Shear-free perfect fluids with a solenoidal electric curvature

We prove that the vorticity or the expansion vanishes for any shear-free perfect fluid solution of the Einstein field equations where the pressure satisfies a barotropic equation of state and the spatial divergence of the electric part of the Weyl tensor is zero.Comment: 9 page

Historische analyse van de Zeeschelde en haar getijgebonden zijrivieren: 19e eeuw tot heden

The River Scheldt currently has a macrotidal estuary with a tidal reach of 160km. It covers a complete salinity gradient with a polyhaline, mesohaline and oligohaline zone and a fresh water part with long and short retention time of chloride. The estuary and its tidal tributaries Durme and Rupel have been heavily influenced by anthropogenic pressures such as land reclamation, harbour expansion, dredging activities, embankments and urbanisation. A good understanding of the impact of human interventions on the ecological functioning of the estuarine ecosystem is required. Based on this knowledge appropriate compensation or mitigation measures can be taken in response or anticipation to future negative effects of anthropogenic changes and relative sea level rise. In this study, all available data on tidal regime (1850-2000), bathymetric charts (1930, 1950, 1960, 1970, 1980, 1990, 2000) historical maps from the beginning of the 19th century onwards and aerial photographs from 1944 onwards were compiled and analysed for the Zeeschelde (Belgian part of the Scheldt estuary) and the tidal tributary river sections.The report deals with three main issues:(1) Ecological changes during the past century are analysed using a hierarchical ecotope approach.The evolution of the acreage of the distinguished physiotopes (physically characterised units) andecotopes such as shallow waters, tidal mudflats and marshes are analysed for each salinity zone or river section;(2) Changes in hydrodynamic and morphological parameters are analysed over the past century;(3) An overview of anthropogenic interventions and natural changes with potential effects onhabitat acreage and quality in the Zeeschelde and its tidal tributaries in the past two centuries. Thenature of the interventions is very diverse: reclamations, diking, land use change, infrastructureworks, dredging activities, canalisations, discharge manipulations and changes in river dynamics.Changes in tidal regimeBecause tidal regime is a key factor in an estuarine ecosystem, we analysed the characteristics ofthe ten year mean high water (MHW), mean low water (MLW), tidal asymmetry and the tidalamplitude. Temporal and spatial patterns of these parameters are examined within differentsalinity zones in the Scheldt, the Durme and the Rupel.MHW shows a gradual increase in the whole Scheldt estuary. The highest rise is located in the freshwater part with short retention time. Maximum MHW shifted 32km upstream from Antwerp to Sint-Amands within one century.MLW and tidal asymmetry show a more irregular pattern and are more sensitive to anthropogenicactivities such as dredging and deepening in the navigation channel. MLW dropped in the pastcentury in the harbour zone of the Scheldt between the Dutch-Belgian border and the confluence ofthe Rupel and along the Rupel. In the fresh water zone with long retention time, MLW and tidalasymmetry changed little till 1970 (1st deepening of the Scheldt). After this period MLW strongly dropped as in most other river sections. MLW and tidal asymmetry in the fresh water zone with short retention time increased between 1900 and 1970 and were presumably affected by river discharge reductions and canalisations. After the 70’s MLW and tidal asymmetry decreased again. In the Durme MLW dropped at the confluence, more upstream it rised because of sedimentation in the riverbed. Discharge deviation, damming and channel straighthening played an important role here as well. At the same time tidal asymmetry increased in the whole Durme.MHW and MLW changes resulted in a high increase of tidal amplitude in the Scheldt and Rupel. In the River Scheldt the highest tidal amplitude was recorded at Lillo (62km to North Sea) around 1890, while in the last ten years the maximum occurs in Temse (98km to Sea); a shift of 36km upstream in a bit more than a century. In the Rupel the amplitude increased overall in time but less so in the upstream part. The Durme has an irreg

Jet triggered Type Ia supernovae in radio-galaxies?

We report the serendipitous discovery of a supernova (SN) in the nearby radio-galaxy 3C 78. Observations obtained with the STIS spectrograph on board the Hubble Space Telescope show, at a distance of 0.54 arcsec (300 pc) from the galaxy nucleus, a second bright source, not present in previous images. As this source was fortuitously covered by the spectrograph slit its spectrum was obtained and it is characteristic of a Type Ia SN. This SN is closely aligned with the radio-jet of 3C 78. Analysis of historical records shows that such a close association between jet and supernova occurred in 6 of the 14 reported SNe in radio-galaxies. The probability that this results from a random distribution of SN in the host galaxy is less than 0.05%. We then argue that jets might trigger supernova explosions.Comment: 9 pages, 3 figures, 1 table, to appear in ApJL, 20 Jul 200

Shearfree perfect fluids with solenoidal magnetic curvature and a gamma-law equation of state

We show that shearfree perfect fluids obeying an equation of state p=(gamma -1) mu are non-rotating or non-expanding under the assumption that the spatial divergence of the magnetic part of the Weyl tensor is zero.Comment: 11 page

Mergers of Globular Clusters

Globular clusters with composite color-magnitude diagrams, such as NGC 1851, NGC 2808 and Fornax No. 3, might have formed by mergers. It is suggested that each of these objects had two parent clusters, one with a red horizontal branch and another with a blue horizontal branch. Such mergers could have occurred if both ancestral objects were originally members of a single dwarf spheroidal galaxy in which the internal velocity dispersion was low.Comment: 10 pages, no figures. Also available at http://www.dao.nrc.ca/DAO/SCIENCE/science.htm

General non-rotating perfect-fluid solution with an abelian spacelike C_3 including only one isometry

The general solution for non-rotating perfect-fluid spacetimes admitting one Killing vector and two conformal (non-isometric) Killing vectors spanning an abelian three-dimensional conformal algebra (C_3) acting on spacelike hypersurfaces is presented. It is of Petrov type D; some properties of the family such as matter contents are given. This family turns out to be an extension of a solution recently given in \cite{SeS} using completely different methods. The family contains Friedman-Lema\^{\i}tre-Robertson-Walker particular cases and could be useful as a test for the different FLRW perturbation schemes. There are two very interesting limiting cases, one with a non-abelian G_2 and another with an abelian G_2 acting non-orthogonally transitively on spacelike surfaces and with the fluid velocity non-orthogonal to the group orbits. No examples are known to the authors in these classes.Comment: Submitted to GRG, Latex fil

Purely radiative perfect fluids

We study `purely radiative' (div E = div H = 0) and geodesic perfect fluids with non-constant pressure and show that the Bianchi class A perfect fluids can be uniquely characterized --modulo the class of purely electric and (pseudo-)spherically symmetric universes-- as those models for which the magnetic and electric part of the Weyl tensor and the shear are simultaneously diagonalizable. For the case of constant pressure the same conclusion holds provided one also assumes that the fluid is irrotational.Comment: 12 pages, minor grammatical change