Christoph U. Keller
Professor of Experimental Astrophysics
Sterrekundig Instituut Utrecht
Utrecht University, The Netherlands
by Vitas, N., Fischer, C. E., Vögler, A., Keller, C. U., is now available here.
Abstract: Numerical simulations of solar surface convection have predicted the
existence of supersonic horizontal flows in the photospheric
granulation. Recently, the detection of such flows in data from the
Hinode satellite was reported. We study supersonic granular flows in
detail to understand their signatures in spectral lines and to test the
observational detection method used to identify these flows in the
Hinode observations. We perform time-dependent 3D radiative MHD
numerical simulations and synthesize the Fe i 6302 Å spectral
lines at the resolution of the Hinode data for different viewing angles
covering the center-limb variation. There is very large variation in the
detailed shape of the emergent line profiles depending on the viewing
angle and the particular flow properties and orientation. At the full
simulation resolution the supersonic flows can even produce distinct
satellite lines. After smearing to the Hinode resolution sufficient
signature of supersonic motion remains. Our analysis shows that the
detection method used to analyze the Hinode data is indeed applicable.
However, the detection is very sensitive to ad hoc parameter choices and
can also misidentify supersonic flows.
by Makaganiuk, Vitalii, Kochukhov, Oleg, Piskunov, Nikolai, Jeffers, Sandra V., Johns-Krull, Christopher M., Keller, Christoph U., Rodenhuis, Michiel, Snik, Frans, Stempels, Henricus C., Valenti, Jeff A., is now available here.
Abstract: Mercury-manganese (HgMn) stars were considered to be non-magnetic,
showing no evidence of surface spots. However, recent investigations
revealed that some stars in this class possess an inhomogeneous
distribution of chemical elements on their surfaces. According to our
current understanding, the most probable mechanism of spot formation
involves magnetic fields. Taking the advantage of a newly-built
polarimeter attached to the HARPS spectrometer at the ESO
3.6m-telescope, we performed a high-precision spectropolarimetric survey
of a large group of HgMn stars. The main purpose of this study was to
find out how typical it is for HgMn stars to have weak magnetic fields.
We report no magnetic field detection for any of the studied objects,
with a typical precision of the longitudinal field measurements of 10 G
and down to 1 Gauss for some of the stars. We conclude that HgMn stars
lack large-scale magnetic fields typical of spotted magnetic Ap stars
and probably lack any fields capable of creating and sustaining chemical
spots. Our study confirms that alongside the magnetically altered atomic
diffusion, there must be other structure formation mechanism operating
in the atmospheres of late-B main sequence stars.
by Boccaletti, A., Baudoz, P., Mawet, D., Schneider, J., Tinetti, G., Galicher, R., Stam, D., Cavarroc, C., Hough, J., Doel, P., Pinfield, D., Keller, C.-U., Beuzit, J.-L., Udry, S., Ferrari, A., Martin, E., Ménard, F., Sein, E., is now available here.
Abstract: SEE COAST stands for Super Earth Explorer - Coronagraphic Off-Axis Space
Telescope. The concept was initially proposed to ESA for Cosmic Vision.
None of the direct detection exoplanet proposals were selected in 2007
and we are now pursuing our efforts to consolidate the astrophysical
program and the technical developments for the next call for proposal.
The prime objective of SEE COAST is to contribute to the understanding
of the formation and evolution of planetary systems. Exploring the
diversity of these objects is therefore the main driver to define the
instrumentation. In the next decade the improvement of radial velocity
instruments and obviously temporal coverage will provide us with a large
numbers of long period giants as well as telluric planets, namely Super
Earths. Obtaining the spectral and polarimetric signatures of these
objects in the visible range to measure atmospheric parameters
(molecular composition, clouds, soils, …) will be unique and with
important scientific returns. A space mission complementary to near IR
instruments like SPHERE, GPI, JWST and later ELTs for the full
characterization of giants and Super Earths is a first secure step
towards the longer term goal that is the characterization of telluric
planets with mass and atmosphere comparable to that of the Earth. An
overview of the astrophysical motivation and the trade-off that lead to
a simple integrated concept of a space-based high contrast imaging
instrument are given here.
by Canovas, H., Rodenhuis, M., Jeffers, S. V., Min, M., Keller, C. U., is now available here.
