176 research outputs found

    Entropy and Correlations in Lattice Gas Automata without Detailed Balance

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    We consider lattice gas automata where the lack of semi-detailed balance results from node occupation redistribution ruled by distant configurations; such models with nonlocal interactions are interesting because they exhibit non-ideal gas properties and can undergo phase transitions. For this class of automata, mean-field theory provides a correct evaluation of properties such as compressibility and viscosity (away from the phase transition), despite the fact that no H-theorem strictly holds. We introduce the notion of locality - necessary to define quantities accessible to measurements - by treating the coupling between nonlocal bits as a perturbation. Then if we define operationally ``local'' states of the automaton - whether the system is in a homogeneous or in an inhomogeneous state - we can compute an estimator of the entropy and measure the local channel occupation correlations. These considerations are applied to a simple model with nonlocal interactions.Comment: 13 pages, LaTeX, 5 PostScript figures, uses psfig. Submitted to Int. J. Mod. Phys.

    PISCO2: the new speckle camera of the Nice 76-cm refractor

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    We present the new speckle camera PISCO2 made in 2010-2012, for the 76-cm refractor of C\^ote d'Azur Observatory. It is a focal instrument dedicated to the observation of visual binary stars using high angular resolution speckle interferometry techniques to partly overcome the degradation caused by the atmospheric turbulence. Fitted with an EMCCD detector, PISCO2 allows the acquisition of short exposure images that are processed in real time by our specially designed software. Two Risley prisms are used for correcting the atmospheric dispersion. All optical settings are remotely controlled. We have already been able to observe faint, close binary stars with angular separations as small as 0".16, and visual magnitudes of about 16. We also have measured some particularly difficult systems with a magnitude difference between the two components of about 4 magnitudes. This level of performance is very promising for the detection and study of large sets of yet unknown (or partly measured) binaries with close separation and/or large magnitude difference.Comment: 13 pages, 12 figure

    C2PU: 1-Meter Telescopes for the GAIA-FUN

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    3 p.International audienceC2PU stands in french for "Centre Pédagogique Planète Univers" (Planet and Universe Pedagogic Center). It is a project both for pedagogic and research purposes. It relies on the renewal of two 1-meter diameter telescopes. These two telescopes were earlier coupled as part of an interferometric instrument called SOIRDETE (for "Synthèse d'Ouverture en Infra Rouge avec DEux Telescopes"), described in Rabbia et al. 1990

    Thermalizing a telescope in Antarctica: Analysis of ASTEP observations

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    The installation and operation of a telescope in Antarctica represent particular challenges, in particular the requirement to operate at extremely cold temperatures, to cope with rapid temperature fluctuations and to prevent frosting. Heating of electronic subsystems is a necessity, but solutions must be found to avoid the turbulence induced by temperature fluctua- tions on the optical paths. ASTEP 400 is a 40 cm Newton telescope installed at the Concordia station, Dome C since 2010 for photometric observations of fields of stars and their exoplanets. While the telescope is designed to spread star light on several pixels to maximize photometric stability, we show that it is nonetheless sensitive to the extreme variations of the seeing at the ground level (between about 0.1 and 5 arcsec) and to temperature fluctuations between --30 degrees C and --80 degrees C. We analyze both day-time and night-time observations and obtain the magnitude of the seeing caused by the mirrors, dome and camera. The most important effect arises from the heating of the primary mirror which gives rise to a mirror seeing of 0.23 arcsec K--1 . We propose solutions to mitigate these effects.Comment: Appears in Astronomical Notes / Astronomische Nachrichten, Wiley-VCH Verlag, 2015, pp.1-2

    Un coronographe interf\'erentiel achromatique coaxial

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    On-axis achromatic interfero-coronagraph. We present a new type of stellar interfero-coronagraph, the "CIAXE", which is a variant of the "AIC", the Achromatic Interfero-Coronagraph [3,4]. The CIAXE is characterized by a very simple, compact and fully coaxial optical combination. Indeed, contrarily to the classical AIC which has a Michelson interferometer structure, the CIAXE delivers its output beam on the same axis as the input beam. This will ease its insertion in the focal instrumentation of existing telescopes or next generation ones. Such a device could be a step forward in the field of instrumental search for exoplanets. ----- Dans le but deparvenir \`a l'imagerie \`a haute dynamique d'objets comme les exoplan\`etes, nous pr\'esentons ici un nouveau concept de coronographe stellaire interf\'erentiel, le "CIAXE". Il est d\'eriv\'e du "CIA", le Coronographe Interf\'erentiel Achromatique. Le CIAXE se distingue de son pr\'ed\'ecesseur par une combinaison optique originale, simplifi\'ee, tr\`es compacte et totalement coaxiale. En effet, \`a la diff\'erence du CIA classique qui est d\'eriv\'e de l'interf\'erom\`etre de Michelson, le CIAXE d\'elivre son faisceau de sortie sur le m\^eme axe que le faisceau d'entr\'ee, ce qui facilitera grandement son insertion au sein de l'instrumentation focale d'un t\'elescope. Un tel dispositif pourrait constituer une avanc\'ee en mati\`ere d'instrumentation focale pour la recherche d'exoplan\`etes

