545 research outputs found
Ultra deep nulling interferometry using fractal interferometers
No full textInternational audienceThe difficult goal of directly detecting a planet around a star requires the cancellation of, as far as possible, the stellar light and nulling interferometry is one way to do so: the star is put on a central dark fringe while the planet is supposed to be on a bright fringe. One problem is, however, leaks due to the finite angular dimension of the stellar disk, resolved by the interferometer. The solution is to increase the exponent of the term theta which describes the cancellation efficiency with respect to the angular distance to the axis of the central dark fringe. Efficient configurations have been found, using basically guess and check methods until recently. I present here one method to define configurations of telescopes that achieve any given power of theta. The principle is based on a peculiar property of a partition into two sets of the first 2 integers; the partition is built using the Prouhet Thué Morse sequence which presents some fractal properties. A phase shift (0 or pi) between 2 telescopes is applied according to this partition. I first examine 1-D pattern of identical telescopes, then extend the method to 2-D configurations of identical telescopes, to 1-D arrays and 2-D arrays of non-identical telescopes and finally to arrays where the phase shift between n groups of telescopes is 2kpi/n. I examine then how a non-perfect fractal interferometer behaves and show that its robustness with respect to nulling stability is an important advantage. To cite this article: D. Rouan, C. R. Physique 8 (2007)
Role-Playing Game and Learning for Young People About Sustainable Development Stakes: An Experiment in Transferring and Adapting Interdisciplinary Scientific Knowledge
No full textThe study refers to the interactions between socio-economic and natural dynamics in an island biosphere reserve by using companion modelling. This approach provides scientific results and involves interdisciplinarity. In the second phase of the study, we transferred knowledge by adapting the main research output, a role-playing game, to young people. Our goal was to introduce interactions between social and ecological systems, coastal dynamics and integrated management. Adapting the game required close collaboration between the scientists and educators in order to transform both its substance and form and to run it with an easy-to-handle ergonomic platform.Children Education, Multi-Agent Environment, Role-Playing Game
A ground-based study for the 4Q coronagraph
No full textHigh angular resolution provided by adaptive optics on
ground-based telescopes allows detailed study of bright stars
environment. The limitation of dectability of faint companions is
the diffracted light from the star. The 4Q coronagraph
(Rouan et al. 2000; Riaud et al. 2001; Abe et al. 2001)
decreases this diffracted halo
and provides high contrast imaging. The residual speckle pattern
becomes the final limitation for the coronagraphic observations.
We present here, a complete study of the 4Q coronagraph in use
with an high order adaptive optics on the VLT telescope, within
the context of "Planet Finder" ESO project. Practically, a simple
solution making use of commercial half waveplates seems well
suited for a ground-based coronagraphic device
From Fractals to Exoplanets: building ultra-nulling interferometers
No full textThe difficult goal of directly detecting a planet around a star requires
to cancel as much as possible the stellar light.
Since the first proposal by Bracewell of a nulling interferometer, where the
star is put on a central dark fringe, several interferometric
configurations have been presented in order to improve the quality of
the rejection, especially to avoid the leaks due to the finite angular dimension
of the stellar disk, resolved by the interferometer. In the Bracewell
interferometer, the behaviour of the nulling efficiency vs the angular
distance θ to the star is as (1-cosθ)
∝ θ2. One goal is to increase the exponent of the term θn which
gives the cancellation efficiency.
I present one method to define configurations of telescopes positions, sizes and
phase-shift that can
achieve any given power of θ. The principle is based on a peculiar property found
by Prouhet
of a partition into two sets of the integers, done according to the Thué-Morse sequence.
2L telescopes regularly spaced on a line, are distributed into two groups, following their rank
in the Thué-Morse sequence and,
to the telescopes of one of the groups, is applied a π phase shift.
The result is a fractal-like distribution of the telescopes where redundancy is minimum and
whose interferometric combination produces a very efficient nulling in θ2L.