Abstract: Context. Imaging polarimetry is a powerful tool for detecting and
characterizing exoplanets and circumstellar environments. Polarimetry
allows a separation of the light coming from an unpolarized source such
as a star and the polarized source such as a planet or a protoplanetary
disk. Future facilities like SPHERE at the VLT or EPICS at the E-ELT
will incorporate imaging polarimetry to detect exoplanets. The Extreme
Polarimeter (ExPo) is a dual-beam imaging polarimeter that can currently
reach contrast ratios of 105, enough to characterize
circumstellar environments. Aims: We present the data-reduction
steps for a dual-beam imaging polarimeter that can reach contrast ratios
of 105. Methods: The data obtained with ExPo at the
William Herschel Telescope (WHT) are analyzed. Instrumental artifacts
and noise sources are discussed for an unpolarized star and for a
protoplanetary disk (AB Aurigae). Results: The combination of
fast modulation and dual-beam techniques allows us to minimize
instrumental artifacts. A proper data processing and alignment of the
images is fundamental when dealing with high contrasts. Imaging
polarimetry proves to be a powerful method to resolve circumstellar
environments even without a coronagraph mask or an adaptive optics
system.
by Canovas, H., Rodenhuis, M., Jeffers, S. V., Min, M., Keller, C. U., is now available here.
Abstract: Imaging polarimetry is a powerful tool for detecting and characterizing
exoplanets and circumstellar environments. Polarimetry allows a
separation of the light coming from an unpolarized source such as a star
and the polarized source such as a planet or a protoplanetary disk.
Future facilities like SPHERE at the VLT or EPICS at the E-ELT will
incorporate imaging polarimetry to detect exoplanets. The Extreme
Polarimeter (ExPo) is a dual-beam imaging polarimeter that currently can
reach contrast ratios of 10^5, enough to characterize circumstellar
environments. We present the data reduction steps for a dual-beam
imaging polarimeter that can reach contrast ratios of 10^5. The data
obtained with ExPo at the William Herschel Telescope (WHT) are analyzed.
Instrumental artifacts and noise sources are discussed for an
unpolarized star and for a protoplanetary disk (AB Aurigae). The
combination of fast modulation and dual-beam techniques allow us to
minimize instrumental artifacts. A proper data processing and alignment
of the images is fundamental when dealing with large contrasts. Imaging
polarimetry proves to be a powerful method to resolve circumstellar
environments even without a coronagraph mask or an Adaptive Optics
system.
by Kochukhov, O., Makaganiuk, V., Piskunov, N., Snik, F., Jeffers, S. V., Johns-Krull, C. M., Keller, C. U., Rodenhuis, M., Valenti, J. A., is now available here.
Abstract: The application of high-resolution spectropolarimetry has led to major
progress in understanding the magnetism and activity of late-type stars.
During the last decade, magnetic fields have been discovered and mapped
for many types of active cool stars using spectropolarimetric data.
However, these observations and modeling attempts are fundamentally
incomplete since they are based on the interpretation of the circular
polarization alone. Taking advantage of the newly built HARPS
polarimeter, we have obtained the first systematic observations of
several cool active stars in all four Stokes parameters. Here we report
the detection of magnetically induced linear polarization for the
primary component of the very active RS CVn binary HR 1099 and for the
moderately active K dwarf ɛ Eri. For both stars the amplitude of
linear polarization signatures is measured to be ~10-4
of the unpolarized continuum, which is approximately a factor of 10
lower than for circular polarization. This is the first detection of the
linear polarization in line profiles of cool active stars. Our
observations of the inactive solar-like star α Cen A show neither
circular nor linear polarization above the level of
~10-5, indicating the absence of a net longitudinal
magnetic field stronger than 0.2 G.
Based on observations obtained at the European Southern Observatory (ESO
programs 083.D-1000(A) and 084.D-0338(A)).
by Makaganiuk, V., Kochukhov, O., Piskunov, N., Jeffers, S. V., Johns-Krull, C. M., Keller, C. U., Rodenhuis, M., Snik, F., Stempels, H. C., Valenti, J. A., is now available here.
