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Christoph U. Keller
Professor of Experimental Astrophysics

Leiden Observatory
Leiden University, The Netherlands

BP Piscium: its flaring disc imaged with SPHERE/ZIMPOL

23 Dec 2016

by de Boer, J., Girard, J. H., Canovas, H., Min, M., Sitko, M., Ginski, C., Jeffers, S. V., Mawet, D., Milli, J., Rodenhuis, M., Snik, F., Keller, C. U., is now available here.

Abstract: Not Available

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

23 Dec 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: Context. Evaporating rocky exoplanets, such as KIC 12557548b, eject large amounts of dust, 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. Aims: 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. Methods: Using a self-consistent numerical model of the dust dynamics and sublimation, we calculated 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 generated synthetic light curves (incorporating the effects of extinction and angle-dependent scattering), which were then compared with the phase-folded Kepler light curve. We explored the free-parameter space thoroughly using a Markov chain Monte Carlo method. Results: 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 μm and dust mass loss rates of 0.6 to 15.6 M⊕ Gyr-1 (2σ 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. Conclusions: 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.

Multiple rings in the transition disk and companion candidates around RX J1615.3-3255. High contrast imaging with VLT/SPHERE

23 Dec 2016

by de Boer, J., Salter, G., Benisty, M., Vigan, A., Boccaletti, A., Pinilla, P., Ginski, C., Juhasz, A., Maire, A.-L., Messina, S., Desidera, S., Cheetham, A., Girard, J. H., Wahhaj, Z., Langlois, M., Bonnefoy, M., Beuzit, J.-L., Buenzli, E., Chauvin, G., Dominik, C., Feldt, M., Gratton, R., Hagelberg, J., Isella, A., Janson, M., Keller, C. U., Lagrange, A.-M., Lannier, J., Menard, F., Mesa, D., Mouillet, D., Mugrauer, M., Peretti, S., Perrot, C., Sissa, E., Snik, F., Vogt, N., Zurlo, A., SPHERE Consortium, is now available here.

Abstract: Context. The effects of a planet sculpting the disk from which it formed are most likely to be found in disks that are in transition between being classical protoplanetary and debris disks. Recent direct imaging of transition disks has revealed structures such as dust rings, gaps, and spiral arms, but an unambiguous link between these structures and sculpting planets is yet to be found. Aims: We aim to find signs of ongoing planet-disk interaction and study the distribution of small grains at the surface of the transition disk around RX J1615.3-3255 (RX J1615). Methods: We observed RX J1615 with VLT/SPHERE. From these observations, we obtained polarimetric imaging with ZIMPOL (R'-band) and IRDIS (J), and IRDIS (H2H3) dual-band imaging with simultaneous spatially resolved spectra with the IFS (YJ). Results: We image the disk for the first time in scattered light and detect two arcs, two rings, a gap and an inner disk with marginal evidence for an inner cavity. The shapes of the arcs suggest that they are probably segments of full rings. Ellipse fitting for the two rings and inner disk yield a disk inclination i = 47 ± 2° and find semi-major axes of 1.50 ± 0.01'' (278 au), 1.06 ± 0.01'' (196 au) and 0.30 ± 0.01'' (56 au), respectively. We determine the scattering surface height above the midplane, based on the projected ring center offsets. Nine point sources are detected between 2.1'' and 8.0'' separation and considered as companion candidates. With NACO data we recover four of the nine point sources, which we determine to be not co-moving, and therefore unbound to the system. Conclusions: We present the first detection of the transition disk of RX J1615 in scattered light. The height of the rings indicate limited flaring of the disk surface, which enables partial self-shadowing in the disk. The outermost arc either traces the bottom of the disk or it is another ring with semi-major axis ≳ 2.35'' (435 au). We explore both scenarios, extrapolating the complete shape of the feature, which will allow us to distinguish between the two in future observations. The most attractive scenario, where the arc traces the bottom of the outer ring, requires the disk to be truncated at r ≈ 360 au. If the closest companion candidate is indeed orbiting the disk at 540 au, then it would be the most likely cause for such truncation. This companion candidate, as well as the remaining four, all require follow up observations to determine if they are bound to the system. Based on observations made with ESO Telescopes at the La Silla Paranal Observatory under programme IDs 095.C-0298(A), 095.C-0298(B), and 095.C-0693(A) during guaranteed and open time observations of the SPHERE consortium, and on NACO observations: program IDs: 085.C-0012(A), 087.C-0111(A), and 089.C-0133(A). The reduced images as FITS files are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (http://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/595/A114

