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Christoph U. Keller
Professor of Experimental Astrophysics

Leiden Observatory
Leiden University, The Netherlands

The Hera Saturn entry probe mission

30 Sep 2016

by Mousis, O., Atkinson, D. H., Spilker, T., Venkatapathy, E., Poncy, J., Frampton, R., Coustenis, A., Reh, K., Lebreton, J.-P., Fletcher, L. N., Hueso, R., Amato, M. J., Colaprete, A., Ferri, F., Stam, D., Wurz, P., Atreya, S., Aslam, S., Banfield, D. J., Calcutt, S., Fischer, G., Holland, A., Keller, C., Kessler, E., Leese, M., Levacher, P., Morse, A., Muñoz, O., Renard, J.-B., Sheridan, S., Schmider, F.-X., Snik, F., Waite, J. H., Bird, M., Cavalié, T., Deleuil, M., Fortney, J., Gautier, D., Guillot, T., Lunine, J. I., Marty, B., Nixon, C., Orton, G. S., Sánchez-Lavega, A., is now available here.

Abstract: The Hera Saturn entry probe mission is proposed as an M-class mission led by ESA with a contribution from NASA. It consists of one atmospheric probe to be sent into the atmosphere of Saturn, and a Carrier-Relay spacecraft. In this concept, the Hera probe is composed of ESA and NASA elements, and the Carrier-Relay Spacecraft is delivered by ESA. The probe is powered by batteries, and the Carrier-Relay Spacecraft is powered by solar panels and batteries. We anticipate two major subsystems to be supplied by the United States, either by direct procurement by ESA or by contribution from NASA: the solar electric power system (including solar arrays and the power management and distribution system), and the probe entry system (including the thermal protection shield and aeroshell). Hera is designed to perform in situ measurements of the chemical and isotopic compositions as well as the dynamics of Saturn's atmosphere using a single probe, with the goal of improving our understanding of the origin, formation, and evolution of Saturn, the giant planets and their satellite systems, with extrapolation to extrasolar planets. Hera's aim is to probe well into the cloud-forming region of the troposphere, below the region accessible to remote sensing, to the locations where certain cosmogenically abundant species are expected to be well mixed. By leading to an improved understanding of the processes by which giant planets formed, including the composition and properties of the local solar nebula at the time and location of giant planet formation, Hera will extend the legacy of the Galileo and Cassini missions by further addressing the creation, formation, and chemical, dynamical, and thermal evolution of the giant planets, the entire solar system including Earth and the other terrestrial planets, and formation of other planetary systems.

Design trade-off and proof of concept for LOUPE, the Lunar Observatory for Unresolved Polarimetry of Earth

30 Sep 2016

by Hoeijmakers, H. J., Arts, M. L. J., Snik, F., Keller, C. U., Kuiper, J. M., is now available here.

Abstract: Not Available

Direct detection of scattered light gaps in the transitional disk around HD 97048 with VLT/SPHERE

30 Sep 2016

by Ginski, C., Stolker, T., Pinilla, P., Dominik, C., Boccaletti, A., de Boer, J., Benisty, M., Biller, B., Feldt, M., Garufi, A., Keller, C. U., Kenworthy, M., Maire, A. L., Ménard, F., Mesa, D., Milli, J., Min, M., Pinte, C., Quanz, S. P., van Boekel, R., Bonnefoy, M., Chauvin, G., Desidera, S., Gratton, R., Girard, J. H. V., Keppler, M., Kopytova, T., Lagrange, A.-M., Langlois, M., Rouan, D., Vigan, A., is now available here.

Abstract: We studied the well known circumstellar disk around the Herbig Ae/Be star HD 97048 with high angular resolution to reveal undetected structures in the disk, which may be indicative of disk evolutionary processes such as planet formation. We used the IRDIS near-IR subsystem of the extreme adaptive optics imager SPHERE at the ESO/VLT to study the scattered light from the circumstellar disk via high resolution polarimetry and angular differential imaging. We imaged the disk in unprecedented detail and revealed four ring-like brightness enhancements and corresponding gaps in the scattered light from the disk surface with radii between 39 au and 341 au. We derived the inclination and position angle as well as the height of the scattering surface of the disk from our observational data. We found that the surface height profile can be described by a single power law up to a separation ~270 au. Using the surface height profile we measured the scattering phase function of the disk and found that it is well consistent with theoretical models of compact dust aggregates. We discuss the origin of the detected features and find that low mass (< 1 M_Jup) nascent planets are a possible explanation.

