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.
by Snellen, I., de Kok, R., Birkby, J. L., Brandl, B., Brogi, M., Keller, C., Kenworthy, M., Schwarz, H., Stuik, R., is now available here.
Abstract: Context. Ground-based high-dispersion (R ~ 100 000) spectroscopy (HDS) is proving to be a powerful technique with which to characterize extrasolar planets. The planet signal is distilled from the bright starlight, combining ral and time-differential filtering techniques. In parallel, high-contrast imaging (HCI) is developing rapidly, aimed at spatially separating the planet from the star. While HDS is limited by the overwhelming noise from the host star, HCI is limited by residual quasi-static speckles. Both techniques currently reach planet-star contrast limits down to ~10-5, albeit for very different types of planetary systems. Aims: In this work, we discuss a way to combine HDS and HCI (HDS+HCI). For a planet located at a resolvable angular distance from its host star, the starlight can be reduced up to several orders of magnitude using adaptive optics and/or coronography. In addition, the remaining starlight can be filtered out using high-dispersion spectroscopy, utilizing the significantly different (or Doppler shifted) high-dispersion spectra of the planet and star. In this way, HDS+HCI can in principle reach contrast limits of ~10-5 × 10-5, although in practice this will be limited by photon noise and/or sky-background. In contrast to current direct imaging techniques, such as Angular Differential Imaging and Spectral Differential Imaging, it will work well at small working angles and is much less sensitive to speckle noise. For the discovery of previously unknown planets HDS+HCI requires a high-contrast adaptive optics system combined with a high-dispersion R ~ 100 000 integral field spectrograph (IFS). This combination currently does not exist, but is planned for the European Extremely Large Telescope. Methods: We present simulations of HDS+HCI observations with the E-ELT, both probing thermal emission from a planet at infrared wavelengths, and starlight reflected off a planet atmosphere at optical wavelengths. For the infrared simulations we use the baseline parameters of the E-ELT and METIS instrument, with the latter combining extreme adaptive optics with an R = 100 000 IFS. We include realistic models of the adaptive optics performance and atmospheric transmission and emission. For the optical simulation we also assume R = 100 000 IFS with adaptive optics capabilities at the E-ELT. Results: One night of HDS+HCI observations with the E-ELT at 4.8 μm (Δλ = 0.07 μm) can detect a planet orbiting α Cen A with a radius of R = 1.5 Rearth and a twin-Earth thermal spectrum of Teq = 300 K at a signal-to-noise (S/N) of 5. In the optical, with a Strehl ratio performance of 0.3, reflected light from an Earth-size planet in the habitable zone of Proxima Centauri can be detected at a S/N of 10 in the same time frame. Recently, first HDS+HCI observations have shown the potential of this technique by determining the spin-rotation of the young massive exoplanet β Pictoris b. Conclusions: The exploration of the planetary systems of our neighbor stars is of great scientific and philosophical value. The HDS+HCI technique has the potential to detect and characterize temperate rocky planets in their habitable zones. Exoplanet scientists should not shy away from claiming a significant fraction of the future ELTs to make such observations possible.
by Kochukhov, O., Rusomarov, N., Valenti, J. A., Stempels, H. C., Snik, F., Rodenhuis, M., Piskunov, N., Makaganiuk, V., Keller, C. U., Johns-Krull, C. M., is now available here.
Abstract: Context. Intermediate-mass, magnetic chemically peculiar (Ap) stars provide a unique opportunity to study the topology of stellar magnetic fields in detail and to investigate magnetically driven processes of spot formation. Aims: Here we aim to derive the surface magnetic field geometry and chemical abundance distributions for the extraordinary Ap star HD 75049. This object hosts a surface field of ~30 kG, one of the strongest known for any non-degenerate star. Methods: We used time-series of high-resolution HARPS intensity and circular polarisation observations. These data were interpreted with the help of magnetic Doppler imaging and model atmospheres incorporating effects of a non-solar chemical composition and a strong magnetic field. Results: Based on high-precision measurements of the mean magnetic field modulus, we refined the rotational period of HD 75049 to Prot = 4.048267 ± 0.000036 d. We also derived basic stellar parameters, Teff = 10 250 ± 250 K and log g = 4.3 ± 0.1. Magnetic Doppler imaging revealed that the field topology of HD 75049 is poloidal and dominated by a dipolar contribution with a peak surface field strength of 39 kG. At the same time, deviations from the classical axisymmetric oblique dipolar configuration are significant. Chemical surface maps of Si, Cr, Fe, and Nd show abundance contrasts of 0.5-1.4 dex, which is low compared with many other Ap stars. Of the chemical elements, Nd is found to be enhanced close to the magnetic pole, whereas Si and Cr are concentrated predominantly at the magnetic equator. The iron distribution shows low-contrast features both at the magnetic equator and the pole. Conclusions: The morphology of the magnetic field and the properties of chemical spots in HD 75049 are qualitatively similar to those of Ap stars with weaker fields. Consequently, whatever mechanism forms and sustains global magnetic fields in intermediate-mass main-sequence stars, it operates in the same way over the entire observed range of magnetic field strengths. Based on observations collected at the European Southern Observatory, Chile (ESO programs 084.D-0338, 085.D-0296, 086.D-0240, 088.D-0066, 090.D-0256, 078.D-0192, 080.D-0170).
