Fractal Structures Driven by Self-gravity: Molecular Clouds and the Universe

In the interstellar medium, as well as in the Universe, large density fluctuations are observed, that obey power-law density distributions and correlation functions. These structures are hierarchical, chaotic, turbulent, but are also self-organizing. The apparent disorder is not random noise, but can be described by a fractal, with a deterministic fractal dimension. We present a new theory of the self-gravity thermodynamics, that could explain the existence of these fractal structures, and predict their fractal dimension. The media obeying scaling laws can be considered critical, as in second order phase transitions for instance. This revised version was published online in July 2006 with corrections to the Cover Date.     

Variations of the stratospheric temperature along the limb of Uranus: Results of the 22 April 1982 stellar occultation

Stratospheric temperature profiles of Uranus were derived from the stellar occultation of 22 April 1982 in the pressure range 5–30 μbar. The observations were made at the European Southern Observatory, Chile, and at the Observatoire du Pic du Midi et de Toulouse, France with two telescopes in both sites. The study of these profiles confirms that Uranus' stratosphere is warmer than had been expected from radiative models (J. F. Appleby, 1980, Atmospheric Structures of the Giant Planets from Radiative-Convective Equilibrium Models. PhD. Thesis, State University of New York at Stony Brook) and that there has been a general increase of temperature since 1977 (R. G. French, J. L. Elliot, E. W. Dunham, D. A. Allen, J. H. Elias, J. A. Frogel, and W. Liller, 1983, Icarus53, 399–414). Furthermore, the profiles exhibit a nonisothermal feature with a maximum temperature around the 8-μbar pressure level. The amplitude of this feature increases linearly with the diurnally averaged insolation 〈D〉 up to the observed value 〈D〉 ～ 0.15. Moreover, the temperature at 8 μbar, as well as the mean stratospheric temperature, reaches a plateau around the equator of the planet which is far from maximum insolation. For a nominal abundance of methane η_CH4 ～ 3 × 10^?5 and normal incidence, the UV absorption could compete with the IR methane absorption bands at the pressure level 8 μbar. However, the high temperatures observed even at grazing incidence imply important circulation phenomena to isothermalize distant regions of the planet. Alternatively, the observed profiles may suggest that an optically thin aerosol layer distributed over one scale height is responsible for the temperature maximum at 8 μbar. The total mass of dust necessary to heat this region up significantly would be a small fraction (6 × 10¹⁰ g vs 5 × 10¹⁸ g) of the Uranian ring system, which appears then as a possible reservoir of dust. However, a falling rate of ～1 msec^?1 would deplete the rings in a short time (≈2 × 10⁵ years) so that a dynamical process is needed to sustain the aerosol layer.     

A study of the 3ν3CH4 region in a high-resolution spectrum of Jupiter recorded by Fourier transform spectroscopy

A spectrum of Jupiter between 6000 and 12 000 cm^? at high resolution (0.05 cm^?) was recorded with a Michelson interferometer at Palomar Mountain in October 1974. An analysis of the R branch of the 3ν₃CH₄ band with the reflecting-layer model, taking into account the H₂ absorption which occurs in the same spectral range, leads to a Lorentzian half-width of 0.09 ± 0.02 cm^?1, a rotational temperature of 175 ± 10° K, and a CH₄ abundance of order 52m atm. Five lines of the ¹³CH₄3ν₃ band have been identified; a comparison with new laboratory spectra indicates that the ¹³CH₄/¹²CH₄ ratio in the Jupiter atmosphere is close to the terrestrial ratio.     

Active faulting in the western Pyrénées (France): Paleoseismic evidence for late Holocene ruptures

We have identified a 50-km-long active fault scarp, called herewith the Lourdes Fault, between the city of Lourdes and Arette village in the French Pyrénées. This region was affected by large and moderate earthquakes in 1660 (Io = VIII–IX, MSK 64,), in 1750 (Io = VIII, MSK 64) and in 1967 (M_d = 5.3, Io = VIII, MSK 64). Most earthquakes in this area are shallow and the few available focal mechanism solutions do not indicate a consistent pattern of active deformation. Field investigations in active tectonics indicate an East–West trending and up to 50-m-high fault scarp, in average, made of 3 contiguous linear fault sub-segments. To the north, the fault controls Quaternary basins and shows uplifted and tilted alluvial terraces. Deviated and abandoned stream channels of the southern block are likely due to the successive uplift of the northern block of the fault. Paleoseismic investigations coupled with geomorphic studies, georadar prospecting and trenching along the fault scarp illustrate the cumulative fault movements during the late Holocene. Trenches exhibit shear contacts with flexural slip faulting and thrust ruptures showing deformed alluvial units in buried channels. ¹⁴C dating of alluvial and colluvial units indicates a consistent age bracket from two different trenches and shows that the most recent fault movements occurred between 4221 BC and 2918 BC. Fault parameters and paleoseismic results imply that the Lourdes Fault and related sub-segments may produce a M_W 6.5 to 7.1 earthquake. Fault parameters imply that the Lourdes Fault segment corresponds to a major seismic source in the western Pyrénées that may generate earthquakes possibly larger than the 1660 historical event.     