Dusty tails of evaporating exoplanets. II. Physical modelling of the KIC 12557548b light curve

30 Sep 2016

by van Lieshout, R., Min, M., Dominik, C., Brogi, M., de Graaff, T., Hekker, S., Kama, M., Keller, C. U., Ridden-Harper, A., van Werkhoven, T. I. M., is now available here.

Abstract: Evaporating rocky exoplanets, such as KIC 12557548b, eject large amounts of dust grains, which can trail the planet in a comet-like tail. When such objects occult their host star, the resulting transit signal contains information about the dust in the tail. We aim to use the detailed shape of the Kepler light curve of KIC 12557548b to constrain the size and composition of the dust grains that make up the tail, as well as the mass loss rate of the planet. Using a self-consistent numerical model of the dust dynamics and sublimation, we calculate the shape of the tail by following dust grains from their ejection from the planet to their destruction due to sublimation. From this dust cloud shape, we generate synthetic light curves (incorporating the effects of extinction and angle-dependent scattering), which are then compared with the phase-folded Kepler light curve. We explore the free-parameter space thoroughly using a Markov chain Monte Carlo method. Our physics-based model is capable of reproducing the observed light curve in detail. Good fits are found for initial grain sizes between 0.2 and 5.6 micron and dust mass loss rates of 0.6 to 15.6 M_earth/Gyr (2-sigma ranges). We find that only certain combinations of material parameters yield the correct tail length. These constraints are consistent with dust made of corundum (Al2O3), but do not agree with a range of carbonaceous, silicate, or iron compositions. Using a detailed, physically motivated model, it is possible to constrain the composition of the dust in the tails of evaporating rocky exoplanets. This provides a unique opportunity to probe to interior composition of the smallest known exoplanets.

Search for an exosphere in sodium and calcium in the transmission spectrum of exoplanet 55 Cancri e

22 Aug 2016

by Ridden-Harper, A. R., Snellen, I. A. G., Keller, C. U., de Kok, R. J., Di Gloria, E., Hoeijmakers, H. J., Brogi, M., Fridlund, M., Vermeersen, B. L. A., van Westrenen, W., is now available here.

Abstract: [Abridged] The aim of this work is to search for an absorption signal from exospheric sodium (Na) and singly ionized calcium (Ca$^+$) in the optical transmission spectrum of the hot rocky super-Earth 55 Cancri e. Although the current best-fitting models to the planet mass and radius require a possible atmospheric component, uncertainties in the radius exist, making it possible that 55 Cancri e could be a hot rocky planet without an atmosphere. High resolution (R$\sim$110000) time-series spectra of five transits of 55 Cancri e, obtained with three different telescopes (UVES/VLT, HARPS/ESO 3.6m & HARPS-N/TNG) were analysed. Targeting the sodium D lines and the calcium H and K lines, the potential planet exospheric signal was filtered out from the much stronger stellar and telluric signals, making use of the change of the radial component of the orbital velocity of the planet over the transit from -57 to +57 km/sec. Combining all five transit data sets, we detect a signal potentially associated with sodium in the planet exosphere at a statistical significance level of 3$\sigma$. Combining the four HARPS transits that cover the calcium H and K lines, we also find a potential signal from ionized calcium (4.1 $\sigma$). Interestingly, this latter signal originates from just one of the transit measurements - with a 4.9$\sigma$ detection at this epoch. Unfortunately, due to the low significance of the measured sodium signal and the potentially variable Ca$^+$ signal, we estimate the p-values of these signals to be too high (corresponding to <4$\sigma$) to claim unambiguous exospheric detections. By comparing the observed signals with artificial signals injected early in the analysis, the absorption by Na and Ca$^+$ are estimated to be at a level of approximately 2.3$\times 10^{-3}$ and 7.0$\times 10^{-2}$ respectively, relative to the stellar spectrum.