by Di Noia, A., Hasekamp, O. P., van Harten, G., Rietjens, J. H. H., Smit, J. M., Snik, F., Henzing, J. S., de Boer, J., Keller, C. U., Volten, H., is now available here.
Abstract: In this paper, the use of a neural network algorithm for the retrieval of the aerosol properties from ground-based spectropolarimetric measurements is discussed. The neural network is able to retrieve the aerosol properties with an accuracy that is almost comparable to that of an iterative retrieval. By using the outcome of the neural network as first guess in the iterative retrieval scheme, the accuracy of the retrieved fine- and coarse-mode optical thickness is further improved, while for the other parameters the improvement is small or absent. The resulting scheme (neural network + iterative retrieval) is compared to the original one (look-up table + iterative retrieval) on a set of simulated ground-based measurements, and on a small set of real observations carried out by an accurate ground-based spectropolarimeter. The results show that the use of a neural-network-based first guess leads to an increase in the number of converging retrievals, and possibly to more accurate estimates of the aerosol effective radius and complex refractive index.
by van Harten, G., de Boer, J., Rietjens, J. H. H., Di Noia, A., Snik, F., Volten, H., Smit, J. M., Hasekamp, O. P., Henzing, J. S., Keller, C. U., is now available here.
Abstract: Characterization of atmospheric aerosols is important for understanding their impact on health and climate. A wealth of aerosol parameters can be retrieved from multi-angle, multi-wavelength radiance and polarization measurements of the clear sky. We developed a ground-based SPEX instrument (groundSPEX) for accurate spectropolarimetry, based on the passive, robust, athermal, and snapshot spectral polarization modulation technique, and is hence ideal for field deployment. It samples the scattering phase function in the principal plane in an automated fashion, using a motorized pan/tilt unit and automatic exposure time detection. Extensive radiometric and polarimetric calibrations were performed, yielding values for both random noise and systematic uncertainties. The absolute polarimetric accuracy at low degrees of polarization is established to be ~5 × 10-3. About 70 measurement sequences have been performed throughout four clear-sky days at Cabauw, the Netherlands. Several aerosol parameters were retrieved: aerosol optical thickness, effective radius, and complex refractive index for fine and coarse mode. The results are in good agreement with the colocated AERONET products, with a correlation coefficient of ρ = 0.932 for the total aerosol optical thickness at 550 nm.
by Jeffers, S. V., Min, M., Waters, L. B. F. M., Canovas, H., Pols, O. R., Rodenhuis, M., de Juan Ovelar, M., Keller, C. U., Decin, L., is now available here.
Abstract: Aims: Understanding the formation of planetary nebulae remains elusive because in the preceding asymptotic giant branch (AGB) phase these stars are heavily enshrouded in an optically thick dusty envelope. Methods: To further understand the morphology of the circumstellar environments of AGB stars we observe the closest carbon-rich AGB star IRC+10216 in scattered light. Results: When imaged in scattered light at optical wavelengths, IRC+10216 surprisingly shows a narrow equatorial density enhancement, in contrast to the large-scale spherical rings that have been imaged much further out. We use radiative transfer models to interpret this structure in terms of two models: firstly, an equatorial density enhancement, commonly observed in the more evolved post-AGB stars, and secondly, in terms of a dust rings model, where a local enhancement of mass-loss creates a spiral ring as the star rotates. Conclusions: We conclude that both models can be used to reproduce the dark lane in the scattered light images, which is caused by an equatorially density enhancement formed by dense dust rather than a bipolar outflow as previously thought. We are unable to place constraints on the formation of the equatorial density enhancement by a binary system. Final reduced images (FITS) are available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://126.96.36.199) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/572/A3Based on observations made with the William Herschel Telescope operated on the island of La Palma by the Isaac Newton Group in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias.
by Snik, Frans, Rietjens, Jeroen H. H., Apituley, Arnoud, Volten, Hester, Mijling, Bas, Di Noia, Antonio, Heikamp, Stephanie, Heinsbroek, Ritse C., Hasekamp, Otto P., Smit, J. Martijn, Vonk, Jan, Stam, Daphne M., Harten, Gerard, Boer, Jozua, Keller, Christoph U., is now available here.