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

23 Dec 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: Aims: 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. Methods: 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. Results: 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 consistent with theoretical models of compact dust aggregates. We discuss the origin of the detected features and find that low mass (≤1 MJup) nascent planets are a possible explanation. Based on data collected at the European Southern Observatory, Chile (ESO Programs 096.C-0248, 096.C-0241, 077.C-0106).

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

23 Dec 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: Context. The atmospheric and surface characterization of rocky planets is a key goal of exoplanet science. Unfortunately, the measurements required for this are generally out of reach of present-day instrumentation. However, the planet Mercury in our own solar system exhibits a large exosphere composed of atomic species that have been ejected from the planetary surface by the process of sputtering. Since the hottest rocky exoplanets known so far are more than an order of magnitude closer to their parent star than Mercury is to the Sun, the sputtering process and the resulting exospheres could be orders of magnitude larger and potentially detectable using transmission spectroscopy, indirectly probing their surface compositions. Aims: 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. Methods: High resolution (R ~ 110 000) time-series spectra of five transits of 55 Cancri e, obtained with three different telescopes (UVES/VLT, HARPS/ESO 3.6 m and 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 s-1. Results: Combining all five transit data sets, we detect a signal potentially associated with sodium in the planet exosphere at a statistical significance level of 3σ. Combining the four HARPS transits that cover the calcium H and K lines, we also find a potential signal from ionized calcium (4.1σ). Interestingly, this latter signal originates from just one of the transit measurements - with a 4.9σ 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 <4σ) 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 ~2.3 × 10-3 and ~7.0 × 10-2 respectively, relative to the stellar spectrum. Conclusions: If confirmed, the 3σ signal would correspond to an optically thick sodium exosphere with a radius of 5 R⊕, which is comparable to the Roche lobe radius of the planet. The 4.9σ detection of Ca+ in a single HARPS data set would correspond to an optically thick Ca+ exosphere approximately five times larger than the Roche lobe radius. If this were a real detection, it would imply that the exosphere exhibits extreme variability. Although no formal detection has been made, we advocate that probing the exospheres of hot super-Earths in this way has great potential, also knowing that Mercury's exosphere varies significantly over time. It may be a fast route towards the first characterization of the surface properties of this enigmatic class of planets. Based on observations collected at the European Organisation for Astronomical Research in the Southern Hemisphere under ESO programmes 092.C-0178 and 288.C-5010 and the Telescopio Nazionale Galileo under programme CAT13B_33.

Novel instrument concepts for characterizing directly imaged exoplanets

23 Dec 2016

by Keller, Christoph U., is now available here.

Abstract: Current high-contrast exoplanet imagers are optimized to find new exoplanets; they minimize diffracted starlight in a large area around a star. I present four novel instrumental approaches that are optimized to characterize these discoveries by minimizing starlight in a small area around the known location of an exoplanet: 1) coronagraphs that remove virtually all starlight over an octave in wavelength while transmitting more than 90% of the exoplanet signal; 2) holographic wavefront sensors that measure aberrations in the science focal plane; 3) ultra-fast adaptive optics systems that minimize these aberrations; and 4) direct minimization of the remaining starlight. By integrating these technologies with a high spectral- resolution, integral-field spectrograph that can resolve the Doppler shift and the polarization difference between the starlight and the reflected light from the exoplanet, it will be possible to determine the atmospheric composition, temperature and velocity structures of exoplanets and their spin rotation rate and orbital velocity. This will ultimately allow the upcoming extremely large telescopes to characterize rocky exoplanets in the habitable zone to look for signatures of life.

The Leiden EXoplanet Instrument (LEXI): a high-contrast high-dispersion spectrograph

23 Dec 2016

by Haffert, S. Y., Wilby, M. J., Keller, C. U., Snellen, I. A. G., is now available here.