FlySPEX: a flexible multi-angle spectropolarimetric sensing system

22 Aug 2016

by Snik, Frans, Keller, Christoph U., Wijnen, Merijn, Peters, Hubert, Derks, Roy, Smulders, Edwin, is now available here.

Abstract: Accurate multi-angle spectropolarimetry permits the detailed and unambiguous characterization of a wide range of objects. Science cases and commercial applications include atmospheric aerosol studies, biomedical sensing, and food quality control. We introduce the FlySPEX spectropolarimetric fiber-head that constitutes the essential building block of a novel multi-angle sensing system. A combination of miniaturized standard polarization optics inside every fiber-head encodes the full linear polarization information as a spectral modulation of the light that enters two regular optical fibers. By orienting many FlySPEX fiber-heads in any desired set of directions, a fiber bundle contains the complete instantaneous information on polarization as a function of wavelength and as a function of the set of viewing directions. This information is to be recorded by one or several multi-fiber spectrometers. Not only is this system flexible in the amount of viewing directions and their configuration, it also permits multiplexing different wavelength ranges and spectral resolutions by implementing different spectrometers. We present the design and prototyping for a FlySPEX fiber-head that is optimized for both polarimetric accuracy and commercial series production. We integrate the polarimetric calibration of each FlySPEX fiber-head in the manufacturing process.

Accurate spectrally modulating polarimeters for atmospheric aerosol characterization

22 Aug 2016

by Rietjens, Jeroen H. H., Smit, Martijn, van Harten, Gerard, Di Noia, Antonio, Hasekamp, Otto P., de Boer, Jos, Volten, Hester, Snik, Frans, Keller, Christoph U., is now available here.

Abstract: Highly accurate multi-angle polarimeters are essential for taking the next step in global characterization of atmospheric aerosol. Spectral polarization modulation enables highly accurate snapshot polarimetry and is very suitable for ground-, air- and space-based instrumentation. In this paper we present two instruments that employ this technology, the SPEX prototype and groundSPEX. We have performed ground-based measurements at the CESAR Observatory in the Netherlands with these two instruments. We compare the measured degree of linear polarization of co-located measurements, which show an rms difference of 0.005. Aerosol microphysical properties that have been retrieved from these measurements agree well with similar retrievals from AERONET measurements. Finally, we discuss the current efforts to upgrade the SPEX prototype to an autonomous instrument suitable for flying on NASA's ER-2 high altitude aircraft.

Instrumemtation

22 Aug 2016

by Keller, Christoph U., Snik, Frans, Harrington, David M., Packham, Chris, is now available here.

Abstract: Not Available

Inversions of High-Cadence SOLIS-VSM Stokes Observations

22 Aug 2016

by Fischer, C. E., Keller, C. U., Snik, F., is now available here.

Abstract: We have processed full-Stokes observations made with the SOLIS-VSM using Fe I 630.15 and Fe I 630.25 nm. The data have high spectral and temporal resolution, moderate spatial resolution, and large polarimetric sensitivity and accuracy. We use the code LILIA, an LTE inversion code written by Socas-Navarro (2001), in order to invert the data in vector magnetic fields. The 180-degree ambiguity in magnetic field orientation is solved by using the Non-Potential Field Calculation (NPFC) method of Georgoulis (2005). The output product are maps of the fullmagnetic field vector at the photospheric level, as illustrated in Fig. 1. We performed such inversions for observations of active region NOAA 10808 taken during an X-class flare in September 2005. Details of the data processing and the first results are given in the proceedings of the Fifth Solar PolarizationWorkshop (ASP Conf. Ser., in press).

The Hera Saturn Entry Probe Mission

21 Nov 2015

by Mousis, O., Atkinson, D. H., Spilker, T., Venkatapathy, E., Poncy, J., Frampton, R., Coustenis, A., Reh, K., Lebreton, J.-P., Fletcher, L. N., Hueso, R., Amato, M. J., Colaprete, A., Ferri, F., Stam, D., Wurz, P., Atreya, S., Aslam, S., Banfield, D. J., Calcutt, S., Fischer, G., Holland, A., Keller, C., Kessler, E., Leese, M., Levacher, P., Morse, A., Munoz, O., Renard, J.-B., Sheridan, S., Schmider, F.-X., Snik, F., Waite, J. H., Bird, M., Cavalié, T., Deleuil, M., Fortney, J., Gautier, D., Guillot, T., Lunine, J. I., Marty, B., Nixon, C., Orton, G. S., Sanchez-Lavega, A., is now available here.

