The intracluster iron distribution around 4C+55.16

We report on the metal distribution in the intracluster medium around the radio galaxy 4C+55.16     observed with the Chandra X-ray Observatory . The radial metallicity profile shows a dramatic change at 10 arcsec (∼50 kpc) in radius from half solar to twice solar at inner radii. Also found was a plume-like feature located at ∼3 arcsec to the south-west of the centre of the galaxy, which is mostly accounted for by a strong enhancement of iron L emission. The X-ray spectrum of the plume is characterized by the metal abundance pattern of Type Ia supernovae (SNeIa), i.e. large ratios of Fe to α elements, with the iron metallicity being unusually high at     solar (90 per cent error). How the plume has been formed is not entirely clear. The inhomogeneous iron distribution suggested in this cluster provides important clues to understanding the metal enrichment process of the cluster medium.     

Porous interstellar grains

Recent observations of stellar composition suggest that elements in the Sun are significantly more abundant than in other stars. The reduction in the available element budget implies a drastic revision in current models of interstellar dust. Theoretical models are therefore exploring fluffy, porous physical structure for the grain material. Since a detailed exact treatment of extinction cross-sections is mandatory for a correct understanding of the nature of interstellar dust, we present a technique based on the multipole expansions of the electromagnetic field, which has proven to be general, flexible and powerful in treating scattering of light by porous, composite, arbitrarily shaped particles. The results of this study speak in favour of core–mantle structures characterized by the presence of porosities.     

Deforestation and leaching of nitrogen as nitrates into underground water in intertropical zones: The example of Côte d'lvoire

High nitrate contents (up to 200 mgl) have been observed in wells drilled into fractured aquifers lying beneath layers of weathered and decayed rock in the humid tropics where annual rainfall is over 1000 mm and where there is no notable pollution. The source of the nitrates is linked mainly with localized deforestation by man.     

Dissolution and depletion of ferromagnesian minerals from Holocene tephra layers in an acid bog,New Zealand,and implications for tephra correlation

This study examines the depletion of ferromagnesian silicate minerals from a sequence of thin, distal, mainly rhyolitic tephra layers of Holocene age preserved in an acid peat bog (Kopouatai), North Island, New Zealand. The rate of such depletion has been fast, as indicated by the complete loss of biotite from one tephra layer (Kaharoa Tephra), in which it is normally dominant, in only ca. 770 yr. Chemical dissolution is advocated as the likely cause for the depletion, with amphiboles and other mineral grains commonly showing etch pits, microcaves, and other characteristic surface solution features. Theoretical thermodynamic and kinetic models show a marked increase in the rate of dissolution of all ferromagnesian minerals under conditions of low pH (< 4), but that where silica concentrations in solution are high the relative proportions of minerals remaining are unaffected. However, where concentrations of dissolved silica are low, as in most bog environments, the relative proportions of ferromagnesian minerals are affected as well as absolute amounts being decreased. Amphiboles are depleted relative to pyroxenes, consistent with kinetic studies. The results show that the identification and correlation of tephras on the basis of relative abundances of ferromagnesian minerals alone may be unreliable, and emphasise the need to use multiple criteria in such studies.     

Response of circular footings and anchor plates in non-homogeneous elastic soils

The axisymmetric elastic response of circular footings and anchor plates in a linearly non-homogeneous elastic soil is analysed. It is assumed that footings/anchors are flexible and subjected to axisymmetric vertical loads. The response of the footing/anchor is modelled by using the classical Poisson–Kirchhoff thin plate theory. A variational technique is used to analyse the interaction problem. A representation for the contact stress is established by using a fundamental solution corresponding to a unit vertical pressure acting over an annular region in the interior of the non-homogeneous soil. The fundamental solution can be derived by using rigorous analytical procedures. The influence of the footing flexibility and the degree of soil non-homogeneity on the displacements, bending moments and contact stresses of a surface footing is examined over a wide range of governing parameters. In the case of anchor plates the influence of depth of embedment, degree of soil non-homogeneity and anchor flexibility on the anchor displacement is investigated.     