Abstract: To assess the impact of atmospheric aerosols on health, climate, and air traffic, aerosol properties must be measured with fine spatial and temporal sampling. This can be achieved by actively involving citizens and the technology they own to form an atmospheric measurement network. We establish this new measurement strategy by developing and deploying iSPEX, a low-cost, mass-producible optical add-on for smartphones with a corresponding app. The aerosol optical thickness (AOT) maps derived from iSPEX spectropolarimetric measurements of the daytime cloud-free sky by thousands of citizen scientists throughout the Netherlands are in good agreement with the spatial AOT structure derived from satellite imagery and temporal AOT variations derived from ground-based precision photometry. These maps show structures at scales of kilometers that are typical for urban air pollution, indicating the potential of iSPEX to provide information about aerosol properties at locations and at times that are not covered by current monitoring efforts.
by Di Noia, A., Hasekamp, O. P., van Harten, G., Rietjens, J. H. H., Smit, J. M., Snik, F., Henzing, J. S., de Boer, J., Keller, C. U., Volten, H., is now available here.
Abstract: In this paper, the use of a neural network algorithm for the retrieval of the aerosol properties from ground-based spectropolarimetric measurements is discussed. The neural network is able to retrieve the aerosol properties with an accuracy that is almost comparable to that of an iterative retrieval. By using the outcome of the neural network as a first guess of the iterative retrieval scheme, the accuracy of the fine and coarse mode optical thickness are further improved while for the other parameters the improvement is small or absent. The resulting scheme (neural network + iterative retrieval) is compared to the original one (look-up table + iterative retrieval) on a set of simulated ground-based measurements, and on a small set of real observations carried out by an accurate ground-based spectropolarimeter. The results show that the use of a neural network based first guess leads to an increase in the number of converging retrievals, and possibly to more accurate estimates of the aerosol effective radius and complex refractive index.
by Korkiakoski, Visa, Keller, Christoph U., Doelman, Niek, Kenworthy, Matthew, Otten, Gilles, Verhaegen, Michel, is now available here.
Abstract: We investigate methods to calibrate the non-common path aberrations at an adaptive optics system having a wavefront-correcting device working with an extremely high resolution (larger than 150x150 correcting elements). We use focal-plane images collected successively, the corresponding phase-diversity information and numerically efficient algorithms to calculate the required wavefront updates. Different approaches are considered in numerical simulations, and laboratory experiments are shown to confirm the results. We compare the performances of the standard Gerchberg-Saxton algorithm, Fast and Furious (use of small-phase assumption to take advantage of linearisation) and recently proposed phase-retrieval methods based on convex optimisation. The results indicate that the calibration task is easiest with algorithms similar to Fast and Furious, at least in the framework we considered.
by Heikamp, Stephanie, Brandl, Bernhard R., Keller, Christoph U., Venema, Lars, Pantin, Eric, Siebenmorgen, Ralf, Ives, Derek, Kerber, Florian, is now available here.
Abstract: Accurate calibration of ground-based, mid-infrared observations is challenging due to the strong and rapidly varying thermal background emission. The classical solution is the chopping/nodding technique where the secondary mirror and the telescope are being moved by several tens of arcseconds on the sky. However, chopping is generally inefficient and limited in accuracy and frequency by the mass and size of the secondary mirror. A more elegant solution is a drift scan where the telescope slowly drifts across or around the region of interest; the source moves on the detector by at least one FWHM of the PSF within the time over which the detector performance and the background emission can be considered stable. The final image of a drift scan is mathematically reconstructed from a series of adjacent short exposures. The drift scan approach has recently received a lot of interest, mainly for two reasons: first, some of the new, large-format mid-IR Si:As detectors (AQUARIUS) suffer from excess low frequency noise (ELFN). To reach the nominal performance limit of the detectors, chopping would have to be performed at a high frequency, faster than what most telescopes can handle; second, the next generation of extremely large telescopes will not offer chopping/nodding, and alternative methods need to be developed and tested. In this paper we present the results from simulated drift scan data. We use drift scanning to simultaneously obtain an accurate detector flat field and the sky background. The results are relevant for the future operation and calibration of VISIR at the VLT as well as for METIS, the thermal infrared instrument for the E-ELT.
by de Boer, Jozua, Girard, Julien H., Mawet, Dimitri, Snik, Frans, Keller, Christoph U., Milli, Julien, is now available here.