Abstract: The Hera Saturn entry probe mission is proposed as an M--class mission led by ESA with a contribution from NASA. It consists of one atmospheric probe to be sent into the atmosphere of Saturn, and a Carrier-Relay spacecraft. In this concept, the Hera probe is composed of ESA and NASA elements, and the Carrier-Relay Spacecraft is delivered by ESA. The probe is powered by batteries, and the Carrier-Relay Spacecraft is powered by solar panels and batteries. We anticipate two major subsystems to be supplied by the United States, either by direct procurement by ESA or by contribution from NASA: the solar electric power system (including solar arrays and the power management and distribution system), and the probe entry system (including the thermal protection shield and aeroshell). Hera is designed to perform in situ measurements of the chemical and isotopic compositions as well as the dynamics of Saturn's atmosphere using a single probe, with the goal of improving our understanding of the origin, formation, and evolution of Saturn, the giant planets and their satellite systems, with extrapolation to extrasolar planets. Hera's aim is to probe well into the cloud-forming region of the troposphere, below the region accessible to remote sensing, to the locations where certain cosmogenically abundant species are expected to be well mixed. By leading to an improved understanding of the processes by which giant planets formed, including the composition and properties of the local solar nebula at the time and location of giant planet formation, Hera will extend the legacy of the Galileo and Cassini missions by further addressing the creation, formation, and chemical, dynamical, and thermal evolution of the giant planets, the entire solar system including Earth and the other terrestrial planets, and formation of other planetary systems.

