perihelion distance of any other bound minor planet. It is the first solar system object discovered between the Kuiper Belt and the Oort Cloud, and may represent a transition population between the two. The reflectance spectrum of (90377) Sedna appears largely featureless at the current signal-to-noise ratio, suggesting a surface likely to be highly processed by cosmic rays. For large grain models (100 micron to 1 cm) we find that (90377) Sedna must have less than 70% surface fraction of water ice and less than 60% surface fraction of methane ice to 3 sigma confidence. Minor planet (90482) Orcus shows strong water ice absorption corresponding to less than 50% surface fraction for grain models 25 micron and larger. Orcus cannot have more than 30% of its surface covered by large (100 mm to 1 cm) methane grains to 3 sigma confidence.Comment: Accepted for publication in the Astrophysical Journa

7-Li(p,n) Nuclear Data Library for Incident Proton Energies to 150 MeV

We describe evaluation methods that make use of experimental data, and nuclear model calculations, to develop an ENDF-formatted data library for the reaction p + Li7 for incident protons with energies up to 150 MeV. The important 7-Li(p,n_0) and 7-Li(p,n_1) reactions are evaluated from the experimental data, with their angular distributions represented using Lengendre polynomial expansions. The decay of the remaining reaction flux is estimated from GNASH nuclear model calculations. The evaluated ENDF-data are described in detail, and illustrated in numerous figures. We also illustrate the use of these data in a representative application by a radiation transport simulation with the code MCNPX.Comment: 11 pages, 8 figures, LaTeX, submitted to Proc. 2000 ANS/ENS International Meeting, Nuclear Applications of Accelerator Technology (AccApp00), November 12-16, Washington, DC, US

The Complex Wind Torus and Jets of PSR B1706-44

We report on Chandra ACIS imaging of the pulsar wind nebula (PWN) of the young Vela-like PSR B1706-44, which shows the now common pattern of an equatorial wind and polar jets. The structure is particularly rich, showing a relativistically boosted termination shock, jets with strong confinement, a surrounding radio/X-ray PWN and evidence for a quasi-static `bubble nebula'. The structures trace the pulsar spin geometry and illuminate its possible relation to SNR G343.1-2.3. We also obtain improved estimates of the pulsar flux and nebular spectrum, constraining the system age and energetics.Comment: To appear in the Astrophysical Journal. 15pp, 4 figures in 7 file

Thermoelastic Damping in Micro- and Nano-Mechanical Systems

The importance of thermoelastic damping as a fundamental dissipation mechanism for small-scale mechanical resonators is evaluated in light of recent efforts to design high-Q micrometer- and nanometer-scale electro-mechanical systems (MEMS and NEMS). The equations of linear thermoelasticity are used to give a simple derivation for thermoelastic damping of small flexural vibrations in thin beams. It is shown that Zener's well-known approximation by a Lorentzian with a single thermal relaxation time slightly deviates from the exact expression.Comment: 10 pages. Submitted to Phys. Rev.

Separating N2O production and consumption in intact agricultural soil cores at different moisture contents and depths

Agricultural soils are a major source of the potent greenhouse gas and ozone depleting substance, N2O. To implement management practices that minimise microbial N2O production and maximise its consumption (i.e. complete denitrification) we must understand the interplay between simultaneously occurring biological and physical processes, especially how this changes with soil depth. Meaningfully disentangling these processes is challenging and typical N2O flux measurement techniques provide little insight into subsurface mechanisms. Additionally, denitrification studies are often conducted on sieved soil in altered O2 environments which relate poorly to in situ field conditions. Here, we use a novel dual headspace system with field-relevant O2 concentrations to incubate intact sandy clay loam textured agricultural topsoil (0-10 cm) and subsoil (50-60 cm) cores for 3-4 d at 50 and 70% water filled pore space (WFPS), respectively. 15N-N2O pool dilution and an SF6 tracer were used to determine the relative diffusivity (Ds/D0) and the net N2O emission and gross N2O emission and consumption rates. The relationship between calculated fluxes from the below- and above- soil core headspaces confirmed that the system performed well. We found no difference in Ds/D0 between soil depth fractions, which was probably because of the preservation of preferential flow pathways in intact cores. Both gross N2O emissions and uptake were not different with depth but were higher in the 50% WFPS, contrary to expectation. We attribute this to aerobic denitrification and simultaneously occurring denitrification and nitrification for the gross consumption and emission of N2O, respectively. Here, we developed a novel system that allows careful control of conditions and, with a headspace below and above a soil core, a more realistic reconstruction of in situ gas dynamics. We provide further evidence of substantial N2O consumption in drier soil and without net negative N2O emissions. The results from this study are important for the future application of the 15N-N2O pool dilution method and N budgeting and modelling, as required for improving management to minimise N2O losses

