Prospects for the characterisation of exo-zodiacal dust with the VLTI

Exo-zodiacal dust, exozodi for short, is warm (～300 K) or hot (up to ～2000 K) dust found in the inner regions of planetary systems around main sequence stars. In analogy to our own zodiacal dust, it may be located in or near the habitable zone or closer in, down to the dust sublimation distance. The study of the properties, distribution, and evolution of exozodis can inform about the architecture and dynamics of the innermost regions of planetary systems, close to their habitable zones. On the other hand, the presence of large amounts of exo-zodiacal dust may be an obstacle for future space missions aiming to image Earth-like exoplanets. The dust can be the most luminous component of extrasolar planetary systems, but predominantly emits in the near- to mid-infrared where it is outshone by the host star. Interferometry provides a unique method of separating the dusty from the stellar emission. We discuss the prospects of exozodi observations with the next generation VLTI instruments and summarize critical instrument specifications.     

Study on the Effects of Waves and Dispersant on the Submergence of Spilled Oil

Water Resources - The baffled flasks were used to simulate regular wave and breaking wave at different oscillation intensities to study the effects of wave strength, reaction time, and dispersant...     

The Preflight Photometric Calibration of the Extreme-Ultraviolet Imaging Telescope EIT

This paper presents the preflight photometric calibration of the Extreme-ultraviolet Imaging Telescope (EIT) aboard the Solar and Heliospheric Observatory (SOHO). The EIT consists of a Ritchey–Chrétien telescope with multilayer coatings applied to four quadrants of the primary and secondary mirrors, several filters and a backside-thinned CCD detector. The quadrants of the EIT optics were used to observe the Sun in 4 wavelength bands that peak near 171, 195, 284, and 304 Å. Before the launch of SOHO, the EIT mirror reflectivities, the filter transmissivities and the CCD quantum efficiency were measured and these values are described here. The instrumental throughput in terms of an effective area is presented for each of the various mirror quadrant and filter wheel combinations. The response to a coronal plasma as a function of temperature is also determined and the expected count rates are compared to the count rates observed in a coronal hole, the quiet Sun and an active region.     

Detailed Study of Three Contiguous Segments of the Mid-Atlantic Ridge,South of the Azores (37° N to 38°30′ N), Using Acoustic Imaging Coupled with Submersible Observations

Using a new tool of seafloor characterisation (sonar images from FARA-SIGMA cruise; Needham et al., 1992), coupled with submersible observations (DIVA1 cruise) we compare, at different scales of observation, three contiguous segments of the Mid-Atlantic Ridge, South of the Azores Triple Junction, between 37° N and 38°30 N.The two northernmost segments (38°20 N and Menez-Gwen) show unusual morphological features for the MAR; the rift valley is absent and the present-day magmatism is focused on shallow axial volcanoes. On the third segment (Lucky Strike), the morphology is the one usually found on the MAR. On the Menez-Gwen and 38°20 N segments, volcanic constructional activity can obliterate, during periods of high magmatic supply, the morphology inherited from tectonic activity. The dive results constrain the recent evolution of each segment and show that a temporal variability in volcanic dynamics exists. On the three segments, outcrops of eruptive lavas alternate with large areas of explosive volcanic ejecta. This cycle in volcanic activity is influenced by changes in water depth, both spatially (i.e. between segments) and temporally (i.e. for the same segment through time).Each segment has known a specific history in its accretionary processes with a succession of tectonic and volcanic predominance and changes in its volcanic phases between volcanic ejecta and effusive dynamics.The hydrothermal activity is focused at the central part of each segment and is controlled by the presence of fresh lava and major tectonic features.     

Observations of the OH radical in comets at 18 cm wavelength

We present recent observations of the OH radical at λ 18 cm with the Nançay radio telescope in comets Meier (1978 XXI), Bradfield (1979 X), Meier (1980q), P/Encke (1980), and Bradfield (1980t). The analysis of the OH radio line shape is a powerful tool to study the kinematics of the coma. The expansion velocity of the OH molecules is found to be ≈1.5 km sec^?1 at r_h = 1 AU, and decreases with increasing heliocentric distance. The line profile is generally asymmetric, which demonstrates the Greenstein effect on the fluorescent excitation mechanism and/or anisotropic outgassing of the nucleus. In several cases, especially for comet Meier (1978 XXI), an asymmetry is also found in the east-west brightness distribution of the OH line, showing again the Greenstein effect and/or anisotropic outgassing. An excitation model by uv pumping and fluorescence of the OH radical, which agrees with the observations at least in the first order, and the application of Haser's model lead to the production rate of the parent molecule of OH. There is a close correlation between this gas production rate and the visual brightness of the comet. Our estimates of gas production rates are smaller than or equal to those obtained from uv measurements, but both radio and uv estimates depend heavily on the parameters used in Haser's models.     

