2,292 research outputs found
Detecting a rotation in the epsilon Eridani debris disc
The evidence for a rotation of the epsilon Eridani debris disc is examined.
Data at 850 micron wavelength were previously obtained using the Submillimetre
Common User Bolometer Array (SCUBA) over periods in 1997-1998 and 2000-2002. By
chi-square fitting after shift and rotation operations, images from these two
epochs were compared to recover proper motion and orbital motion of the disc.
The same procedures were then performed on simulated images to estimate the
accuracy of the results.
Minima in the chi-square plots indicate a motion of the disc of approximately
0.6'' per year in the direction of the star's proper motion. This
underestimates the true value of 1'' per year, implying that some of the
structure in the disc region is not associated with epsilon Eridani,
originating instead from background galaxies. From the chi-square fitting for
orbital motion, a counterclockwise rotation rate of ~2.75 degrees per year is
deduced. Comparisons with simulated data in which the disc is not rotating show
that noise and background galaxies result in approximately Gaussian
fluctuations with a standard deviation +/-1.5 degrees per year. Thus
counterclockwise rotation of disc features is supported at approximately a
2-sigma level, after a 4-year time difference. This rate is faster than the
Keplerian rate of 0.65 degrees per year for features at ~65 AU from the star,
suggesting their motion is tracking a planet inside the dust ring.
Future observations with SCUBA-2 can rule out no rotation of the epsilon
Eridani dust clumps with ~4-sigma confidence. Assuming a rate of about 2.75
degrees per year, the rotation of the features after a 10-year period could be
shown to be >1 degree per year at the 3-sigma level.Comment: 8 pages, 6 figure
Effect of pressure on the quantum spin ladder material IPA-CuCl3
Inelastic neutron scattering and bulk magnetic susceptibility studies of the
quantum S=1/2 spin ladder system IPA-CuCl3 are performed under hydrostatic
pressure. The pressure dependence of the spin gap is determined. At
GPa it is reduced to meV from meV at
ambient pressure. The results allow us to predict a soft-mode quantum phase
transition in this system at P GPa. The measurements are
complicated by a proximity of a structural phase transition that leads to a
deterioration of the sample.Comment: 5 pages, 4 figure
Neutron scattering study of a quasi-2D spin-1/2 dimer system Piperazinium Hexachlorodicuprate under hydrostatic pressure
We report inelastic neutron scattering study of a quasi-two-dimensional S=1/2
dimer system Piperazinium Hexachlorodicuprate under hydrostatic pressure. The
spin gap {\Delta} becomes softened with the increase of the hydrostatic
pressure up to P= 9.0 kbar. The observed threefold degenerate triplet
excitation at P= 6.0 kbar is consistent with the theoretical prediction and the
bandwidth of the dispersion relation is unaffected within the experimental
uncertainty. At P= 9.0 kbar the spin gap is reduced to 0.55 meV from 1.0 meV at
ambient pressure.Comment: 4 pages, 5 figure
Why do we make changes to the long-term experiments at Rothamsted?
