188 research outputs found

    Imaging the heart of astrophysical objects with optical long-baseline interferometry

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    The number of publications of aperture-synthesis images based on optical long-baseline interferometry measurements has recently increased due to easier access to visible and infrared interferometers. The interferometry technique has now reached a technical maturity level that opens new avenues for numerous astrophysical topics requiring milli-arcsecond model-independent imaging. In writing this paper our motivation was twofold: 1) review and publicize emblematic excerpts of the impressive corpus accumulated in the field of optical interferometry image reconstruction; 2) discuss future prospects for this technique by selecting four representative astrophysical science cases in order to review the potential benefits of using optical long baseline interferometers. For this second goal we have simulated interferometric data from those selected astrophysical environments and used state-of-the-art codes to provide the reconstructed images that are reachable with current or soon-to-be facilities. The image reconstruction process was "blind" in the sense that reconstructors had no knowledge of the input brightness distributions. We discuss the impact of optical interferometry in those four astrophysical fields. We show that image reconstruction software successfully provides accurate morphological information on a variety of astrophysical topics and review the current strengths and weaknesses of such reconstructions. We investigate how to improve image reconstruction and the quality of the image possibly by upgrading the current facilities. We finally argue that optical interferometers and their corresponding instrumentation, existing or to come, with 6 to 10 telescopes, should be well suited to provide images of complex sceneries.Comment: Acccepted to Astronomy and Astrophysics Revie

    Enabling planetary science across light-years. Ariel Definition Study Report

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    Ariel, the Atmospheric Remote-sensing Infrared Exoplanet Large-survey, was adopted as the fourth medium-class mission in ESA's Cosmic Vision programme to be launched in 2029. During its 4-year mission, Ariel will study what exoplanets are made of, how they formed and how they evolve, by surveying a diverse sample of about 1000 extrasolar planets, simultaneously in visible and infrared wavelengths. It is the first mission dedicated to measuring the chemical composition and thermal structures of hundreds of transiting exoplanets, enabling planetary science far beyond the boundaries of the Solar System. The payload consists of an off-axis Cassegrain telescope (primary mirror 1100 mm x 730 mm ellipse) and two separate instruments (FGS and AIRS) covering simultaneously 0.5-7.8 micron spectral range. The satellite is best placed into an L2 orbit to maximise the thermal stability and the field of regard. The payload module is passively cooled via a series of V-Groove radiators; the detectors for the AIRS are the only items that require active cooling via an active Ne JT cooler. The Ariel payload is developed by a consortium of more than 50 institutes from 16 ESA countries, which include the UK, France, Italy, Belgium, Poland, Spain, Austria, Denmark, Ireland, Portugal, Czech Republic, Hungary, the Netherlands, Sweden, Norway, Estonia, and a NASA contribution

    Das beobachtbare Erscheinungsbild von Trümmerscheiben

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    Debris disks are dusty circumstellar disks around main-sequence stars and natural by-products of the planet formation process. With an almost gas-free environment, dust-replenishing parent bodies orbit their host star and most likely continuously supply fine dust through mutual collisions. Thus, debris disks comprise solids ranging from kilometer-sized planetesimals down to micrometer-sized dust. Due to the large surface to volume ratio, dust grains are efficient radiators of thermal re-emission and scatterers of incident radiation from the stellar source. Dust grains are, therefore, readily detectable in a planetary system. Consequently, debris disk observables mainly depend on dust properties and the disk structure, as well as stellar properties. In this dissertation, an observational appearance of debris disks is investigated. This allows the verification of predictions made concerning the spatial structure, underlying dynamical processes, and optical properties of the dust in the debris disk system. In particular, the potential for multi-wavelength and spatially resolved observations in numerical studies are conducted to constrain the observational appearance of debris disks with the physical properties and dynamics of dust grains. To develop observational strategies of disk observations, a new tool has been developed for analytical modeling of debris disks and the interpretation of results from the collisional approach. The dependence of the observational appearance of debris disks on essential collisional parameters, such as the eccentricity of the parent belt, the dispersion of the eccentricities of parent belt bodies, and the critical specific energy for fragmentation of dust particles, is investigated. Furthermore, the feasibility of detecting water ice in typical debris disk systems, assuming various ice destruction mechanisms and dust mixtures with various internal structures, is investigated. Additionally, the multi-wavelength modeling of debris disks in η Chameleontis cluster is investigated to constrain the physical parameters and properties of the disks, such as the range of possible radial locations and total dust mass, based on observation from the APEX/LABOCA and the archival Gaia/DR2 data. Finally, a model based on a planetesimal mass distribution function is investigated to discuss the flattening of the spectral energy distribution of HD 107146 at mm wavelength with the NIKA2 observation