Real-time difference imaging analysis of MOA Galactic bulge observations during 2000

We describe observations carried out by the MOA group of the Galactic bulge during 2000 that were designed to detect efficiently gravitational microlensing of faint stars in which the magnification is high and/or of short duration. These events are particularly useful for studies of extrasolar planets and faint stars. Approximately 17 deg² were monitored at a sampling rate of up to six times per night. The images were analysed in real time using a difference imaging technique. 20 microlensing candidates were detected, of which eight were alerted to the microlensing community whilst in progress. Approximately half of the candidates had high magnifications (≳10), at least one had very high magnification (≳50), and one exhibited a clear parallax effect. The details of these events are reported here, together with details of the on-line difference imaging technique. Some nova-like events were also observed and these are described, together with one asteroid.     

A ROSAT WFC observation of SW UMa: the EUV behaviour of dwarf novae in superoutburst explained

During re-processing and analysis of the entire ROSAT Wide Field Camera (WFC) pointed observations data base, we discovered a serendipitous, off-axis detection of the cataclysmic variable SW UMa at the onset of its 1997 October superoutburst. Although long outbursts in this SU UMa-type system are known to occur every ∼ 450 d, none had ever been previously observed in the extreme ultra-violet (EUV) by ROSAT . The WFC observations began just ≈13 hr after the optical rise was detected. With a peak count rate of ∼ 4.5 count s⁻¹ in the S1 filter, SW UMa was temporarily the third brightest object in the sky in this waveband. Over the next ≈19 hr the measured EUV flux dropped to < 2 count s⁻¹, while the optical brightness remained essentially static at m _v∼11 . Similar behaviour has also been recently reported in the EUV light curve of the related SU UMa-type binary OY Car during superoutburst, as reported by Mauche & Raymond. In contrast, U Gem-type dwarf novae show no such early EUV dip during normal outbursts. Therefore, this feature may be common in superoutbursts of SU UMa-like systems. We expand on ideas first put forward by Osaki and Mauche & Raymond and offer an explanation for this behaviour by examining the interplay between the thermal and tidal instabilities that affect the accretion discs in these systems.     

Seismology of stellar envelopes: probing the outer layers of a star through the scattering of acoustic waves

The outer layers of Sun-like stars are regions of rapid spatial variation which modulate the p-mode frequencies by partially reflecting the constituent acoustic waves. With the accuracy that has been achieved by current solar observations, and that is expected from imminent stellar observations, this modulation can be observed from the spectra of the low-degree modes. We present a new and simple theoretical calculation to determine the leading terms in an asymptotic expansion of the outer phase of these modes, which is determined by the structure of the surface layers of the star. Our procedure is to compare the stellar envelope with a plane-parallel polytropic envelope, which we regard as a smooth reference background state. Then we can isolate a seismic signature of the acoustic phase and relate it to the stratification of the outer layers of the convection zone. One can thereby constrain theories of convection that are used to construct the convection zones of the Sun and Sun-like stars. The accuracy of the diagnostic is tested in the solar case by comparing the predicted outer phase with an exact numerical calculation.     

Time signatures of impulsively generated waves in a coronal plasma

Impulsively generated waves in solar coronal loops are numerically simulated in the frame-work of cold magnetohydrodynamics. Coronal inhomogeneities are approximated by gas density slabs embedded in a uniform magnetic field. The simulations show that an initially excited pulse results in the propagation of wave packets which correspond to both trapped and leaky waves. Whereas the leaky waves propagate outside the slab, the trapped waves occur as a result of a total reflection from the slab walls. Time signatures of these waves are made by a detection of the trapped waves at a fixed spatial location. For waves excited within the slab, time signatures exhibit periodic, quasi-periodic and decay phases. The time signatures for waves excited outside the slab, or for a multi-series of variously shaped impulses generated at different places and times, can possess extended quasi-periodic phases. The case of parallel slabs, when the presence of a second slab influences the character of wave propagation in the first slab, exhibits complex time signatures as a result of solitary waves interaction.