The long-term field experiments at Rothamsted in south-east England (UK) are an important resource that has been used extensively to study the effects of land management, atmospheric pollution and climate change on soil fertility and the sustainability of crop yields. However, for these and other long-term experiments around the world to remain useful, changes are sometimes needed. These changes may be required to ensure that the experiment is not compromised by e.g. acidification or weeds, but often they are needed to ensure that the experiment remains relevant to current agricultural practice, e.g. the introduction of new cultivars and the judicious use of pesticides. However, changes should not be made just for the sake of change or to investigate aspects of management that could be better resolved in a short-term experiment. Rather, modifications should only be made after carefully considered discussion, involving scientists from different disciplines. It must be remembered however that there are limitations to what can be achieved in one experiment. In this paper we give examples of why certain changes were made to the Rothamsted experiments and what the results of those changes have been. We also highlight the value of archiving crop and soil samples for future studies
Lattice collapse and quenching of magnetism in CaFe2As2 under pressure: A single crystal neutron and x-ray diffraction investigation
Single crystal neutron and high-energy x-ray diffraction have identified the
phase lines corresponding to transitions between the ambient-pressure
tetragonal (T), the antiferromagnetic orthorhombic (O) and the non-magnetic
collapsed tetragonal (cT) phases of CaFe2As2. We find no evidence of additional
structures for pressures up to 2.5 GPa (at 300 K). Both the T-cT and O-cT
transitions exhibit significant hysteresis effects and we demonstrate that
coexistence of the O and cT phases can occur if a non-hydrostatic component of
pressure is present. Measurements of the magnetic diffraction peaks show no
change in the magnetic structure or ordered moment as a function of pressure in
the O phase and we find no evidence of magnetic ordering in the cT phase. Band
structure calculations show that the transition results in a strong decrease of
the iron 3d density of states at the Fermi energy, consistent with a loss of
the magnetic moment.Comment: List of authors in metadata and typos in labeling of inset in Fig.
1(a) corrected. One ref. added. No other change
Development of a modified SEDEX phosphorus speciation method for ancient rocks and modern iron-rich sediments
We report the development of a modified method for evaluating different reservoirs of sedimentary phosphorus (P) in ancient marine sedimentary rocks and in modern Fe-rich sediments. Utilising the existing SEDEX scheme for P partitioning in modern sediments, we initially demonstrate limitations in the application of the original scheme to sediments and rocks containing crystalline hematite and magnetite. We tested additional extractions for these crystalline Fe phases, using both synthetic minerals, and modern and ancient sediments. The addition of 6 h oxalate and 6 h citrate-dithionate-acetate extractions considerably enhanced the total recovery of synthetic magnetite and hematite to 88.7 ± 1.1% and 76.9 ± 3.8%, respectively. In addition, application of the 6 h oxalate extraction to synthetic P-containing magnetite recovered 93.9 ± 1.7% of the Fe present and 88.2 ± 12.8% of the co-precipitated P. Based upon these results we developed a modified SEDEX extraction scheme. The modified scheme was applied to modern Fe-rich sediments from Golfo Dulce, Costa Rica, which resulted in 16% higher Fe-bound P recovery. Application of the scheme to a variety of ancient marine rocks increased the recovery of Fe-bound P by up to 22%. We also highlight the potential for authigenic carbonate fluorapatite to convert to more crystalline apatite in ancient rocks during deep burial and metamorphism. We suggest that in such systems minimum and maximum estimates of the total reactive P pool may be calculated with and without the inclusion of crystalline P. It is noted that the application of the revised method may have important implications for understanding the cycling of P in ancient marine environments
High-frequency homogenization for periodic media
This article is available open access through the publisher’s website at the link below. Copyright @ 2010 The Royal Society.An asymptotic procedure based upon a two-scale approach is developed for wave propagation in a doubly periodic inhomogeneous medium with a characteristic length scale of microstructure far less than that of the macrostructure. In periodic media, there are frequencies for which standing waves, periodic with the period or double period of the cell, on the microscale emerge. These frequencies do not belong to the low-frequency range of validity covered by the classical homogenization theory, which motivates our use of the term ‘high-frequency homogenization’ when perturbing about these standing waves. The resulting long-wave equations are deduced only explicitly dependent upon the macroscale, with the microscale represented by integral quantities. These equations accurately reproduce the behaviour of the Bloch mode spectrum near the edges of the Brillouin zone, hence yielding an explicit way for homogenizing periodic media in the vicinity of ‘cell resonances’. The similarity of such model equations to high-frequency long wavelength asymptotics, for homogeneous acoustic and elastic waveguides, valid in the vicinities of thickness resonances is emphasized. Several illustrative examples are considered and show the efficacy of the developed techniques.NSERC (Canada) and the EPSRC
- …