Sensitivity of LAGEOS to changes in Earth's (2, 2) gravity coefficients

In recent years it has become of geophysical interest to detect a possible time variation of the low-degree coefficients of the Earth's gravity field, in particular the (2, 2) tesseral harmonic. We investigate the possibility of detecting such a phenomenon via analysis of the tracking data from LAGEOS, the passive geodynamics satellite tracked by laser stations on the ground. For this purpose the main problems are caused: (i) for short orbital arcs, by the irregular distribution in time of the tracking data, and by the dynamical effects of oceanic tides, which cannot be easily separated from the effects of geopotential changes; (ii) for long orbital arcs, by the difficulty of reliably predicting the variable drag-like force that is causing a slow semimajor axis decay. We estimate that a relative accuracy of the order of 10^–4 in the (2, 2) coefficients can be reached provided that a larger number of higher technology laser stations is available, and that better modelling is possible of the drag-like force. Both these conditions seem quite unrealistic at present. If a relative accuracy better than 10^–4 has to be reached, an effective separation of the tidal perturbations is also needed, since they give rise to perturbations with a similar signature.     

The motion of rotating spheroidal bodies in general relativity

The motion of two rotating spheroidal bodies, constituting the components of a binary system in a weak gravitational field, has been considered up to terms of the second order in the small parameterV/c, whereV denotes the velocity of the bodies andc is the velocity of light.The following simplifying assumptions, consistent with a problem of astronomical interest, have been made: (1) the dimensions of the bodies are small compared with their mutual distance; (2) the bodies consist of matter in the fluid state with internal hydrostatic pressure and their oblateness is due to their own rotation; (3) there exist axial symmetry about the axis of rotation and symmetry with respect to the equatorial plane, the same symmetry properties apply to mass densities and stress tensors.The Fock-Papapetrou method was used to ascertain those terms in the equations of motion which are due to the rotation and to the oblateness of each component. Approximate solutions to the Poisson and wave equations were obtained to express the potential and retarded potential at large distances from the bodies generating them. The explicit evaluation of certain integrals has necessitated the use of the Laplace-Clairaut theory for the equibrium configuration of rotating bodies. The final expressions require the knowledge of the mass density as a function of the mean radius of the equipotential surfaces.As an interpretation of the results, the Lagrangian perturbation equations were employed to evaluate the secular motion of the nodal line for the relative orbit of the two components. The results constitute a generalization of Fock's work and furnish the contribution of the mass distribution to the rotation effect of general relativity.     

Long term spectral variability in the soft gamma-ray repeater SGR 1900+14

We present a systematic analysis of all the BeppoSAX data of SGR1900+14. The observations spanning five years show that the source was brighter than usual on two occasions: ～20 days after the August 1998 giant flare and during the 10⁵?s long X-ray afterglow following the April 2001 intermediate flare. In the latter case, we explore the possibility of describing the observed short term spectral evolution only with a change of the temperature of the blackbody component. In the only BeppoSAX observation performed before the giant flare, the spectrum of the SGR1900+14 persistent emission was significantly harder and detected also above 10 keV with the PDS instrument. In the last BeppoSAX observation (April 2002) the flux was at least a factor 1.2 below the historical level, suggesting that the source was entering a quiescent period.     

NGC 3741: the dark halo profile from the most extended rotation curve

We present new H  i observations of the nearby dwarf galaxy NGC 3741. This galaxy has an extremely extended H  i disc, which allows us to trace the rotation curve out to unprecedented distances in terms of the optical disc: we reach 42 B -band exponential scalelengths or about 7 kpc. The H  i disc is strongly warped, but the warp is very symmetric. The distribution and kinematics are accurately derived by building model data cubes, which closely reproduce the observations. In order to account for the observed features in the data cube, radial motions of the order of 5–13 km s⁻¹ are needed. They are consistent with an inner bar of several hundreds of pc and accretion of material in the outer regions.
The observed rotation curve was decomposed into its stellar, gaseous and dark components. The Burkert dark halo (with a central constant density core) provides very good fits. The dark halo density distribution predicted by the Λ cold dark matter (CDM) theory fails to fit the data, unless NGC 3741 is a 2.5σ exception to the predicted relation between concentration parameter and virial mass and at the same time a high value of the virial mass (though poorly constrained) of  10¹¹ M_⊙  . Noticeably, modified Newtonian dynamics (MOND) seems to be consistent with the observed rotation curve. Scaling up the contribution of the gaseous disc also gives a good fit.     