News ArchiveThe Hera Saturn entry probe missionDesign trade-off and proof of concept for LOUPE, the Lunar Observatory for Unresolved Polarimetry of EarthDirect detection of scattered light gaps in the transitional disk around HD 97048 with VLT/SPHEREDusty tails of evaporating exoplanets. II. Physical modelling of the KIC 12557548b light curveSearch for an exosphere in sodium and calcium in the transmission spectrum of exoplanet 55 Cancri eFlySPEX: a flexible multi-angle spectropolarimetric sensing systemAccurate spectrally modulating polarimeters for atmospheric aerosol characterizationInstrumemtationInversions of High-Cadence SOLIS-VSM Stokes ObservationsThe Hera Saturn Entry Probe MissionCombining high-dispersion spectroscopy with high contrast imaging: Probing rocky planets around our nearest neighborsMagnetic field topology and chemical spot distributions in the extreme Ap star HD 75049Use of neural networks in ground-based aerosol retrievals from multi-angle spectropolarimetric observationsAtmospheric aerosol characterization with a ground-based SPEX spectropolarimetric instrumentSurprising detection of an equatorial dust lane on the AGB star IRC+10216Mapping atmospheric aerosols with a citizen science network of smartphone spectropolarimetersUse of neural networks in ground-based aerosol retrievals from multi-angle spectropolarimetric observationsFocal-plane wavefront sensing with high-order adaptive optics systemsDrift scanning technique for mid-infrared background subtractionCharacterizing instrumental effects on polarization at a Nasmyth focus using NaCoThe ZIMPOL high contrast imaging polarimeter for SPHERE: system test resultsLOUPE: Spectropolarimetry of the Earth from the surface of the MoonTowards Polarimetric Exoplanet Imaging with ELTsAstronomical Polarimetry: Polarized Views of Stars and PlanetsSearching for signs of habitability with LOUPE, the Lunar Observatory of Unresolved Polarimetry of EarthFocal-plane wavefront sensing with high-order adaptive optics systemsFast & Furious focal-plane wavefront sensingSpectral line polarimetry with a channeled polarimeterOptimization-based wavefront sensorless adaptive optics for multiphoton microscopyAtmospheric aerosol characterization with a ground-based SPEX spectropolarimetric instrumentFive-dimensional optical instrumentation: combining polarimetry with time-resolved integral-field spectroscopySnapshot coherence-gated direct wavefront sensing for multi-photon microscopyInstrumental polarisation at the Nasmyth focus of the E-ELTImaging the circumstellar environment of the young T Tauri star SU AurigaeAnalysis and interpretation of 15 quarters of Kepler data of the disintegrating planet KIC 12557548 bCrowdsourced aerosol measurements using smartphone spectropolarimetersRemote sensing of atmospheric aerosols with the SPEX spectropolarimeterCalibrating a high-resolution wavefront corrector with a static focal-plane cameraIterative linear focal-plane wavefront correctionSPHERE-ZIMPOL system testing: status report on polarimetric high contrast resultsCalibrating a high-resolution wavefront corrector with a static focal-plane cameraThree-dimensional magnetic and abundance mapping of the cool Ap star HD 24712 . I. Spectropolarimetric observations in all four Stokes parametersAre there tangled magnetic fields on HgMn stars?The color dependent morphology of the post-AGB star HD 161796Observing Circumstellar Neighbourhoods with the Extreme PolarimeterSterrekundig Instituut Utrecht: The Last YearsAstronomical Polarimetry: Polarized Views of Stars and PlanetsThe color dependent morphology of the post-AGB star HD161796Magnetically Controlled Accretion on the Classical T Tauri Stars GQ Lupi and TW HydraeHARPS Spectropolarimetry of the Classical T Tauri Stars GQ Lup and TW HyaAstronomical Polarimetry: Polarized Views of Stars and PlanetsObserving the Earth as an exoplanet with LOUPE, the lunar observatory for unresolved polarimetry of EarthSemidefinite programming for model-based sensorless adaptive opticsUnusual Stokes V profiles during flaring activity of a delta sunspotPotential of phase-diversity for metrology of active instrumentsModeling the instrumental polarization of the VLT and E-ELT telescopes with the M&m's codeA spectro-polarimetric integral field spectrograph for EPICS-EPOLThe extreme polarimeter: design, performance, first results and upgradesSearching for signs of habitability with LOUPE, the Lunar Observatory of Unresolved Polarimetry of EarthEvidence for the disintegration of KIC 12557548 bSterrekundig Instituut Utrecht: The Last YearsLinear analytical solution to the phase diversity problem for extended objects based on the Born approximationBilinear solution to the phase diversity problem for extended objects based on the Born approximationExperimental validation of optimization concepts for focal-plane image processing with adaptive opticsFOAM: the modular adaptive optics frameworkExtremely fast focal-plane wavefront sensing for extreme adaptive opticsEvidence for the disintegration of KIC 12557548 bExtremely fast focal-plane wavefront sensing for extreme adaptive opticsConstraining the circumbinary envelope of Z Canis Majoris via imaging polarimetryData driven identification and aberration correction for model-based sensorless adaptive opticsHARPS spectropolarimetry of classical T Tauri starsSPEX2Earth, a novel spectropolarimeter for remote sensing of aerosols and cloudsiSPEX: everybody can measure atmospheric aerosols