Resonant excitation of white dwarf oscillations in compact object binaries – I. The no back reaction approximation

We consider the evolution of white dwarfs with compact object companions (specifically black holes with masses up to  ∼10⁶  M_⊙  , neutron stars, and other white dwarfs). We suppose that the orbits are initially quite elliptical and then shrink and circularize under the action of gravitational radiation. During this evolution, the white dwarfs will pass through resonances when harmonics of the orbital frequency match the stellar oscillation eigenfrequencies. As a star passes through these resonances, the associated modes will be excited and can be driven to amplitudes that are so large that there is a back reaction on the orbit which, in turn, limits the growth of the modes. A formalism is presented for describing this dynamical interaction for a non-rotating star in the linear approximation when the orbit can be treated as non-relativistic. A semi-analytical expression is found for computing the resonant energy transfer as a function of stellar and orbital parameters for the regime where back reaction may be neglected. This is used to calculate the results of passage through a sequence of resonances for several hypothetical systems. It is found that the amplitude of the  ℓ= m = 2  f -mode can be driven into the non-linear regime for appropriate initial conditions. We also discuss where the no back reaction approximation is expected to fail, and the qualitative effects of back reaction.     

Natural and human-induced sinkhole hazards in Saudi Arabia: distribution,investigation, causes and impacts

Approximately 60 % of the 2,150,000 km² area of Saudi Arabia is underlain by soluble sediments (carbonate and evaporite rock formations, salt diapirs, sabkha deposits). Despite its hyper-arid climate, a wide variety of recent sinkholes have been reported in numerous areas, involving significant property losses. Human activities, most notably groundwater extraction, have induced unstable conditions on pre-existing cavities. This work provides an overview of the sinkhole hazard in Saudi Arabia, a scarcely explored topic. It identifies the main karst formations and the distribution of the most problematic sinkhole areas, illustrated through several case studies covering the wide spectrum of subsidence mechanisms. Some of the main investigation methods are presented through selected examples, including remote sensing, trenching and geophysics. Based on the available data, the main causal factors are identified and further actions that should be undertaken to better assess and manage the risk are discussed.     

Assessment of Coastal Inundation due to Sea-Level Rise along the Mediterranean Coast of Egypt

If the rising sea level due to climate change proceeds in the future with the rate observed in the past four decades, it could inundate some coastal lowlands. The aim of this paper is to assess future risk of sea-level rise (SLR) on the Nile delta of Egypt located along the Mediterranean Sea. Digital Elevation Models (DEMs) are verified, against ground control points, and used to identify areas susceptible to inundation due to future SLR. Analysis of DEMs maps and cross-shore profiles has identified locations that are vulnerable to SLR including coastal wetlands, agriculture areas, and urban neighborhoods. The results have revealed that about 7% of the Nile delta area is at risk of inundation due to future SLR. This information could be used by coastal zone managers in planning and protection of coastal areas.     

Rudists (Bivalvia) from a Cretaceous Platform in Northern Egypt:Taxonomy and Paleobiogeography

New well-preserved rudist materials come from the Barremian–Turonian marine formations in the Yelleg, Minsherah, Maaza, and Raghawi sections in North Sinai, northern Egypt. There, 17 discovered rudist species belonging to 13 genera and seven taxonomic families are described in detail as follows: Eoradiolites plicatus (Conrad), Eoradiolites liratus (Conrad), Praeradiolites ponsianus (?Archiac), Archaeoradiolites sp., Bournonia africana Douvillé, Bournonia fourtaui Douvillé, Biradiolites lombricalis (?Orbigny), Biradiolites zumoffeni Douvillé, Radiolites lewyi lewyi Parnes, Radiolites sauvagesi (?Hombres-Firmas), Durania arnaudi (Choffat), Toucasia carinata (Matheron), Toucasia sp., Neocaprina raghawiensis Steuber and Bachmann, Sellaea sp., Ichthyosarcolites sp. and Horoiopleura sp.. The Cenomanian deposits in the northern Eastern desert of Egypt only contain E. liratus. The domination of the Cenomanian rudist species in North Sinai, however, is attributed to changes in the platform, which passes mainly from carbonate in the north to siliciclastics in the south. The presence of Horiopleura sp. in the late Barremian–early Aptian deposits is documented for the first time, which suggests the expansion of taxa of the genus Horiopleura Douvillé to North Sinai during this interval. The age of the rudist species is documented from the late Barremian–Turonian formations, with their geographic distribution in the Mediterranean region considered..