Abstract: Context. According to our current understanding, a subclass of the upper
main-sequence chemically peculiar stars, called mercury-manganese
(HgMn), is non-magnetic. Nevertheless, chemical inhomogeneities were
recently discovered on their surfaces. At the same time, no global
magnetic fields stronger than 1-100 G are detected by systematic
studies. Aims: The goals of our study are to search for a
magnetic field in the HgMn binary system 66 Eri and to investigate
chemical spots on the stellar surfaces of both components.
Methods: Our analysis is based on high-quality spectropolarimetric
time-series observations obtained during 10 consecutive nights with the
HARPSpol instrument at the ESO 3.6-m telescope. To increase the
sensitivity of the magnetic field search we employed a least-squares
deconvolution (LSD). We used spectral disentangling to measure radial
velocities and study the line profile variability. Chemical spot
geometry was reconstructed using multi-line Doppler imaging.
Results: We report a non-detection of magnetic field in 66 Eri, with
error bars 10-24 G for the longitudinal field. Circular polarization
profiles also do not indicate any signatures of complex surface magnetic
fields. For a simple dipolar field configuration we estimated an upper
limit of the polar field strength to be 60-70 G. For the HgMn component
we found variability in spectral lines of Ti, Ba, Y, and Sr with the
rotational period equal to the orbital one. The surface maps of these
elements reconstructed with the Doppler imaging technique show a
relative underabundance on the hemisphere facing the secondary
component. The contrast of chemical inhomogeneities ranges from 0.4 for
Ti to 0.8 for Ba.
Based on observations collected at the European Southern Observatory,
Chile (ESO program 084.D-0338).
by Bettonvil, F. C. M., Collados, M., Feller, A., Gelly, B. F., Keller, C. U., Kentischer, T. J., López Ariste, A., Pleier, O., Snik, F., Socas-Navarro, H., is now available here.
Abstract: EST, the European Solar Telescope, is a 4-m class solar telescope, which
will be located at the Canary Islands. It is currently in the conceptual
design phase as a European funded project. In order to fulfill the
stringent requirements for polarimetric sensitivity and accuracy, the
polarimetry has been included in the design work from the very
beginning. The overall philosophy has been to use a combination of
techniques, which includes a telescope with low (and stable)
instrumental polarization, optimal full Stokes polarimeters,
differential measurement schemes, fast modulation and demodulation, and
accurate calibration, and at the same time not giving up flexibility.
The current baseline optical layout consists of a 14-mirror layout,
which is polarimetrically compensated and non-varying in time. In the
polarization free F2 focus ample space is reserved for calibration and
modulators and a polarimetric switch. At instrument level the s-, and
p-planes of individual components are aligned, resulting in a system in
which eigenvectors can travel undisturbed through the system.
by Snik, F., Kochukhov, O., Piskunov, N., Rodenhuis, M., Jeffers, S., Keller, C., Dolgopolov, A., Stempels, E., Makaganiuk, V., Valenti, J., Johns-Krull, C., is now available here.
Abstract: We recently commissioned the polarimetric upgrade of the HARPS
spectrograph at ESO's 3.6-m telescope at La Silla, Chile. The HARPS
polarimeter is capable of full Stokes spectropolarimetry with large
sensitivity and accuracy, taking advantage of the large spectral
resolution and stability of HARPS. In this paper we present the
instrument design and its polarimetric performance. The first HARPSpol
observations show that it can attain a polarimetric sensitivity of
˜10-5 (after addition of many lines) and that no
significant instrumental polarization effects are present.
by Piskunov, N., Snik, F., Dolgopolov, A., Kochukhov, O., Rodenhuis, M., Valenti, J., Jeffers, S., Makaganiuk, V., Johns-Krull, C., Stempels, E., Keller, C., is now available here.