Abstract: The Leiden EXoplanet Instrument (LEXI) will be the first instrument designed for high-contrast, high-dispersion integral field spectroscopy at optical wavelengths. High-contrast imaging (HCI) and high-dispersion spectroscopy (HDS) techniques are used to reach contrasts of 10-7. LEXI will be a bench-mounted, high dispersion integral field spectrograph that will record spectra in a small area around the star with high spatial resolution and high dynamic range. A prototype is being setup to The Leiden EXoplanet Instrument (LEXI) will be the first instrument designed for high-contrast, high-dispersion integral field spectroscopy at optical wavelengths. High-contrast imaging (HCI) and high-dispersion spectroscopy (HDS) techniques are used to reach contrasts of 10-7. LEXI will be a bench-mounted, high dispersion integral field spectrograph that will record spectra in a small area around the star with high spatial resolution and high dynamic range. A prototype is being setup to test the combination of HCI+HDS and its first light is expected in 2016.

End-to-end simulations of the E-ELT/METIS coronagraphs

23 Dec 2016

by Carlomagno, Brunella, Absil, Olivier, Kenworthy, Matthew, Ruane, Garreth, Keller, Christoph U., Otten, Gilles, Feldt, Markus, Hippler, Stefan, Huby, Elsa, Mawet, Dimitri, Delacroix, Christian, Surdej, Jean, Habraken, Serge, Forsberg, Pontus, Karlsson, Mikael, Vargas Catalan, Ernesto, Brandl, Bernhard R., is now available here.

Abstract: The direct detection of low-mass planets in the habitable zone of nearby stars is an important science case for future E-ELT instruments such as the mid-infrared imager and spectrograph METIS, which features vortex phase masks and apodizing phase plates (APP) in its baseline design. In this work, we present end-to-end performance simulations, using Fourier propagation, of several METIS coronagraphic modes, including focal-plane vortex phase masks and pupil-plane apodizing phase plates, for the centrally obscured, segmented E-ELT pupil. The atmosphere and the AO contributions are taken into account. Hybrid coronagraphs combining the advantages of vortex phase masks and APPs are considered to improve the METIS coronagraphic performance.

A "Fast and Furious'" solution to the low-wind effect for SPHERE at the VLT

23 Dec 2016

by Wilby, M. J., Keller, C. U., Sauvage, J.-F., Fusco, T., Mouillet, D., Beuzit, J.-L., Dohlen, K., is now available here.

Abstract: We present a potential non-invasive solution to sensing the so-called low-wind effect (LWE) seen in the SPHERE instrument at the VLT, based on the "Fast and Furious (F&F) sequential phase diversity wavefront reconstruction algorithm. This uses non-coronagraphic focal-plane images available from the near-infra-red Differential Tip-Tilt Sensor (DTTS), with the closed-loop correction cycle itself providing the necessary phase diversity between frames required to reconstruct the full wavefront phase. Crucially, this means F&F does not need to apply large artificial phase probes as required by standard phase diversity algorithms, allowing it to operate in a real-time ( 10 Hz) correction mode without impacting science observations. In this paper we present the results of realistic closed-loop AO simulations designed to emulate SPHERE/DTTS observations of the LWE. With this we demonstrate that the F&F algorithm is capable of effective removal of the characteristic point-spread function (PSF) aberrations of strongly LWE-affected images within a few closed-loop iterations, with the final wavefront quality limited only by the corrective order of the deformable mirror. The ultimate goal of this project is to provide an independent, real-time and focal-plane wavefront sensor for SPHERE which is capable of detecting and directly compensating the LWE as it arises, thus improving coronagraph performance under the best 15-20 % of observing conditions where the effect is most pronounced.

Focal-plane electric field sensing with pupil-plane holograms

23 Dec 2016

by Por, Emiel H., Keller, Christoph U., is now available here.