Abstract: We propose a new calibration scheme to determine the instrumental polarization (IP) and crosstalk induced by either the telescope or an instrument at Nasmyth focus. We measure the polarized blue sky at zenith with VLT/UT4/NaCo for different NaCo derotator and telescope azimuth angles. Taking multiple measurements after rotating both the instrument and the telescope with angles of 90° allows use to determine the IP and most crosstalk components separately for the telescope and the instrument. This separation of the Mueller matrices of UT4 and the NaCo is especially important for measurements taken in the conventional polarimetric mode (field stabilized), because the rotation of the instrument with respect to M3 causes a variation in the IP and crosstalk throughout the measurement. The technique allows us to determine the IP with an accuracy of 0.4%, and constrain or determine lower or upper limits for most crosstalk components. Most notably, the UT4 U --> V crosstalk is substantially larger than theory predicts. An angular offset in NaCo's half wave plate orientation is a possible source of systematic errors. We measure this offset to be 1.8° +/- 0.5°.
by Roelfsema, Ronald, Bazzon, Andreas, Schmid, Hans Martin, Pragt, Johan, 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 new instrument for the VLT 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 (600-900 nm) which is best suited to detect the very faint reflected and hence polarized visible light from extra solar planets. It has an instantaneous Field of View of 3 x 3 arcsec2 (extendable to 8 arcsec diameter) with an angular resolution of 14 mili-arcsec. We discuss the results that are obtained from the full SPHERE-ZIMPOL system testing. In particular the optical, polarimetric and high contrast performance.
by Hoeijmakers, H. J., Snik, F., Stam, D. M., Keller, C. U., is now available here.
Abstract: We present our prototype for the LOUPE instrument: A small and robust imaging spectropolarimeter that can observe the Earth from the surface of the moon, with as primary objective to characterize the Earth's linear polarization spectrum throughout the Earth's daily rotation and monthly phase angle changes. The purpose of LOUPE is to provide benchmark data for future polarization observations of possibly habitable exoplanets. Our instrument concept has been proven to work in a laboratory setting, and efforts are being made to design and produce a flight model.
by Keller, C. U., Korkiakoski, V., Rodenhuis, M., Snik, F., is now available here.
Abstract: A prime science goal of Extremely Large Telescopes (ELTs) is the detection and characterization of exoplanets to answer the question: are we alone? ELTs will obtain the first direct images of rocky exoplanets in the habitable zone and search for atmospheric biomarkers. However, the required instrumental technologies are not yet at a level where an instrument could be built that would achieve this goal. Polarimetry will be an important ingredient in future high-contrast instruments as it will provide a major contrast improvement for planets located within the first two Airy rings and offers unique diagnostic capabilities for liquid water (ocean glint, water clouds and their rainbows), hazes and dust in exoplanetary atmospheres. We will describe novel instrumental approaches to improving subsystems, in particular polarimetry, wavefront sensing and adaptive optics control. To reach contrasts of 10-9 and beyond to image rocky exoplanets from the ground, a series of individually optimized subsystems cannot succeed; rather, entire combinations of subsystems must be optimized together. We will describe our efforts at measuring and controlling wavefronts with 40'000 degrees of freedom, reaching the photon-noise limit in high-contrast polarimetric imaging at telescopes and our plans to reach a contrast of at least 10-9 in broadband light under realistic, simulated ground-based conditions in the laboratory and to test new approaches at telescopes, in particular achromatic aperture and focal-plane coronagraphs, focal-plane wavefront-sensing and speckle suppression, integral-field polarimetry and high-contrast data reduction algorithms.
by Snik, Frans, Keller, Christoph U., is now available here.
Abstract: Polarization is a fundamental property of light from astronomical objects, and measuring that polarization often yields crucial information, which is unobtainable otherwise.This chapter reviews the useful formalisms for describing polarization in the optical regime, the mechanisms for the creation of such polarization, and methods for measuring it. Particular emphasis is given on how to implement a polarimeter within an astronomical facility, and on how to deal with systematic effects that often limit the polarimetric performance.
by Karalidi, T., Stam, D. M., Snik, F., Bagnulo, S., Sparks, W. B., Keller, C. U., is now available here.
Abstract: We present LOUPE, a novel type of spectropolarimeter to measure the flux and state of polarization of sunlight that is reflected by the Earth from 0.4 to 0.8 μm. LOUPE has been designed as payload of a lunar lander. From the moon, the Earth can be observed as a whole, during its daily rotation and at all phase angles, just as if it were an exoplanet. LOUPE will provide benchmark data for the development of instruments for Earth-like exoplanet characterization, and for the testing of numerical retrieval algorithms.