Review of the dynamics in the Kirkwood gaps

The study of mean motion resonance dynamics was motivated by the search for an explanation for the puzzling problem of the Kirkwood gaps. The most important contributions in this field within the last 32 years are reviewed here. At the beginning of that period, which coincides with the first long-term numerical investigations of resonant motion, different hypotheses (collisional, gravitational, statistical and cosmological) to explain the origin of the gaps were still competing with each other. At present, a general theory, based on gravitational mechanisms only, is capable of explaining in a uniform way all the Kirkwood gaps except the 2/1 one. Indeed, in the 4/1, 3/1, 5/2 and 7/3 mean motion commensurabilities, the overlap of secular resonances leads to almost overall chaos where asteroids undergo large and wild variations in their orbital elements. Such asteroids, if not thrown directly into the Sun, are sooner or later subject to strong close encounters with the largest inner planets, the typical time scale of the whole process being of the order of a few million years. Unfortunately, this mechanism is not capable of explaining the 2/1 gap where the strong chaos produced by the overlapping secular resonances does not attain orbits with moderate eccentricity, of low inclination and with low to moderate amplitude of libration. In the light of the most recent studies, it appears that the 2/1 gap is the global consequence of slow diffusive processes. At present, the origin of these processes remains under study.     

The transition from alkali basalts to kimberlites: Isotope and trace element evidence from melilitites

Eighteen Cenozoic melilitite samples from Spain, France, West Germany and Czechoslovakia have been analyzed for major and trace elements (including REE) together with their Sr and Nd isotopic compositions. Leaching experiments produced significant shifts of their⁸⁷Sr/⁸⁶Sr ratio indicative of a contamination by a crustal component. Most samples fall within the Sr-Nd mantle array with ?_Nd values in the 1.5–6 range. These values are considered as minimum for the melilitite mantle source hence demonstrating its time integrated LRE depletion. The Ni and Cr contents of the samples are typical of primary magmas and exclude extensive crystallization of olivine and pyroxene in a closed system. However, the chemical relationships suggest that dilution of the liquids by mafic minerals of the conduits during their ascent has been important. The REE patterns show some variations which are interpreted by this dilution effect. Once normalized to Yb they are closely similar and perfectly distinguishable from those of alkali basalts and kimberlites. All of these rocks have Ce/Yb ratios which are high but distinctive for each rock type: 40 to 200 times the chondritic ratio for kimberlites, 20 to 30 for melilitites, 8 to 15 for alkali basalts. As contamination is likely to have modified somewhat the isotopic characteristics of most of these rocks, there is no overwhelming evidence that their source is chemically different. The Ba and Rb contents together with the REE patterns of the melilitites would constrain the degree of melting to be very small (<0.2%). The calculation of batch melting and steady zone refining models suggests that kimberlites, melilitites and alkali basalts may have been derived by equilibration of deep melts with different upper mantle levels characterized by decreasing garnet/clinopyroxene ratios. The strongly incompatible elements are enriched in the melt during its ascent by leaching of the wall rocks. For the steady zone refining model, the degree of melting concept loses its significance and the difficult requirement of extracting small liquid fractions from a molten source disappears. Within the frame of this model, the preenrichment of the kimberlite, melilitite and alkali basalts source in incompatible elements by metasomatic fluids is no longer necessary.     

Âge des orthogneiss de la zone axiale de la Montagne Noire (France) par la méthode 87Rb-87Sr

The basement of Montagne Noire (South France), as represented by the orthogneisses of the axial zone, has been dated by means of whole rocks isochron at 530 m.y. This age is discussed with the assistance of petrographic observations and compared to similar results obtained by other authors in the Massif Armoricain and eastern Pyrénées ranges. The high initial ratio ⁸⁷Sr/⁸⁶Sr allows one to hypothesize that these igneous masses were originated from a segment of a preexisting crust.

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Contribution I.P.G.N.S. No 8.     