Excursion set approach to the clustering of dark matter haloes in Lagrangian space

We present a stochastic approach to the spatial clustering of dark matter haloes in Lagrangian space. Our formalism is based on a local formulation of the 'excursion set' approach by Bond et al., which automatically accounts for the 'cloud-in-cloud' problem in the identification of bound systems. Our method allows us to calculate correlation functions of haloes in Lagrangian space using either a multidimensional Fokker–Planck equation with suitable boundary conditions, or an array of Langevin equations with spatially correlated random forces. We compare the results of our method with theoretical predictions for the halo autocorrelation function considered in the literature, and find good agreement with the results recently obtained within a treatment of halo clustering in terms of 'counting fields' by Catelan et al. Finally, the possible effect of spatial correlations on numerical simulations of halo merger trees is discussed.     

Galaxy formation and evolution: low-surface-brightness galaxies

We investigate in detail the hypothesis that low-surface-brightness galaxies (LSBs) differ from ordinary galaxies simply because they form in haloes with large spin parameters. We compute star formation rates using the Schmidt law, assuming the same gas infall dependence on surface density as used in models of the Milky Way. We build stellar population models, predicting colours, spectra and chemical abundances. We compare our predictions with observed values of metallicity and colours for LSBs, and find excellent agreement with all observables. In particular, integrated colours, colour gradients, surface brightness and metallicity match very well to the observed values of LSBs for models with ages larger than 7 Gyr and high values (λ > 0.05) for the spin parameter of the haloes. We also compute the global star formation rate (SFR) in the Universe due to LSBs, and show that it has a flatter evolution with redshift than the corresponding SFR for normal discs. We furthermore compare the evolution in redshift of [ Zn / H ] for our models to those observed in damped Lyman α systems by Pettini et al. and show that damped Lyman α system abundances are consistent with the predicted abundances at different radii for LSBs. Finally, we show how the required late redshift of collapse of the halo may constrain the power spectrum of fluctuations.     

Microflares and hot component in solar active regions

Open-shutter RHESSI observations of 3–15 keV X-rays are found to exhibit active-region transient brightenings and microflares at a rate of a least 10 per hour occurring even during the periods of lowest solar activity so far in the mission. A thermal component fitted by temperatures of 6–14 MK dominates from 3 keV to about 9 keV, but can be traced up to 14 keV in some cases, and has an average duration of 131(±103) s at 7–8 keV. The duration increases with decreasing photon energy. The peak count rate defined by cross-correlation is delayed at low energies. The temperature peaks early in the event and then decreases, whereas the emission measure increases throughout the event. The properties are consistent with thermal conduction dominating the evolution. In some of the bigger events, a second component was found in the 11–14 keV range extending down to 8 keV in some cases. The duration is typically 3 times shorter and ends near the peak time of the thermal component consistent with the Neupert effect of regular flares. Therefore the second component is suggested to be of non-thermal origin, presumably causing the beam-driven evaporation of the first component. The two components can be separated and analyzed in detail for the first time. Low-keV measurements allow a reliable estimate of the energy input by microflares necessary to assess their relevance for coronal heating. Supplementary material to this paper is available in electronic form at http://dx.doi.org/10.1023/A:1022496515506     

An astrophysical oscillator strength for the S ii 94.7-nm resonance line and S abundances in DLAs

By using UV spectra for the O star HD 93521 taken with the ORFEUS II echelle spectrograph, we determine an 'astrophysical' f value for the S  ii   λ 94.7-nm line: f =0.00498_−0.00138^+0.00172 , error at 1 σ level. This is almost a factor of 30 smaller than the guessed value found in the Kurucz data base (  f =0.1472) , which was until now the only one available for this transition. We use our 'astrophysical' f to investigate the S abundance in two damped Ly α absorption systems (DLAs) observed with the UV–Visual Echelle Spectrograph (UVES) at the European Southern Observatory's 8.2-m Kueyen telescope. In the case of the absorber at z _abs=3.02486 towards QSO 0347-3819, we find a sulphur column density which is consistent, within errors, with that determined by Centurión et al. by means of the λ 125.9-nm line, thus providing an external check on the accuracy of our f value. For the damped absorber at z _abs=4.4680 towards BR J0307-4945, we determine a high value of the S abundance, which, however, is probably the result of blending with Ly α forest lines.     