I first examine 1-D patterns of identical telescopes, then extend the method to 2-D configurations, then show that the latter
can be used to define 1-D arrays of non identical telescopes, according to some
algebra of interferometers. The generalization
to arrays where the phase shift between n groups of telescopes is 2kπ/n is
finally proposed
Adaptive Optics spectro-imaging of starburst galaxies
No full textAs part of an ongoing program started at the Canada--France--Hawaii Telescope, we have observed Arp 299 and NGC 7469 in broad and narrow bands with PUEO, the facility adaptive optics system. Locating precisely the regions where starburst activity occurs can help to understand the events that may have led to enhanced star formation. Previous results [Rouan, 1998a], [Lai etal, 1998a] have shown that starburst activity is fragmented, either in very dense star clusters or in well defined regions such as rings or interface regions. We report on new images of Arp 299 and NGC 7469 obtained in narrow band filters (H 2 , Brfl, both redshifted at z=0.01). Such an attempt to obtain spectral information at high angular resolution allows us to identify the contribution of each hot spot in terms of star formation -- more accurately than broad band color-color diagrams (chromatography): does the flux come from a population of hot recently-formed young stars? from ionized gas? from an older population..
Achromatic four quadrants phase mask coronagraph using the dispersion of form birefringence
No full textWe describe an alternative design for the 4-quadrant phase mask coronagraph described recently by Rouan et al. 2000. Based on the same principle, i.e. producing a very efficient nulling by mutually destructive interferences of the coherent light from the main source, our mask realises the pi phase shift using some properties of ZOGs (Zeroth Order Gratings) and according to an original scheme respecting the 4-quadrant symmetry. When the period of the one-dimension grating structure is smaller than the wavelength of the incident light, the structure becomes birefringent. The effective refractive indices depend on the wavelength. Using this feature, we can design a mask whose nulling e±ciency is maintained within a wide wavelength range. Numerical simulations were made according to the RCWT (Rigorous Coupled Wave Theory)
Does Your Species Have Memory? Analysing Capture-Recapture Data with Memory Models.
No full text1. We examine memory models for multi-site capture-recapture data. This is an important topic,as animals may exhibit behaviour that is more complex than simple first-order Markov movement between sites, when it is necessary to devise and fit appropriate models to data.
2. We consider the Arnason-Schwarz model for multi-site capture-recapture data, which incorporates just first-order Markov movement, and also two alternative models that allow for memory, the Brownie model and the Pradel model. We use simulation to compare two alternative tests which may be undertaken to determine whether models for multi-site capture-recapture data need to incorporate memory.
3. Increasing the complexity of models runs the risk of introducing parameters that cannot be estimated, irrespective of how much data are collected, a feature which is known as parameter redundancy. Rouan et al (JABES, 2009, pp 338-355) suggest a constraint that may be applied to overcome parameter redundancy when it is present in multi-site memory models. For this case, we apply symbolic methods to derive a simpler constraint, which allows more parameters to be estimated, and give general results not limited to a particular configuration. We also consider the effect sparse data can have on parameter redundancy, and recommend minimum sample sizes.
4. Memory models for multi-site capture-recapture data can be highly complex, and difficult to fit to data. We emphasise the importance of a structured approach to modelling such data, by considering a priori which parameters can be estimated, which constraints are needed in order for estimation to take place, and how much data need to be collected. We also give guidance on the amount of data needed to use two alternative families of tests for whether models for multi-site capture-recapture data need to incorporate memory
Diophantine Optics
No full textWhat I call Diophantine optics is the exploitation in optics of some remarkable algebraic relations between powers of integers. The name comes from Diophantus of Alexandria, a greek mathematician, known as the father of algebra. He studied polynomial equations with integer coefficients and integer solutions, called diophantine equations. Since constructive or destructive interferences are playing with optical path differences which are multiple integer (odd or even) of λ/2 and that the complex amplitude is a highly non-linear function of the optical path difference (or equivalently of the phase), one can understand that any Taylor development of this amplitude implies powers of integers. This is the link with Diophantine equations. We show how, especially in the field of interferometry, remarkable relations between powers of integers can help to solve several problems, such as achromatization of a phase shifter or deep nulling efficiency. It appears that all the research that was conducted in this frame of thinking, relates to the field of detection of exoplanets, a very active domain of astrophysics today
Astrophysique observationnelle
No full textCours et séminaires Le cours de cette année a encore porté sur les progrès actuels et prévisibles de l’observation astronomique à haute résolution angulaire. Le travail entamé pour définir et construire les premiers exemplaires, au sol et dans l’espace, d’une nouvelle génération d’instruments géants, les hypertélescopes, est décrit. Des séminaires ont été donnés par F. Bouchy, D. Rouan, N. Behara, N. Nardetto, G. Hebrard, P. Riaud, A. Leger, H. Le Coroller, O. Absil. Trois des cours et sémina..