Cyanophycées marines endolithes colonisant les surfaces rocheuses dénudées (Etages Supralittoral et Médiolittoral de la région de Marseille)

Résumé Etude des Cyanophycées endolithes, recolonisant les surfaces rocheuses dénudées, au niveau des Etages Supralittoral et Médiolittoral de la région de Marseille. Apparition de différentes formes évolutives de l’espèceEntophysalis deusta en fonction de l’intensité des facteurs du milieu. Niveau de l’Etage Supralittoral, humectation réduite, formes épilithes et peu pénétrantes; niveau de l’Etage Médiolittoral, humectation intense, formes endolithes filamenteuses. Il est à noter que ces formes se succèdent toujours dans le même ordre, seul le stade final est différent selon la condition du milieu envisagé. Ceci va donc à l’encontre d’une espèce unique avec divers stades évolutifs. La position extrême adoptée parF. Drouet etW.A. Daily au sujet de la systématique des Cyanophycées endolithes entra?ne de nombreuses réserves et seule une étude basée sur des cultures unialgales permettra de conna?tre avec précision la marge de variation des taxons.

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Summary The object of this study was the endolithic blue-green algae of the upper and middle litoral zone of the rocky coast near Marseille. Difficulties in determining these algae cropped up very soon because of their strong plasticity. To ascertain whether there were different species of algae or only different ecological types of the same species, two series of experiments were carried out in the region around Marseille: 1. small rock splinters from various places were cemented into place in the above-mentioned zones. 2. The regrowth of rock surfaces that were cleared of algae was observed. Moreover cultures of the individual species were made to find an unequivocal solution to the systematic and ecological problems of these algae. Only the results of the tests on recolonization of the rock surface by algae are dealt with in this paper. According to our observations, theEntophysalis deusta (Men.) Drouet and Daily developed in different patterns depending on the degree of moisture. In the upper litoral zone where the amount of moisture is less significant it takes on epilithic forms that do not penetrate into the rock. In the more humid middle zone they prevail in an endolithic stringy form. It is interesting to note that the various forms always appear in the same order; thus with increasing moisture one finds:Gloeocapsa, Entophysalis, Scopulonema (forms of the upper litoral zone) andHyella caespitosa, Hyella tenuior (forms of the middle zone). Other species such as theHormathonema that grow in the same biotope as theEntophysalis seem to react differently to outside stimuli. The conjectures concerning the plasticity of theEntophysalis deusta confirmDrouet andDaily’s theory. Yet this hypothesis needs to be confirmed through cultures of the forms observed.

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Zusammenfassung Die endolithischen Blaualgen der supralitoralen und mediolitoralen Zone an der felsigen Küste in der Region von Marseille wurden untersucht. Schon zu Beginn der Arbeit zeigten sich Schwierigkeiten bei der Bestimmung dieser Algen, da sie zum Teil eine starke Plastizit?t aufzuweisen scheinen. Um abzukl?ren, ob verschiedene Arten oder nur unterschiedliche ?kotypen ein, und derselben Art vorliegen, wurden zwei Untersuchungsserien in der Region von Marseille durchgeführt: 1. Kleine Felssplitter verschiedener Herkunft wurden in den genannten Zonen festzementiert. 2. Es wurde beobachtet, wie Stellen, die man von Algen entbl?sste, wieder besiedelt wurden. Ferner begann man Kulturen der einzelnen Typen herzustellen, um die systematisch-?kologischen Probleme dieser, Blaualgen eindeutiger zu l?sen. In dieser Mitteilung sind nur die Resultate derWiederbesiedlungsversuche dargestellt. Die Beobachtungen ergaben im wesentlichen, dass die ArtEntophysalis deusta (Men.) Drouet et Daily sich je nach Intensit?t der Benetzung in verschiedenen Formen entwickelt. So tritt sie in der supralitoralen Zone, die einen reduzierten Feuchtigkeitsgrad aufweist, in epilithischen Formen auf, die wenig in den Fels eindringen. In der mediolitoralen Zone mit grosser Feuchtigkeit findet man sie als endolithische f?dige Formen. Interessant ist, dass diese verschiedenen Erscheinungsformen stets in gleicher Reihenfolge auftreten, und zwar begegnet man mit zunehmender Feuchtigkeit folgenden Formen:Gloeocapsa, Entophysalis, Scopulonema (= Formen der supralitoralen Zone),Hyella caespitosa, Hyella tenuior (= Formen der mediolitoralen Zone). Andere Gattungen, wie z. B.Hormathonema, die im gleichen Biotop wieEntophysalis auftreten, scheinen anders auf die Umweltfaktoren zu reagieren. Die Vermutungen über die Plastizit?t vonEntophysalis deusta best?tigen eigentlich die Auffassung vonDrouet undDaily. Doch diese Hypothese verlangt eine Best?tigung mittels Kulturen der beobachteten Erscheinungsformen.

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Contribution à l’étude des Cyanophycées lithophytes des Etages Supralittoral et Médiolittoral (région de Marseille), Tethys1 (1), 119–172 (1969).