    Interferometric observations to analyze circumstellar environments and planetary formation

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    Protoplanetary disks have a rich structure, with different physics playing a role in different regions of the disk. They are under constant evolution, due to a combination of various physical and chemical processes, e.g., accretion, photo-evaporation, gas-dust interactions, grain growth, and the interaction with protoplanets. The dynamic ranges involved span orders of magnitudes on spatial scales, orbital times, temperatures, and dust- or gas-densities. The extreme dynamic ranges involved in the structure and composition of these objects mean that very different observational techniques have to be combined together to probe their various regions. This work makes use of new interferometric and spectroscopic measurements in the infrared, together with published mid-infrared images and spectral energy distribution fluxes from UV to mm-wavelength, to instruct a new comprehension of the well-known IRS48 object, and uncover part of the delicate balance of physical processes at stake. This PhD reports the first direct imaging of the full extents of a polycyclic aromatic hydrocarbon and very small grains ring in a young circumstellar disk, presents a revised model for the IRS48 object to explain the rich and complex dust- and gas-environment observed from near-infrared to centimeter wavelengths. Also, the spectral type of the spectroscopic binary MWC361 is determined. This will lead to a precise characterization of the stellar parameters of this binary, opening a new window on the studying of the disappearance of the circumsecondary disk of the binary, while the circumprimary disk is still present. The leitmotif throughout this thesis is the understanding of the last moments of circumstellar disks, and the search for the processes which dissipate them. This particular step of the disk-evolution is one the most mysterious to date, yet it sets critical constraints on the by-product of circumstellar disks, exoplanets

    High-contrast imaging study of exoplanets and circumstellar disks

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    High-contrast imaging provides an excellent tool to detect and characterise exoplanets and circumstellar disks. Understanding the connection between them is key for the improvement of planet formation and evolution theories. In this thesis, I analyse near-infrared (NIR) observations obtained with the Spectro-Polarimetric High-contrast Exoplanet REsearch instrument (SPHERE) to look into various stages of the evolution of planetary systems. I combine the high-contrast imaging technique with observations in the millimetre continuum, hydrodynamical simulations, and radiative transfer models, as well as atmospheric retrievals and self-consistent models to analyse and interpret the different systems. Starting with protoplanetary disks as the birthplaces of planets, I study the morphology of the disk around WaOph 6 at different wavelengths (NIR and millimetre continuum) and find the presence of spiral arms in scattered light for the first time in such a young disk. Additionally, I test the hypothesis of a planet driving the architecture of the disk through hydrodynamical simulations and radiative transfer. Moving on to more evolved systems, I first demonstrate the use of the high-contrast imaging technique to characterise companion candidates and to determine their membership to the system. Furthermore, I analyse spectro-photometric data of the exoplanet 51 Eridani b and apply an atmospheric retrieval to estimate the physical parameters of the planet, revisiting previously reported values and finding a cloud-free atmosphere. Finally, I analyse a sample of debris disks with a double belt architecture inferred via SED modelling. I present mass and location estimates of planets that may be orbiting in the gaps between the belts, as well as detection limits from the observations and plans for future research. This thesis illustrates the current challenges in our understanding of planet formation and evolution and provides possible paths to overcome them

    NAS technical summaries. Numerical aerodynamic simulation program, March 1992 - February 1993

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    NASA created the Numerical Aerodynamic Simulation (NAS) Program in 1987 to focus resources on solving critical problems in aeroscience and related disciplines by utilizing the power of the most advanced supercomputers available. The NAS Program provides scientists with the necessary computing power to solve today's most demanding computational fluid dynamics problems and serves as a pathfinder in integrating leading-edge supercomputing technologies, thus benefitting other supercomputer centers in government and industry. The 1992-93 operational year concluded with 399 high-speed processor projects and 91 parallel projects representing NASA, the Department of Defense, other government agencies, private industry, and universities. This document provides a glimpse at some of the significant scientific results for the year