Structure of Mn and Fe oxides and oxyhydroxides: A topological approach by EXAFS

The structure of Mn and Fe oxides and oxyhydroxides has been probed by EXAFS. It is shown that EXAFS spectroscopy is sensitive to the nature of interpolyhedral linkages relying on metal-two nearest metal distances. Spectra recorded at 290 K and 30 K indicate that intercationic distances can be determined by EXAFS with a good accuracy (0.02 Å) assuming a purely Gaussian distribution function, even at room temperature. Although the accuracy on atomic numbers determination is fair for these disordered systems, EXAFS can differentiate structures with contrasted edge- over corner-sharing ratio like pyrolusite, ramsdellite, todorokite and lithiophorite or lepidocrocite and goethite. A direct application of this result has shown that the proportion of pyrolusite domains within the lattice of nsutite from Ghana is equal to 35±15 percent. The systematic study of Mn dioxides also put forward the sensitivity of EXAFS to the presence of corner-sharing octahedra, with a detection limit found to be less than 8 percent. In spite of their similar XRD patterns, the EXAFS study of todorokite and asbolane confirms that they possess a distinct structure; that is, a tunnel structure for the former and a layered structure for the second. Such a topological approach has been used to probe the structure of ferruginous vernadite; a highly disordered iron-bearing Mn oxide. Fe and Mn K-edges EXAFS spectra are very dissimilar, traducing a different short range order. The Mn phase is constituted by MnO₂ layers. Its large local structural order contrasts with the short range disorder of the iron phase. This hydrous Fe oxyhydroxide is constituted by face-, edge- and corner-sharing octahedra. This iron phase possesses the same local order as feroxy-hyte, but is long range disordered. The presence of face-sharing Fe(O,OH)₆ octahedra prevents its direct solid-state transformation into well crystallized oxyhydroxides, and explains the necessary dissolution-reprecipitation mechanism generally invoked for the hydrous ferric gel → goethite transformation.     

Eddy properties in the Southern California Current System

The California Current System (CCS) is an eastern boundary upwelling system characterized by strong eddies that are often generated at the coast. These eddies contribute to intense, long-distance cross-shelf transport of upwelled water with enhanced biological activity. However, the mechanisms of formation of such coastal eddies, and more importantly their capacity to trap and transport tracers, are poorly understood. Their unpredictability and strong dynamics leave us with an incomplete picture of the physical and biological processes at work, their effects on coastal export, lateral water exchange among eddies and their surrounding waters, and how long and how far these eddies remain coherent structures. Focusing our analysis on the southern part of the CCS, we find a predominance of cyclonic eddies, with a 25-km radius and a SSH amplitude of 6 cm. They are formed near shore and travel slightly northwest offshore for ~?190 days at ~?2 km day^?1. We then study one particular, representative cyclonic eddy using a combined Lagrangian and Eulerian numerical approach to characterize its kinematics. Formed near shore, this eddy trapped a core made up of ~?67% California Current waters and ~?33% California Undercurrent waters. This core was surrounded by other waters while the eddy detached from the coast, leaving the oldest waters at the eddy’s core and the younger waters toward the edge. The eddy traveled several months as a coherent structure, with only limited lateral exchange within the eddy.     

Mineralogical confirmation of a near-P:Fe = 1:2 limiting stoichiometric ratio in colloidal P-bearing ferrihydrite-like hydrous ferric oxide

We present a chemical and mineralogical explanation, derived from powder X-ray diffraction and Mössbauer spectroscopy measurements of synthetic samples, of the P:Fe = 1:2 limiting ratio of P incorporation (as PO₄) that was previously observed in natural aquatic oxic iron precipitates. The ⁵⁷Fe Mössbauer hyperfine parameters are interpreted with the help of state-of-the-art ab initio electronic structure calculations. We find that there is a strong tendency for solid solution P-Fe mixing in the P-bearing hydrous ferric oxide (P-HFO) aqueous coprecipitate system, interpreted as occurring between the P-free (ferrihydrite) end-member and an inferred P:Fe = 1:2 end-member beyond which P is not incorporated in the structure of the P-HFO solid. Up to and somewhat beyond the limiting end-member P:Fe ratio, all available P is scavenged by the coprecipitation reaction, suggesting strong P-Fe complexation in the precipitation-precursor dissolved species. The P-HFO solids are more stable (i.e., have stronger chemical bonds) than the P-free ferrihydrite end-member. We show that in coprecipitation the P specifically incorporates within the nanoparticle structure rather than complexing to the nanoparticle surface. Our results are relevant to the question of the mechanisms of coupling between the Fe and P cycles in natural aqueous environments and highlight a strong affinity between Fe and P in aqueous environments.     

Molecular absorptions in high-z objects

Molecular absorption lines measured along the line of sight of distant quasars are important probes of the gas evolution in galaxies as a function of redshift. A review is made of the handful of molecular absorbing systems studied so far, with the present sensitivity of mm instruments. They produce information on the chemistry of the ISM at z～1, the physical state of the gas, in terms of clumpiness, density and temperature. The CMB temperature can be derived as a function of z, and also any possible variations of fundamental constants can be constrained. With the sensitivity of ALMA, many more absorbing systems can be studied, for which some predictions and perspectives are described.     

Chemodynamical evolution of interacting galaxies

We have undertaken numerical simulations of galaxy interactions and mergers, coupling the dynamics with the star formation history and the chemical evolution. The self-gravity of stars and gas is taken into account through a tree-code algorithm, the gas hydrodynamics through SPH, and an empirical law such as a local Schmidt law is used to compute star formation. The gas and stellar metallicity is computed at each position, according to assumed yields, and the dust amount is monitored. At each step the spectra of galaxies are computed, according to simple radiative transfer and dust models. Initial conditions for these simulations will be taken from a large-scale cosmological frame-work. The aim is to build a statistically significant library of merger histories. The first results of the project will be discussed, in particular on predictions about galaxy surveys at high redshift. This revised version was published online in September 2006 with corrections to the Cover Date.