The composition of Alpine marine sediments (Bündnerschiefer Formation,W Alps) and the mobility of their chemical components during orogenic metamorphism

The Bündnerschiefer of the Swiss-Italian Alps is a large sedimentary complex deposited on the Piemonte–Liguria and Valais oceans and associated continental margins from the upper Jurassic to Eocene. It is made of a large variety of sequences associated or not with an ophiolitic basement. The Bündnerschiefer makes an accretionary prism that developed syn-tectonically from the onset of alpine subduction, and it records orogenic metamorphism following episodes of HP metamorphism. The Bündnerschiefer shares important similarities with the Otago schists of New Zealand and with the Wepawaug schists of Connecticut, both of which form accretionary prisms and have an orogenic metamorphic imprint.With the aim of testing the hypothesis of mobility of chemical components as a function of metamorphic grade, in this work I present fifty-five bulk chemical analyses of various lithological facies of the Bündnerschiefer collected along the well-studied field gradient of the Lepontine dome of Central Switzerland, in the Prättigau half window of East Switzerland, and in the Tsaté Nappe of Valle d'Aosta (Italy). The dataset includes the concentration of major components, large ion lithophile elements (Rb, Sr, Ba, Cs), high field strength elements (Zr, Ti, Nb, Th, U, Ta, Hf), fluid-mobile light elements (B, Li), volatiles (CO₂, S), REEs, and Y, V, Cr, Co, Sn, Pb, Cu, Zn, Tl, Sb, Be, and Au. These data are compared against the compositions of the global marine sediment reservoir, typical crustal reservoirs, and against the previously measured compositions of Otago and Wepawaug schists. Results reveal that, irrespective of their metamorphic evolution, the bulk chemical compositions of orogenic metasediments are characterized by mostly constant compositional ratios (e.g., K₂O/Al₂O₃, Ba/Al₂O₃, Sr/CaO, etc.), whose values in most cases are undistinguishable from those of actual marine sediments and other crustal reservoirs. For these rocks, only volatile concentrations decrease dramatically as a function of metamorphic temperature, and significant deviations from the reservoir signatures are evident for SiO₂, B, and Li. These results are interpreted as an indication of residual enrichment in the sediments, a process taking place during syn-metamorphic dehydration from the onset of metamorphism in a regime of chemical immobility. Residual enrichment increased the absolute concentrations of the chemical components of these rocks, but did not modify significantly their fundamental ratios. This poor compositional modification of the sediments indicates that orogenic metamorphism in general does not promote significant mass transfer from accretionary prisms. In contrast, mass transfer calculations carried out in a shear zone crosscutting the Bündnerschiefer shows that significant mass transfer occurs within these narrow zones, resulting in gains of H₂O, SiO₂, Al₂O₃, K₂O, Ba, Y, Rb, Cu, V, Tl, Mo, and Ce during deformation and loss of Na₂O, CO₂, S, Ni, B, U, and Pb from the rock. These components were presumably transported by an aquo-carbonic fluid along the shear zone. These distinct attitudes to mobilize chemical elements from orogenic sediments may have implications for a potentially large number of geochemical processes in active continental margins, from the recycling of chemical components at plate margins to the genesis of hydrothermal ore deposits.     

A new macromodel for the assessment of the seismic response of infilled RC frames

Numerous research studies have proved that numerical models aiming at an accurate evaluation of the seismic response of RC framed buildings cannot ignore the inelastic behaviour of infills and the interaction between infill and frame elements. To limit the high computational burden of refined non-linear finite element models, in the latest decades, many researchers have developed simplified infill models by means of single or multiple strut-elements. These models are low time-consuming and thus adequate for static and dynamic analyses of multi-storey structures. However, their simulation of the seismic response is sometimes unsatisfying, particularly in the presence of infill walls with regular or (particularly) irregular distributions of openings. This paper presents a new 2D plane macro-element, which provides a refined simulation of the non-linear cyclic response of infilled framed structures at the expense of a limited computational cost. The macro-element consists of an articulated quadrilateral panel, a single 1D diagonal link, and eight 2D links and is able to model the shear and flexural behaviour of the infill and the non-linear flexural/sliding interaction between infill and surrounding frame. The proposed macro-element has been implemented into the open source software OpenSees and used to simulate the response of single-storey, single-span RC infilled frame prototypes tested by other authors. The above prototypes are selected as made of different masonry units and characterised by full or open geometric configuration.