with a smartphone spectropolarimeteriSPEX: the creation of an aerosol sensor network of smartphone spectropolarimetersObserving the Earth as an exoplanetMultiwavelength imaging polarimetry of Venus at various phase anglesMagnetism, chemical spots, and stratification in the HgMn star ϕ PhoenicisCoherence-gated wavefront sensing for microscopy using fringe analysisSignatures of Water Clouds on Exoplanets: Numerical Simulations.New Insights into Stellar Magnetism from the Spectropolarimetry in All Four Stokes ParametersInnovative Imaging of Young Stars: First Light ExPo ObservationsSimulating Polarized Light from ExoplanetsData Reduction Approach for the Extreme PolarimeterDesign and Prototype Results of the ExPo Imaging PolarimeterPlanetary science: In search of biosignaturesObserving the Earth as an exoplanet with LOUPE, the Lunar Observatory for Unresolved Polarimetry of EarthDirect imaging of a massive dust cloud around R Coronae BorealisSignatures of Water Clouds on Exoplanets: Numerical Simulations.Joint optimization of phase diversity and adaptive optics: demonstration of potentialThe effects of disk and dust structure on observed polarimetric images of protoplanetary disksMagnetism, chemical spots, and stratification in the HgMn star phi PhoenicisSpectropolarimeter for planetary exploration (SPEX): performance measurements with a prototypeNo magnetic field in the spotted HgMn star μ LeporisPrototyping for the Spectropolarimeter for Planetary EXploration (SPEX): calibration and sky measurementsM&m's: an error budget and performance simulator code for polarimetric systemsThe ZIMPOL high contrast imaging polarimeter for SPHERE: sub-system test resultsFast horizontal flows in a quiet sun MHD simulation and their spectroscopic signaturesThe search for magnetic fields in mercury-manganese starsSpectral and polarimetric characterization of gazeous and telluric planets with SEE COASTData-reduction techniques for high-contrast imaging polarimetry. Applications to ExPoData Reduction Techniques for High Contrast Imaging Polarimetry. Applications to ExPoFirst Detection of Linear Polarization in the Line Profiles of Active Cool StarsChemical spots in the absence of magnetic field in the binary HgMn star 66 EridaniThe Polarization Optics for the European Solar TelescopeThe HARPS PolarimeterHARPSpol — The New Polarimetric Mode for HARPSEPOL: the exoplanet polarimeter for EPICS at the E-ELTImaging polarimetry of protoplanetary disks: feasibility and usabilityImaging polarimetry of circumstellar environments with the Extreme PolarimeterThe search for magnetic fields in mercury-manganese starsThe search for magnetic fields in mercury-manganese starsThe HARPS polarimeterObservations of solar scattering polarization at high spatial resolutionThe polarization optics for the European Solar Telescope (EST)EPOL: the exoplanet polarimeter for EPICS at the E-ELTThe ZIMPOL high-contrast imaging polarimeter for SPHERE: design, manufacturing, and testingEPICS: direct imaging of exoplanets with the E-ELTSPEX: the spectropolarimeter for planetary explorationObservations of solar scattering polarization at high spatial resolutionInversions of High-Cadence SOLIS-VSM Stokes ObservationsEPICS, the exoplanet imager for the E-ELTTwo Ways of Improving Stokes InversionsStatistics of Convective Collapse Events in the Photosphere and Chromosphere Observed with the HINODE SOTThe case for spectropolarimetry with SPEX on EJSMStatistics of convective collapse events in the photosphere and chromosphere observed with the Hinode SOTAn IFU for diffraction-limited 3D spectroscopic imaging: laboratory and on-site testsPolarimetric Measurements of Protoplanetary Disks with ExPo Tim van Werkhoven The Prototype of the Small Synoptic Second Solar Spectrum Telescope (S^5T)Polarimetry from the Ground UpVector Magnetic Field Inversions of High Cadence SOLIS-VSM DataSOLIS Vector Spectromagnetograph: Status and ScienceA Brief History of the Second Solar SpectrumStatistics of convective collapse events in the photosphere and chromosphere observed with the Hinode SOTCharacterization of Extra-solar Planets with Direct-Imaging TechniquesPolarization Properties of Real Aluminum Mirrors, I. Influence of the Aluminum Oxide LayerSuper earth explorer: a coronagraphic off-axis space telescopePolarization properties of real aluminum mirrors; I. Influence of the aluminum oxide layerSpectral modulation for full linear polarimetryThe Prototype of the Small Synoptic Second Solar Spectrum Telescope (S5T)An analytical model to demonstrate the reliability of reconstructed `active longitudes'.Polarimetry of Mars with SPEX, an Innovative SpectropolarimeterDiversity among other worlds: characterization of exoplanets by direct detectionA New Era in Solar Thermal-IR Astronomy: the NSO Array Camera (NAC) on the McMath-Pierce TelescopeSPEX: an in-orbit spectropolarimeter for planetary explorationDesign of a laboratory simulator to test exoplanet imaging polarimetryThe Extreme Polarimeter (ExPo): design of a sensitive imaging polarimeterSPHERE ZIMPOL: overview and performance simulationThe upgrade of HARPS to a full-Stokes high-resolution spectropolarimeterDesign of a laboratory simulator to test exoplanet imaging polarimetryThe Extreme Polarimeter (ExPo): design of a sensitive imaging polarimeterSPHERE ZIMPOL: overview and performance simulationThe upgrade of HARPS to a full-Stokes high-resolution spectropolarimeterPolarimetry from the Ground UpNew web page online