    The Achromatic Interfero Coronagraph

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    We report on the Achromatic Interfero Coronagraph, a focal imaging device which aims at rejecting the energy contribution of a point-like source set on-axis, so as to make detectable its angularly-close environment (applicable to stellar environment: circumstellar matter, faint companions, planetary systems, but also conceivably to Active Galatic Nucleii and multiple asteroids). With AIC, starlight rejection is based on destructive interference, which allows exploration of the star's neighbouring at angular resolution better than the diffraction limit of the hosting telescope. Thanks to the focus crossing property of light, rejection is achromatic thus yielding a large spectral bandwidth of work. Descriptions and comments are given regarding the principle, the device itself, the constraints and limitations, and the theoretical performance. Results are presented which demonstrate the close-sensing capability and which show images of a companion obtained in laboratory and 'on the sky' as well. A short pictorial description of alternative AIC concepts, CIAXE and Open-Air CIAXE, currently under study, is given

    Combined spectroscopy and intensity interferometry to determine the distances of the blue supergiants P Cygni and Rigel

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    In this paper, we report on the spatial intensity interferometry measurements within the Hα\alpha line on two stars: the Luminous Blue Variable (LBV) supergiant \PCygni\,and the late-type B supergiant Rigel. The experimental setup has been upgraded to allow simultaneous measurement of two polarization channels and the zero baseline correlation function. Combined with simultaneous spectra measurements and based on radiative transfer models calculated with the code CMFGEN, we were able to fit our measured visibility curves to extract the stellar distances. Our distance determinations for both \PCygni\ (1.61 ±\pm 0.18 kpc) and Rigel (0.26 ±\pm 0.02 kpc) agree very well with the values provided by astrometry with the Gaia and Hipparcos missions, respectively. This is the first successful step towards extending the application of the Wind Momentum Luminosity Relation method for distance calibration from an LBV supergiant to a more normal late-type B supergiant

    Radar shape modeling and physical characterization of the PHA 1998 OR2

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    On April 29th 2020, the potentially hazardous asteroid (52768) 1998 OR2 (herafter OR2) flew-by the Earth at a distance of 16.4 Lunar distances. OR2 is a near-Earth object of absolute magnitude H = 16 that can experience close approaches to Earth as close as 6 Lunar distances. During this fly-by we obtained high SNR Arecibo S-band (2380 MHz; 12.6 cm) radar delay-Doppler images between Apr 13-23. We also obtained polarimetric observations with the ToPol polarimeter [1] mounted on the Omicron-West 1-m telescope from the Calern Observatory, Nice, France. These polarimetric observations span a range of phase angles from 30° to 77° allowing detailed characterization of the positive branch of 1998 OR2 phase-polarization curve. We also obtained new photometric observations with the Trappist-North telescope [2] located at the Oukaimeden Observatory in Morroco. Using the radar delay-Doppler and the lightcurve observations, we derived a non-convex shape model of 1998 OR2. This model displays a typical top-shape (diamond-like) model with an equivalent diameter of 1.6 km. The spin axis solution is oriented toward 330° +- 10° of ecliptic longitude and 23° +- 10° of ecliptic latitude with a rotation period of 4.1084 +- 0.0001 hours. Top-shape asteroids are typical for near-Earth objects as it had been observed for the asteroids Bennu [3], Ryugu [4], or Moshup [5]. Radar shape modeling has already been proven to be effective in modeling these type of asteroid as the radar shape model of Bennu [3], that was obtained before the Osiris-Rex mission, proved to be highly accurate.OR2 delay-Doppler images are characterized by the presence of a large feature visible on the leading edge of the echo. The shape model shows that this feature is a crater located near the equator. The shape model also shows that there are other craters/concavities formations mostly located on one-side of OR2 while the other side is lacking large scale structures. The polarimetric observations of OR2 displays a phase-polarization curve typical of moderate albedo objects with a polarization of 8.3% at a phase angle of 77.5°. The degree of linear polarization (especially at large phase angles) of an asteroid is directly correlated with its albedo. At a similar phase angle, the low albedo asteroid (3200) Phaethon (pv = 0.107 +- 0.011 [6]) is displaying a polarization of 27.3% [7] while the high albedo object E-type 1998 WT24 (pv = 0.654 +- 0.13 [8]) only displays a polarization around 1.5% [9]. Polarimetric observations were also collected over several hours during the same nights in order to obtain time-series. Folding the polarimetric data according to the rotation period of OR2, we observed that the degree of linear polarization is dependant on the rotation phase angle. Such variation means that the surface of OR2 displays heterogeneities that can be either due to a variation of albedo or grain size over the surface. We also observe that the maximum of the polarization occurs when crater is facing the observer. In conclusion, we obtained new radar, polarimetric, and photometric data of 1998 OR2 during its close approach to Earth on April 2020. These observations allowed us to obtain a non-convex shape model displaying a spin axis orientation of (330°,23°) with a rotation period of P = 4.1084 +- 0.0001 hours. The shape model displays large scale structures like concavities and craters. The polarimetric observations display variation of the polarization locked with the rotation phase angle of 1998 OR2 that seems to be correlated with the large structures observed on the shape model
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