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

Absence of magnetic long range order in Y2_{2}CrSbO7_{7}: bond-disorder induced magnetic frustration in a ferromagnetic pyrochlore

The consequences of nonmagnetic-ion dilution for the pyrochlore family Y$_{2}$($M_{1-x}N_{x}$)$_{2}$O$_{7}$ ($M$ = magnetic ion, $N$ = nonmagnetic ion) have been investigated. As a first step, we experimentally examine the magnetic properties of Y$_{2}$CrSbO$_{7}$ ($x$ = 0.5), in which the magnetic sites (Cr$^{3+}$) are percolative. Although the effective Cr-Cr spin exchange is ferromagnetic, as evidenced by a positive Curie-Weiss temperature, $\Theta_\mathrm{{CW}}$ = 20.1(6) K, our high-resolution neutron powder diffraction measurements detect no sign of magnetic long range order down to 2 K. In order to understand our observations, we performed numerical simulations to study the bond-disorder introduced by the ionic size mismatch between $M$ and $N$. Based on these simulations, bond-disorder ($x_{b}$ $\simeq$ 0.23) percolates well ahead of site-disorder ($x_{s}$ $\simeq$ 0.61). This model successfully reproduces the critical region (0.2 < $x$ < 0.25) for the N\'eel to spin glass phase transition in Zn(Cr$_{1-x}$Ga$_{x}$)$_{2}$O$_{4}$, where the Cr/Ga-sublattice forms the same corner-sharing tetrahedral network as the $M/N$-sublattice in Y$_{2}$($M_{1-x}N_{x}$)$_{2}$O$_{7}$, and the rapid drop in magnetically ordered moment in the N\'eel phase [Lee $et$ $al$, Phys. Rev. B 77, 014405 (2008)]. Our study stresses the nonnegligible role of bond-disorder on magnetic frustration, even in ferromagnets

Co-located wave and offshore wind farms: A preliminary approach to the shadow effect

In recent years, with the consolidation of offshore wind technology and the progress carried out for wave energy technology, the option of combine both technologies has arisen. This combination rest mainly in two main reasons: in one hand, to increase the sustainability of both energies by means of a more rational harnessing of the natural resources; in the other hand, to reduce the costs of both technologies by sharing some of the most important costs of an offshore project. In addition to these two powerful reasons there are a number of technology synergies between wave and wind systems which makes their combination even more suitable. Co-located projects are one of the alternatives to combine wave-wind systems, and it is specially for these project were so-called shadow effect synergy becomes meaningful. In particular, this paper deals with the co-location of Wave Energy Conversion (WEC) technologies into a conventional offshore wind farm. More specifically, an overtopping type of WEC technology was considered in this work to study the effects of its co-location with a conventional offshore wind park. This study aims to give a preliminary approach to the shadow effect and its implications for both wave and offshore wind energies

Predicting the frequencies of diverse exo-planetary systems

Extrasolar planetary systems range from hot Jupiters out to icy comet belts more distant than Pluto. We explain this diversity in a model where the mass of solids in the primordial circumstellar disk dictates the outcome. The star retains measures of the initial heavy-element (metal) abundance that can be used to map solid masses onto outcomes, and the frequencies of all classes are correctly predicted. The differing dependences on metallicity for forming massive planets and low-mass cometary bodies are also explained. By extrapolation, around two-thirds of stars have enough solids to form Earth-like planets, and a high rate is supported by the first detections of low-mass exo-planets.Comment: 5 pages, 2 figures; accepted by MNRA

Forming the first planetary systems: debris around Galactic thick disc stars

The thick disc contains stars formed within the first Gyr of Galactic history, and little is known about their planetary systems. The Spitzer MIPS instrument was used to search 11 of the closest of these old low-metal stars for circumstellar debris, as a signpost that bodies at least as large as planetesimals were formed. A total of 22 thick disc stars has now been observed, after including archival data, but dust is not found in any of the systems. The data rule out a high incidence of debris among star systems from early in the Galaxy's formation. However, some stars of this very old population do host giant planets, at possibly more than the general incidence among low-metal Sun-like stars. As the Solar System contains gas giants but little cometary dust, the thick disc could host analogue systems that formed many Gyr before the Sun.Comment: accepted by MNRAS Letters; 5 pages, 4 figure

The feasibility of sea surface temperature determination using satellite infrared data