Abstract: The HARPS spectrograph can now perform a full polarisation analysis of
spectra. It has been equipped with a polarimetric unit, HARPSpol, which
was jointly designed and produced by Uppsala, Utrecht and Rice
Universities and by the STScI. Here we present the new instrument,
demonstrate its polarisation capabilities and show the first scientific
results.
by Snik, F., Keller, C., Ovelar, M. J., Rodenhuis, M., Korkiakoski, V., Venema, L., Jager, R., Rigal, F., Hanenburg, H., Roelfsema, R., Schmidt, H. M., Verinaud, C., Kasper, M., Martinez, P., Yaitskova, N., is now available here.
Abstract: EPOL is the imaging polarimeter part of EPICS (Exoplanet Imaging Camera
and Spectrograph) for the 42-m E-ELT. It is based on sensitive imaging
polarimetry to differentiate between linearly polarized light from
exoplanets and unpolarized, scattered starlight and to characterize
properties of exoplanet atmospheres and surfaces that cannot be
determined from intensity observations alone. EPOL consists of a
coronagraph and a dual-beam polarimeter with a liquid-crystal retarder
to exchange the polarization of the two beams. The polarimetry thereby
increases the contrast between star and exoplanet by 3 to 5 orders of
magnitude over what the extreme adaptive optics and the EPOL coronagraph
alone can achieve. EPOL operates between 600 and 900 nm, can select more
specific wavelength bands with filters and aims at having an integral
field unit to obtain linearly polarized spectra of known exoplanets. We
present the conceptual design of EPOL along with an analysis of its
performance.
by Min, M., Jeffers, S. V., Rodenhuis, M., Canovas, H., Buenzli, E., Keller, C. U., Waters, L. B. F. M., Dominik, C., is now available here.
Abstract: Imaging polarimetry is one of the most promising tools to map the
structure of faint protoplanetary disks. In this contribution we discuss
the feasibility of imaging polarimetry of protoplanetary disks and the
usability to answer the scientific questions in the field. From the
theoretical side we do this by simulations of disks of various
geometries and dust properties. We model the expected signal and
detailed predictions for current and upcoming imaging polarimeters. This
way we can address the question what the diagnostic value of polarimetry
is for the structure of the disk and the characteristics of the grains
in it. We compare extremely fluffy aggregated grains and compact
homogeneous grains and show that their expected signal is significantly
different. In combination with infrared/mm observations this could allow
us to obtain grain properties in addition to mapping of the disk
geometry. From the observational side we address the issues by
discussing some of the early results from the Extreme Polarimeter
(ExPo). ExPo is a sensitive imaging polarimeter designed to be a
pathfinding instrument for the large imaging polarimetry projects
planned for the VLT and the ELT. Already it proves to be a pioneering
instrument in the field of imaging polarimetry of circumstellar matter.
by Rodenhuis, M., Canovas, H., Jeffers, S. V., Min, M., Keller, C. U., is now available here.
Abstract: Three successful observation campaigns have been conducted with the
Extreme Polarimeter, an imaging polarimeter for the study of
circumstellar environments in scattered light at visible wavelengths. A
contrast ratio between the central star and the circumstellar source of
10-5 can be achieved with polarimetry, with a Lyot coronograph capable
of increasing this contrast by several orders of magnitude. The
instrument currently operates without an adaptive optics system. An
Adaptive Optics system under development for ExPo is expected to
increase the contrast further. The polarimeter uses the dual-beam
exchange technique, in which the two orthogonal polarisation states are
imaged simultaneously after which a polarisation modulator is used to
swap the polarisation states of the two beams before the next image is
taken. The imaging polarimetry technique developed with ExPo will be
used in the polarimetry arm of the EPICS exoplanet characterisation
instrument proposed for the E-ELT. Here we present the results from the
first observation campaigns, highlighting observations of protoplanetary
disks around several young stars. Systematic effects that limit the
polarimetric sensitivity, and the strategies we employ to overcome them,
are discussed in detail. In particular, the advantages of the dual-beam
exchange polarimetry method are demonstrated.
by Makaganiuk, V., Kochukhov, O., Piskunov, N., Jeffers, S. V., Johns-Krull, C. M., Keller, C. U., Rodenhuis, M., Snik, F., Stempels, H. C., Valenti, J. A., is now available here.