Biflavonoied diastereo-isomere en hul biogenetiese voorlopers uit Berchemia SPP.: pogings tot sintese van analoe
No full textAfrikaans: Hierdie studie behels die struktuurondersoek, omskakelingsreaksies en pogings tot sintese van sommige van die mono- en biflavonoïede uit die kernhout van Berchernia zeyheri met 'n aanvullende studie op die kernhoutstowwe van die taksonomies notiverwante spesie; B discolor. Die verbindings uit B. zeyheri (rooi ivoor) kan in twee groepe geklassifiseer word, nl. mono- en biflavonoïede, almal op dieselfde 4' ,5,7- trihidroksi- (of sy ekwivalent) fenoliese patroon gegrond.
Die eerste groep sluit die volgende in: maesopsin {2-(4-hidroksibensiel)-2,4,6-trihidroksibenso[b]furan-3(2H)-oon}; 7-metielmaesopsin (nnut); α,2’,4,4’,6’-pentahidroksicis- en trans-chalkoon; 2,3-trans-aromadendrin (3,4’,5,7-tetrahidroksiflavanoon); kaempferol (3,4’,5,7-tetrahidroksiflavoon); naringenin (4’,5,7-trihidroksiflavanoon) en 4,4’,6-trihidroksiauroon. Uit die beskikbare eksperimentele gegewens kon geen aanduiding van die struktuur van die rooi pigment wat verantwoordelik is vir die besondere voorkoms van die kernhout, verkry word nie.
Die relatiewe hoë opbrengs van maesopsin het toegelaat dat verskeie nuwe omskakelings op die tetra-O-metieleter uitgevoer kan word met die doel om soortgelyke reaksies op daardie biflavonoïede, met een of meer maesopsineenhede waarvan genoegsaam verkry kon word, te herhaal. Reduksie met LiA1H4 het die ooreenstemmende 3-ol gelewer; brominering die 7-broomderivaat; en fotolise die 2’-hidroksi-α-metoksi-cis-chalkoonanaloog sowel as 1-(2-hidroksi-4,6-dimetoksifeniel)-2,2-dimetoksi-3-(4-metoksifeniel)-propan-1-oon. 2-(α-Asetoksi-4-metoksibensiel)-2,4,6-trimetoksi- en 2-(4-metoksibensoïel)-2-asetoksi-4,6-dimetoksibesno[b]furan-3(2H)-oon is vir die eerste keer verkry deur oksidasie van sintetiese 2’-hidroksi-α-metoksi-trans-chalkoon met Pb(0Ac)4 in asynsuur. Die struktuur van die cis- en trans-α-hidroksiechalkone is deur sintese en fotolitiese omskakeling bevestig. Die struktuur van die “konvensionele” chalkoon is deur sintese van 2’-hidroksi-4,4’,6’-trimetoksi-trans-chalkoon bevestig. Oksidasie van laasgenoemde met Pb(OAc)4 en Tl(NO3)3 lewer onderskeidelik tri-0-metielauroon en –isoflavoon.
Al die biflavonoïede is nuwe natuurprodukte en sluit die volgende opties onaktiewe verbindings in (slegs triviale name genoem): zeyherin en isozeyherin, beide enantiomere pare bestaande uit twee I-2,II-7-gekoppelde bensokumaranonieleenheid verbind en neo- en isoneoberchenin wat slegs verskil t.o.v. die I-3,II-5-interflavonoïedbinding van voorafgaande twee. Al vier is enantiomere pare.
‘n Verdere biflavonoïed, rhamnin, saamgestel uit ‘n naringenieleenheid en ‘n terminale α-hidroksichalkooneenheid (I-3,II-3’-gekoppel), lewer vir die eerste keer direkte bewys (met die moontlikheid van ringopening in ag geneem) van inkorporasie van ‘n α-hidroksichalkoon as biogenetiese voorloper. Die bestaan van die verbinding dui aan dat ringsluiting van die terminale eenheid by berchenin en isoberchenin na intermolekulêre koppeling plaasvind.
Twee verdure biflavonoïede, laktoon A en B genoem, identies aan die zeyherins behalwe dat die terminale eenheid (maesopsin) ’n bensielsuuromskakeling onder die toestande van metilering ondergaan het en derhalwe nie as natuurprodukte beskou word nie, is as enantiomorfe mengsels verkry. Hul dien egter as ‘n indirekte bewys vir α-hidroksichalkone as biogenetiese voorlopers, en dat siklisering by die zeyherins soos in die geval van berchenins moontlik eers na intermolekulêre koppeling geskied.
Geeneen van die pare biflavonoïede ondergaan onderlinge omskakeling by ca. 150° (ontbinding) nie, sodat hul nie rotasie isomere verteenwoordig nie, maar wel diastereoisomere.