Sea surface temperature determination feasibility using satellite infrared dat

Transience of hot dust around sun-like stars

There is currently debate over whether the dust content of planetary systems is stochastically regenerated or originates in planetesimal belts evolving in steady state. In this paper a simple model for the steady state evolution of debris disks due to collisions is developed and confronted with the properties of the emerging population of 7 sun-like stars that have hot dust <10AU. The model shows there is a maximum possible disk mass at a given age, since more massive primordial disks process their mass faster. The corresponding maximum dust luminosity is f_max=0.00016r^(7/3)/t_age. The majority (4/7) of the hot disks exceed this limit by >1000 and so cannot be the products of massive asteroid belts, rather the following systems must be undergoing transient events characterized by an unusually high dust content near the star: eta Corvi, HD69830, HD72905 and BD+20307. It is also shown that the hot dust cannot originate in a recent collision in an asteroid belt, since there is also a maximum rate at which collisions of sufficient magnitude to reproduce a given dust luminosity can occur. Further it is shown that the planetesimal belt feeding the dust in these systems must be located further from the star than the dust, typically at >2AU. Other notable properties of the 4 hot dust systems are: two also have a planetesimal belt at >10AU (eta Corvi and HD72905); one has 3 Neptune mass planets at <1AU (HD69830); all exhibit strong silicate features in the mid-IR. We consider the most likely origin for the dust in these systems to be a dynamical instability which scattered planetesimals inwards from a more distant planetesimal belt in an event akin to the Late Heavy Bombardment in our own system, the dust being released from such planetesimals in collisions and possibly also sublimation.Comment: 16 pages, accepted by ApJ, removed HD128400 as hot dust candidat

The replacement histone H2A.Z in a hyperacetylated form is a feature of active genes in the chicken

The replacement histone H2A.Z is variously reported as being linked to gene expression and preventing the spread of heterochromatin in yeast, or concentrated at heterochromatin in mammals. To resolve this apparent dichotomy, affinity-purified antibodies against the N-terminal region of H2A.Z, in both a triacetylatedandnon- acetylatedstate, areusedin native chromatin immmuno-precipitation experiments with mononucleosomes from three chicken cell types. The hyperacetylated species concentrates at the 50 end of active genes, both tissue specific and housekeeping but is absent from inactive genes, while the unacetylated form is absent from both active and inactive genes. A concentration of H2A.Z is also found at insulators under circumstances implying a link to barrier activity but not to enhancer blocking. Although acetylated H2A.Z is widespread throughout the interphase genome, at mitosis its acetylation is erased, the unmodified form remaining. Thus, although H2A.Z may operate as an epigenetic marker for active genes, its N-terminal acetylation does not

The importance of regional variation in the analysis of urbanization-agriculture interactions

La recherche géographique sur l'agriculture dans les régions métropolitaines a été orientée vers l'étude des changements agricoles influencés par l'urbanisation. D'autres processus d'évolution et les facteurs liés aux variations de l'environnement régional ont été négligés. Certaines recherches récentes faisant apparaître l'importance de l'environnement régional à des échelles géographiques différentes sont décrites. Premièrement, une typologie de régions basées sur les régions métropolitaines de recensement du Canada est présentée. Certains groupes de la typologie ont connu des changements agricoles importants qui ne s'expliquent pas par les pressions du développement métropolitain. Des différences régionales dans l'environnement agricole apportent des explications partielles. Deuxièmement, pour la région de Montréal des variables agricoles (de 1961 à 1971) sont analysées avec une analyse factorielle. Les résultats sont interprétés en termes a) de l'urbanisation et b) des variations dans l'environnement à l'intérieur de la région. En dernier lieu, pour une municipalité située près de Toronto, une analyse est faite de la répartition géographique du morcellement des parcelles cadastrales. Une fois encore, des liens sont apparents avec certaines caractéristiques de l'environnement. La conclusion est que les changements agricoles ne sont pas homogènes, soit entre régions, soit à l'intérieur d'une même région, et que l'explication devrait être formulée aussi bien en termes de la variation de l'environnement régional qu'en termes des influences métropolitaines.Research into agriculture in metropolitan regions has concentrated on urban-induced agricultural land use changes. Other processes of change and factors related to variations in the regional environment have been neglected. Some recent research is reported here which points to the importance of the regional environment at a variety of scale levels. First, a typology of regions based on Census Metropolitan Areas in Canada is developed. Some groups of regions experienced significant agricultural changes quite unrelated to metropolitan development pressures. Regional differences in the agricultural environment are suggested as partial explanations. Second, for the Montréal region, a series of agricultural variables (1961 to 1971) are analysed using factor analysis. Results are interpreted in the light of a) urbanisation forces and b) internal variation in the regional environment. Finally, for a township near Toronto, an investigation is made of the distribution of severances. Once more, relationships appear with certain physical characteristics. The paper concludes that agricultural change is not uniform either between regions or within regions, and that part of the variation is related to differences in the "regional" environment and part to metropolitan forces