Daniel Gisler

Shadows cast on the transition disk of HD 135344B - Multiwavelength VLT/SPHERE polarimetric differential imaging

Context. The protoplanetary disk around the F-type star HD 135344B (SAO 206462) is in a transition stage and shows many intriguing structures both in scattered light and thermal (sub-) millimeter emission which are possibly related to planet formation processes. Aims. We aim to study the morphology and surface brightness of the disk in scattered light to gain insight into the innermost disk regions, the formation of protoplanets, planet-disk interactions traced in the surface and midplane layers, and the dust grain properties of the disk surface. Methods. We have carried out high-contrast polarimetric differential imaging (PDI) observations with VLT/SPHERE and obtained polarized scattered light images with ZIMPOL in the R and I-bands and with IRDIS in the Y and J-bands. The scattered light images and surface brightness profiles are used to study in detail structures in the disk surface and brightness variations. We have constructed a 3D radiative transfer model to support the interpretation of several detected shadow features. Results. The scattered light images reveal with unprecedented angular resolution and sensitivity the spiral arms as well as the 25 au cavity of the disk. Multiple shadow features are discovered on the outer disk with one shadow only being present during the second observation epoch. A positive surface brightness gradient is observed in the stellar irradiation corrected (r(2)-scaled) images in southwest direction possibly due to an azimuthally asymmetric perturbation of the temperature and/or surface density by the passing spiral arms. The disk integrated polarized flux, normalized to the stellar flux, shows a positive trend towards longer wavelengths which we attribute to large (2 pi alpha \textgreater= lambda) aggregate dust grains in the disk surface. Part of the non-azimuthal polarization signal in the U-phi image of the J-band observation can be attributed to multiple scattering in the disk. Conclusions. The detected shadow features and their possible variability have the potential to provide insight into the structure of and processes occurring in the innermost disk regions. Possible explanations for the presence of the shadows include a 22 degrees misaligned inner disk, a warped disk region that connects the inner disk with the outer disk, and variable or transient phenomena such as a perturbation of the inner disk or an asymmetric accretion flow. The spiral arms are best explained by one or multiple protoplanets in the exterior of the disk although no gap is detected beyond the spiral arms up to 1.0.

SPHERE ZIMPOL: overview and performance simulation

The ESO planet finder instrument SPHERE will search for the polarimetric signature of the reflected light from extrasolar planets, using a VLT telescope, an extreme AO system (SAXO), a stellar coronagraph, and an imaging polarimeter (ZIMPOL). We present the design concept of the ZIMPOL instrument, a single-beam polarimeter that achieves very high polarimetric accuracy using fast polarization modulation and demodulating CCD detectors. Furthermore, we describe comprehensive performance simulations made with the CAOS problem-solving environment. We conclude that direct detection of Jupiter-sized planets in close orbit around the brightest nearby stars is achievable with imaging polarimetry, signal-switching calibration, and angular differential imaging.

CHEOPS/ZIMPOL: a VLT instrument study for the polarimetric search of scattered light from extrasolar planets

We present results from a phase A study supported by ESO for a VLT instrument for the search and investigation of extrasolar planets. The envisaged CHEOPS (CHaracterizing Extrasolar planets by Opto-infrared Polarization and Spectroscopy) instrument consists of an extreme AO system, a spectroscopic integral field unit and an imaging polarimeter. This paper describes the conceptual design of the imaging polarimeter which is based on the ZIMPOL (Zurich IMaging POLarimeter) technique using a fast polarization modulator combined with a demodulating CCD camera. ZIMPOL is capable of detecting polarization signals on the order of p=0.001% as demonstrated in solar applications. We discuss the planned implementation of ZIMPOL within the CHEOPS instrument, in particular the design of the polarization modulator. Further we describe strategies to minimize the instrumental effects and to enhance the overall measuring efficiency in order to achieve the very demanding science goals.

The ZIMPOL high-contrast imaging polarimeter for SPHERE: design, manufacturing, and testing

ZIMPOL is the high contrast imaging polarimeter subsystem of the ESO SPHERE instrument. ZIMPOL is dedicated to detect the very faint reflected and hence polarized visible light from extrasolar planets. ZIMPOL is located behind an extreme AO system (SAXO) and a stellar coronagraph. SPHERE is foreseen to have first light at the VLT at the end of 2011. ZIMPOL is currently in the manufacturing, integration and testing phase. We describe the optical, polarimetric, mechanical, thermal and electronic design as well as the design trade offs. Specifically emphasized is the optical quality of the key performance component: the Ferro-electric Liquid Crystal polarization modulator (FLC). Furthermore, we describe the ZIMPOL test setup and the first test results on the achieved polarimetric sensitivity and accuracy. These results will give first indications for the expected overall high contrast system performance. SPHERE is an instrument designed and built by a consortium consisting of LAOG, MPIA, LAM, LESIA, Fizeau, INAF, Observatoire de Genève, ETH, NOVA, ONERA and ASTRON in collaboration with ESO.

Search and investigation of extra-solar planets with polarimetry

Light reflected from planets is polarized. This basic property of planets provides the possibility for detecting and characterizing extra-solar planets using polarimetry. The expected polarization properties of extra-solar planets are discussed that can be inferred from polarimetry of “our” solar system planets. They show a large variety of characteristics depending on the atmospheric and/or surface properties. Best candidates for a polarimetric detection are extra-solar planets with an optically thick Rayleigh scattering layer. Even the detection of highly polarized extra-solar planets requires a very sophisticated instrument. We present the results from a phase A (feasibility) study for a polarimetric arm in the ESO VLT planet finder instrument. It is shown that giant planets around nearby stars can be searched and investigated with an imaging polarimeter, combined with a powerful AO system and a coronagraph at an 8 m class telescope. A similar type of polarimeter is also considered for the direct detection of terrestrial planets using an AO system on one of the future Extremely Large Telescopes.