Bibensokumaranonielmetaan, moontlik ‘n rotameriese mengsel volgens duplisering van seine in die KMR-spektrum van die 0-metieleter, bestaan uit twee maesopsineenhede, unike I-5,II-7 oor ‘n CH2-brug verbind. ‘n Verbinding verwant aan laasgenoemde, n. 2,7-bi(4-hidroksibensiel)-2,4,6-trihidroksibenso[b]furan-3(2H)-oon is ook geïsoleer.
Al die biflavonoïede is uniek in die opsig dat hul 5-ledige heterosikliese ring(e) of ‘n α-hidroksichalkoon as eenhede bevat.
As gevolg van ‘n gebrek aan material is slegs enkele omskakelings op sommige van die biflavonoïede uitgevoer. Reduksie van 0-metielzeyherin met KBH4 skakel slegs die karbonielgroep van die terminale eenheid na die alcohol om. Dieselfde resultaat is op die berchenins verkry. Fotolise van 0-metielzeyherin lei tot omskakeling van die terminale eenheid na ‘n ketalstruktuur soortgelyk as die verkry uit die fotolise van 0-metielmaesopsin.
Deur die uitskakeling van suurtoestande en hitte is die 0-metieleters van maesopsin, zeyherin, isozeyherin, berchenin en isoberchenin na voltooiing van die ondersoek as opties aktiewe verbindings geïsoleer na metilering met diasometaan.
In ‘n poging tot sintese van ‘n biflavonoïed is gevind dat wanneer ‘n gesubstitueerde fenol i.p.v. ‘n flavonoïedeenheid gebruik word onder toestande wat fenolkoppeling bevorder [alkalise K3Fe(CN)6], die reaksie van die verwagte afwyk deurdat koppeling in die β- i.p.v. die α-posisie van die chalkoon plaasvind. In die spesifieke geval koppel 2’,4-dihidroksi-4’,6’-dimetoksi-trans-chalkoon in die β-posisie met 3,5-dimetoksifenol in teenwoordigheid van K3Fe(CN)6 om vier diastereoisomere 2-[α-(4-hidroksi-2,6-dimetoksifeniel)-4-hidroksibensiel]-4,6-dimetoksiebenso[b]furan-3(2H)-one en 4’-hidroksie-4,6-dimetoksiauroon in goeie opbrengste te lewer.
Die volgende verbindings is uit die kernhout van die verwante B. discolour geïsoloeer: maesopsin, alfitonien {2-(3,4-dihidroksibensiel)-2,4,6-trihidroksibenso[b]furan-3(2H)-oon}; α,2’,4,4’,6’-pentahidroksi- en α-2’,3,4,4’,6’-heksahidroksitrans-chalkoon (beide nuwe natuurprodukte); (-)-epi- en (+)-katesjien; kaempferol (3,4’,5,7-tetrahidroksiflavoon), kwersitien (3,3’,4’,5,7-pentahidroksiflavoon), 3,4’,5-trihidroksidihidrostilbeen, asook twee verbindings waarvan nog geen struktuurtoeseggings moontlik was nie.
Fotolise van die heksametoksi-trans-chalkoon, derivaat van bogenoemde α-heksahidroksichalkoon, het die cis-isomeer gelewer. Slegs ‘n ketalanaloog is na die foto-oksidatiewe omskakeling van penta-0-metielalfitonien geïsoleer, in teenstelling met die addisionele cis-α-metoksichalkoon verkry by die fotolise van tetra-0-metielmaesopsin.
Skrille kontras bestaan dus tussen die inhoudstowwe van die twee spesies. Benewens ‘n aanvullende katekoloksigeneringspatroon is die opvallendste egter die afwesigheid van die verwagte biflavonoïede by B. discolour op grond van taksonomiese verwantskap. Alhoewel twee α-hidroksichalkone in die kernhout voorkom dui die afwesigheid van biflavonoïede in teenstelling met die geval van B. zeyheri moontlik op ‘n verskil in die redokspotensiaal van die twee spesies se ensiemsisteme en/of dat ‘n α-hidroksichalkoon slegs tesame met ‘n “konvensionele” chalkoon, soos in die geval van rooi ivoor, as biogenetiese voorlopers vir biflavonoïede optree.
Die aanwesigheid van α-hidroksichalkone en verwante hemiketale in hoë konsentrasie dui op ‘n chemotaksonomiese verwantskap tussen twee spesies
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