An assessment of minimum sequence copy thresholds for identifying and reducing the prevalence of artefacts in dietary metabarcoding data

1. Metabarcoding provides a powerful tool for investigating biodiversity and trophic interactions, but the high sensitivity of this methodology makes it vulnerable to errors, resulting in artefacts in the final data. Metabarcoding studies thus often utilise minimum sequence copy thresholds (MSCTs) to remove artefacts that remain in datasets; however, there is no consensus on best practice for the use of MSCTs. 2. To mitigate erroneous reporting of results and inconsistencies, this study discusses and provides guidance for best-practice filtering of metabarcoding data for the ascertainment of conservative and accurate data. Several of the most commonly used MSCTs were applied to example datasets of Eurasian otter Lutra lutra and cereal crop spider (Araneae: Linyphiidae and Lycosidae) diets. 3. Changes in both the method and threshold value considerably affected the resultant data. Of the MSCTs tested, it was concluded that the optimal method for the examples given combined a sample-based threshold with removal of maximum taxon contamination, providing stringent filtering of artefacts while retaining target data. 4. Choice of threshold value differed between datasets due to variation in artefact abundance and sequencing depth, thus studies should employ controls (mock communities, negative controls with no DNA and unused MID tag combinations) to select threshold values appropriate for each individual study

The challenge of modelling nitrogen management at the field scale : simulation and sensitivity analysis of N2O fluxes across nine experimental sites using DailyDayCent

Peer reviewedPublisher PD

Majorana: from atomic and molecular, to nuclear physics

In the centennial of Ettore Majorana's birth (1906-1938?), we re-examine some aspects of his fundamental scientific production in atomic and molecular physics, including a not well known short communication. There, Majorana critically discusses Fermi's solution of the celebrated Thomas-Fermi equation for electron screening in atoms and positive ions. We argue that some of Majorana's seminal contributions in molecular physics already prelude to the idea of exchange interactions (or Heisenberg-Majorana forces) in his later workson theoretical nuclear physics. In all his papers, he tended to emphasize the symmetries at the basis of a physical problem, as well as the limitations, rather than the advantages, of the approximations of the method employed.Comment: to appear in Found. Phy

Global Research Alliance N2O chamber methodology guidelines: Recommendations for deployment and accounting for sources of variability

Adequately estimating soil nitrous oxide (N2O) emissions using static chambers is challenging due to the high spatial variability and episodic nature of these fluxes. This paper discusses how static chamber N2O experiments can be designed, and protocols implemented, to better account for this variability and reduce the uncertainty of N2O emission estimates. It is part of a series of papers in this special issue, each discussing a particular aspect of N2O chamber methodology. Aspects of experimental design and sampling affected by spatial variability include site selection, and chamber layout, size and areal coverage. Where used, treatment application adds a further level of spatial variability. Time of day, frequency and duration of sampling (both in terms of individual chamber closures and overall experiment duration) affect the temporal variability captured. In addition, we present best practice recommendations for experimental chamber installation and sampling protocols to minimise the introduction of further uncertainty. To obtain the best N2O emission estimates, resources should be allocated to minimise the overall uncertainty in line with experiment objectives. In some cases, this will mean prioritising individual flux measurements and increasing their accuracy and precision by, for example, collecting ≥4 headspace samples during each chamber closure. However, where N2O fluxes are exceptionally spatially variable, for example, in heterogeneous agricultural landscapes, such as uneven and woody grazed pastures, using available resources to deploy more chambers with fewer headspace samples per chamber may be beneficial. Similarly, for particularly episodic N2O fluxes, generated for example by irrigation or freeze-thaw cycles, increasing chamber sampling frequency will improve the accuracy and reduce the uncertainty of temporally interpolated N2O fluxe

Helen Chadwick’s ‘Composite Images’

This article traces the considerations of British artist Helen Chadwick (1953–1996) regarding ‘composite images’ and the potential liberation they opened up in the gap between image and form, surface and spectator. These will be discussed as the author follows two apparently contrasting trajectories of her thought; while her considerations of the image, and her own image-making, tend increasingly towards ‘pure surface’, her ambitions for spectatorial positioning and agency increase. In parallel, while the epistemological underpinnings of her thinking become increasingly complex and dynamic, the role of (self)portraiture in her work moves away from the portrayal of her own, and later the recognisably human, body. These trajectories can be mapped (roughly) onto particular projects, beginning with Ego Geometria Sum (1982–1984), developing through Of Mutability (1984–1986) where she first used the photocopier to produce ‘automatic images’ and into her light-based installations, such as Blood Hyphen (1988)