Tests of the demodulating CCDs for the SPHERE / ZIMPOL imaging polarimeter

The imaging polarimeter ZIMPOL is one of three focal plane instruments of the SPHERE / VLT planet finder. ZIMPOL measures the linear polarization based on a fast modulation – demodulation principle using a charge-shifting technique on a masked CCD for separating the photons with opposite polarization direction. This paper describes the on-chip demodulation and the different detector read-out modes which are implemented for the ZIMPOL polarimeter. Test results are presented which allow an evaluation of the performance of the ZIMPOL CCD detectors. The achievable polarization efficiency is close to expectation and the charge trap correction with the two-phase demodulation mode works well. Other detector effects like bias level variations and read-out patterns can be corrected in the data reduction process. The tests demonstrate that the demodulating CCDs fulfill the requirements for the SPHERE project.

Spatial mapping of the Hanle and Zeeman effects on the Sun

Spatial mapping of the Hanle and Zeeman eects on the Sun has been done for the rst time, through Stokes vector imaging with a narrow-band (0.2 A) universal lter. It is shown how the polarization signatures of the Hanle and Zeeman eects can be cleanly distinguished from each other by comparing the Stokes images recorded at dierent, specially selected wavelengths within the Na i D2{D1 line system. Examples of the polarization signatures of sunspots, faculae, the supergranulation network, and large-scale canopy elds are shown. The most striking result of our observations is that the scattering polarization has an extremely intermittent structure rather than being a simple function of limb distance. These intermittent scattering polarization signals are cospatial with the facular and supergranulation network seen both in intensity and circular polarization. The observed pattern can be explained in terms of magnetic enhancement of the scattering polarization in the network and/or Hanle depolarization of the scattering polarization outside the network. Since however no magnetic elds are seen in circular polarization outside the network, the relative absence of linear scattering polarization there may be explained by Hanle depolarization only if the volume lling, depolarizing magnetic eld has mixed polarities on a subarcsec scale that is not resolved.

The ZIMPOL high contrast imaging polarimeter for SPHERE: system test results

SPHERE (Spectro-Polarimetric High Contrast Exoplanet Research) is one of the first instruments which aim for the direct detection from extra-solar planets. The instrument will search for direct light from old planets with orbital periods of several months to several years as we know them from our solar system. These are planets which are in or close to the habitable zone. ZIMPOL (Zurich Imaging Polarimeter) is the high contrast imaging polarimeter subsystem of the ESO SPHERE instrument. ZIMPOL is dedicated to detect the very faint reflected and hence polarized visible light from extrasolar planets. The search for reflected light from extra-solar planets is very demanding because the signal decreases rapidly with the orbital separation. For a Jupiter-sized object and a separation of 1 AU the planet/star contrast to be achieved is on the order of 10-8 for a successful detection. This is much more demanding than the direct imaging of young self-luminous planets. ZIMPOL is located behind an extreme AO system (SAXO) and a stellar coronagraph. SPHERE is foreseen to have first light at the VLT at the end of 2012. ZIMPOL is currently in the subsystem testing phase. We describe the results of verification and performance testing done at the NOVA-ASTRON lab. We will give an overview of the system noise performance, the polarimetric accuracy and the high contrast testing. For the high contrast testing we will describe the impact of crucial system parameters on the contrast performance. SPHERE is an instrument designed and built by a consortium consisting of IPAG, MPIA, LAM, LESIA, Fizeau, INAF, Observatoire de Genève, ETH, NOVA, ONERA and ASTRON in collaboration with ESO.

Imaging spectropolarimetry with two LiNbO3 Fabry Pérot interferometers and a spectrograph

Context. Narrow-band spectropolarimetry is used to obtain information about the velocity and magnetic field structure of the solar atmosphere. Several types of instruments are suited to these observations, each with different advantages and drawbacks. Aims. We set up a novel instrument combination using two LiNbO3 Fabry Perot interferometers (FPI), a high-resolution grating spectrograph, and the ZIMPOL system for polarimetry at IRSOL. With this system, we can carry out imaging spectropolarimetry of any spectral line from 390 to 660 nm, with a spectral resolution of 30 mA at 630 nm. Methods. We describe the setup, its properties, and calculate the limitations induced by the FPI and the spectrograph. We carry out spectropolarimetric observations of the sunspot AR 11087 in different spectral lines with suitable Lande factors that could be used to derive the magnetic field strength in different height ranges of the solar atmosphere. Results. The main advantage of our instrument compared to similar systems is that no special prefilters are required for each spectral line. A slight disadvantage is the spatial smearing induced by the dispersion of the finite transmission profiles of the FPI, which however is of the same magnitude as the seeing-limited resolution of 1–2 �� at IRSOL. Conclusions. We demonstrate that this particular instrument combination is well suited to spectropolarimetry at IRSOL.

FIRST POLARIMETRIC MEASUREMENTS AND MODELING OF THE PASCHEN-BACK EFFECT IN CaH TRANSITIONS

We report the first spectropolarimetric observations and modeling of CaH transitions in sunspots. We have detected strong polarization signals in many CaH lines from the A-X system, and we provide the first successful fit to the observed Stokes profiles using the previously developed theory of the Paschen-Back effect in arbitrary electronic states of diatomic molecules and polarized radiative transfer in molecular lines in stellar atmospheres. We analyze the CaH Stokes profiles together with quasi-simultaneous observations in TiO bands and conclude that CaH provides a valuable diagnostic of magnetic fields in sunspots, starspots, cool stars, and brown dwarfs.