    Statistical analysis of the first results of the SPHERE GTO Survey

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    This dissertation reports about a statistical analysis of the first 88 targets observed by the science channel IFS (Integral-Field-Spectrograph) during the SPHERE Guaranteed Time Survey (GTO). The final goal of this work is to put some initial constraints on the frequency of giant planets in wide orbits, on their mass distribution, on their semi-major axis distribution and possibly on the formation mechanism. In the first chapter we briefly present the two theories aimed at explaining the planet formation mechanism: the “core accretion” and the “disk instability”. In the second chapter we introduce the direct-imaging technique to discover young and self-luminous exoplanets and in the same chapter we present a new instrument optimized to perform direct imaging: SPHERE a Spectro-Polarimetric High-contrast-Exoplanet- REsearch. In the third chapter we describe the Guaranteed Time of Observation (GTO) survey SHINE, which is currently ongoing on SPHERE, the target selection and finally the results of the first two semesters of the survey. In the fourth outline we describe the statistical formalism used for the analysis and in particular the Quick-MESS code (Quick Multipurpose Exoplanet Simulation System): a fast alternative code to the classic Monte-Carlo tools for the statistical analysis of exoplanet direct imaging surveys. The results of our analysis are given in the fifth chapter and finally we compare our data with results from other surveys in sixth chapter. Although still exploratory, because the candidates so far found with SPHERE still require confirmation and only about 1=4 of the targets have been observed, this analysis will provide a first test of the methodology we plan to use once the survey is completed and some very early results. We show that current data are compatible with distributions, from the radial velocities, with only few planets beyound 10 - 20 AU. The peak of the giant planet distribution should then be at a separation not much larger than the snow-line, in agreement with the very recent result obtained by Bryan et al. (2016) from a combination of a radial velocity and the direct imaging data. This is interesting because the selection criteria used in our survey is very different, focusing on young objects and is not biased versus system with closer planets. On the other hand, the number of planets so far detected in our survey, while still compatible with an extrapolation of the results by Bryan et al., is at the lower limit of the acceptable range. Completion of the SPHERE survey will roughly reduce at half the current error bar in the frequency of planets at large separations (> 10 AU). This will be enough to show if this frequency is as high as expected from Bryan et al. analysis, or lower as suggested by our preliminary data. Finally, we propose a distribution of the frequency of giant planets versus the separations. The peak of distribution, in agreement with RV and SHINE data, should be slightly out of the snow-line as predicted by the core accretion scenario. Furthermore we note that the positions of Jupiter and Saturn are compatible with the peak of the overall distribution of giant planets, showing that on this respect the Solar System does not represent an exception

    NAS Technical Summaries, March 1993 - February 1994

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    NASA created the Numerical Aerodynamic Simulation (NAS) Program in 1987 to focus resources on solving critical problems in aeroscience and related disciplines by utilizing the power of the most advanced supercomputers available. The NAS Program provides scientists with the necessary computing power to solve today's most demanding computational fluid dynamics problems and serves as a pathfinder in integrating leading-edge supercomputing technologies, thus benefitting other supercomputer centers in government and industry. The 1993-94 operational year concluded with 448 high-speed processor projects and 95 parallel projects representing NASA, the Department of Defense, other government agencies, private industry, and universities. This document provides a glimpse at some of the significant scientific results for the year

    Resonant thickening of self-gravitating discs: imposed or self-induced orbital diffusion in the tightly wound limit

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    The secular thickening of a self-gravitating stellar galactic disc is investigated using the dressed collisionless Fokker-Planck equation and the inhomogeneous multicomponent Balescu-Lenard equation. The thick WKB limits for the diffusion fluxes are found using the epicyclic approximation, while assuming that only radially tightly wound transient spirals are sustained by the disc. This yields simple quadratures for the drift and diffusion coefficients, providing a clear understanding of the positions of maximum vertical orbital diffusion within the disc, induced by fluctuations either external or due to the finite number of particles. These thick limits also offer a consistent derivation of a thick disc Toomre parameter, which is shown to be exponentially boosted by the ratio of the vertical to radial scale heights. Dressed potential fluctuations within the disc statistically induce a vertical bending of a subset of resonant orbits, triggering the corresponding increase in vertical velocity dispersion. When applied to a tepid stable tapered disc perturbed by shot noise, these two frameworks reproduce qualitatively the formation of ridges of resonant orbits towards larger vertical actions, as found in direct numerical simulations, but overestimates the time-scale involved in their appearance. Swing amplification is likely needed to resolve this discrepancy, as demonstrated in the case of razor-thin discs. Other sources of thickening are also investigated, such as fading sequences of slowing bars, or the joint evolution of a population of giant molecular clouds within the disc.Comment: 31 pages, 19 figure
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