Abstract: The direct detection and spectral characterization of exoplanets requires a coronagraph to suppress the diffracted star light. Amplitude and phase aberrations in the optical train fill the dark zone of the coronagraph with quasi-static speckles that limit the achievable contrast. Focal-plane electric field sensing, such as phase diversity introduced by a deformable mirror (DM), is a powerful tool to minimize this residual star light. The residual electric field can be estimated by sequentially applying phase probes on the DM to inject star light with a well-known amplitude and phase into the dark zone and analyzing the resulting intensity images. The DM can then be used to add light with the same amplitude but opposite phase to destructively interfere with this residual star light. Using a static phase-only pupil-plane element we create holographic copies of the point spread function (PSF), each superimposed with a certain pupil-plane phase probe. We therefore obtain all intensity images simultaneously while still retaining a central, unaltered science PSF. The electric field sensing method only makes use of the holographic copies, allowing for correction of the residual electric field while retaining the central PSF for uninterrupted science data collection. In this paper we demonstrate the feasibility of this method with numerical simulations.

The ZIMPOL high contrast imaging polarimeter for SPHERE: polarimetric high contrast commissioning results

23 Dec 2016

by Roelfsema, Ronald, Bazzon, Andreas, Schmid, Hans Martin, Pragt, Johan, Govaert, Alain, Gisler, Daniel, Dominik, Carsten, Baruffolo, Andrea, Beuzit, Jean-Luc, Costille, Anne, Dohlen, Kjetil, Downing, Mark, Elswijk, Eddy, de Haan, Menno, Hubin, Norbert, Kasper, Markus, Keller, Christoph, Lizon, Jean-Louis, Mouillet, David, Pavlov, Alexey, Puget, Pascal, Salasnich, Bernardo, Sauvage, Jean-Francois, Wildi, Francois, is now available here.

Abstract: SPHERE (Spectro-Polarimetric High-contrast Exoplanet Research) is a second generation VLT instrument aimed at the direct detection of exo-planets. It has received its first light in May 2014. ZIMPOL (Zurich Imaging Polarimeter) is the imaging polarimeter subsystem of the SPHERE instrument. It's capable of both high accuracy and high sensitivity polarimetry but can also be used as a classical imager. It is located behind an extreme AO system and a stellar coronagraph. ZIMPOL operates at visible wavelengths which is best suited to detect the very faint reflected and hence polarized visible light from extra solar planets. During the SPHERE fourth commissioning period (October 2014) we have made deep coronagraphic observations of the bright star alpha Gru (mR = 1.75) to assess the high contrast polarimetric performance of SPHERE-ZIMPOL. We have integrated on the target for a total time of about 45 minutes during the meridian transit in the Very Broad Band filter (600 - 900 nm) with a classical Lyot coronagraph with 3 λ/D radius focal mask. We reduce the data by a combination of Polarized Background subtraction, Polarimetric Differential Imaging (PDI) and Angular Differential Imaging (ADI). We reach contrasts of 10-6 and 10-7 at a radial distances of respectively 7 and 14 lambda/D from the PSF core. At these radial distances we are respectively a factor of 10 and 2 above the photon noise limit. We discuss our results by considering the temporal and spatial speckle behavior close to the PSF core in combination with low order polarimetric aberrations.

Designing and testing the coronagraphic Modal Wavefront Sensor: a fast non-common path error sensor for high-contrast imaging

23 Dec 2016

by Wilby, M. J., Keller, C. U., Haffert, S., Korkiakoski, V., Snik, F., Pietrow, A. G. M., is now available here.

Abstract: Non-Common Path Errors (NCPEs) are the dominant factor limiting the performance of current astronomical high-contrast imaging instruments. If uncorrected, the resulting quasi-static speckle noise floor limits coronagraph performance to a raw contrast of typically 10-4, a value which does not improve with increasing integration time. The coronagraphic Modal Wavefront Sensor (cMWS) is a hybrid phase optic which uses holographic PSF copies to supply focal-plane wavefront sensing information directly from the science camera, whilst maintaining a bias-free coronagraphic PSF. This concept has already been successfully implemented on-sky at the William Herschel Telescope (WHT), La Palma, demonstrating both real-time wavefront sensing capability and successful extraction of slowly varying wavefront errors under a dominant and rapidly changing atmospheric speckle foreground. In this work we present an overview of the development of the cMWS and recent first light results obtained using the Leiden EXoplanet Instrument (LEXI), a high-contrast imager and high-dispersion spectrograph pathfinder instrument for the WHT.

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).

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