Abstract: Context. A subclass of the upper main-sequence chemically peculiar
stars, mercury-manganese (HgMn) stars were traditionally considered to
be non-magnetic, showing no evidence of variability in their spectral
line profiles. However, discoveries of chemical inhomogeneities on their
surfaces imply that this assumption should be investigated. In
particular, spectroscopic time-series of AR Aur, α And, and five
other HgMn stars indicate the presence of chemical spots. At the same
time, no signatures of global magnetic fields have been detected.
Aims: We attempt to understand the physical mechanism that causes the
formation of chemical spots in HgMn stars and gain insight into the
potential magnetic field properties at their surfaces; we performed a
highly sensitive search for magnetic fields for a large set of HgMn
stars. Methods: With the aid of a new polarimeter attached to the
HARPS spectrometer at the ESO 3.6 m-telescope, we obtained high-quality
circular polarization spectra of 41 single and double HgMn stars. Using
a multi-line analysis technique on each star, we co-added information
from hundreds of spectral lines to ensure significantly greater
sensitivity to the presence of magnetic fields, including very weak
fields. Results: For the 47 individual objects studied, including
six components of SB2 systems, we do not detect any magnetic fields at
greater than the 3σ level. The lack of detection in the circular
polarization profiles indicates that if strong fields are present on
these stars, they must have complex surface topologies. For simple
global fields, our detection limits imply upper limits to the fields
present of 2-10 Gauss in the best cases. Conclusions: We conclude
that HgMn stars lack large-scale magnetic fields, which is typical of
spotted magnetic Ap stars, of sufficient strength to form and sustain
the chemical spots observed on HgMn stars. Our study confirms that in
addition to magnetically altered atomic diffusion, there exists another
differentiation mechanism operating in the atmospheres of late-B main
sequence stars that can produce compositional inhomogeneities on their
surfaces.
Based on observations collected at the European Southern Observatory,
Chile (ESO programs 083.D-1000, 084.D-0338, 085.D-0296).Figure 5 is only
available in electronic form at http://www.aanda.org
by Makaganiuk, V., Kochukhov, O., Piskunov, N., Jeffers, S. V., Johns-Krull, C. M., Keller, C. U., Rodenhuis, M., Snik, F., Stempels, H. C., Valenti, J. A., is now available here.
Abstract: We performed a highly sensitive search for magnetic fields on a large
set of HgMn stars. With the aid of a new polarimeter attached to the
HARPS spectrometer at the ESO 3.6m-telescope, we obtained high-quality
circular polarization spectra of 41 single and double HgMn stars. Using
a multi-line analysis technique on each star, we co-added information
from hundreds of spectral lines resulting in significantly greater
sensitivity to the presence of magnetic fields, including very weak
fields. For the 47 individual objects studied, including 6 components of
SB2 systems, we do not detect any magnetic fields at greater than the 3
sigma level. The lack of detection in the circular polarization profiles
indicates that if strong fields are present on these stars, they must
have complex surface topologies. For simple global fields, our detection
limits imply upper limits to the fields present of 2-10 Gauss in the
best cases. We conclude that HgMn stars lack large-scale magnetic
fields, typical for spotted magnetic Ap stars, sufficient to form and
sustain the chemical spots observed on HgMn stars. Our study confirms
that in addition to magnetically altered atomic diffusion, there exists
another differentiation mechanism operating in the atmospheres of late-B
main sequence stars which can compositional inhomogeneities on their
surfaces.
by Snik, Frans, Kochukhov, Oleg, Piskunov, Nikolai, Rodenhuis, Michiel, Jeffers, Sandra, Keller, Christoph, Dolgopolov, Andrey, Stempels, Eric, Makaganiuk, Vitaly, Valenti, Jeff, Johns-Krull, Christopher, is now available here.
Abstract: We recently commissioned the polarimetric upgrade of the HARPS
spectrograph at ESO's 3.6-m telescope at La Silla, Chile. The HARPS
polarimeter is capable of full Stokes spectropolarimetry with large
sensitivity and accuracy, taking advantage of the large spectral
resolution and stability of HARPS. In this paper we present the
instrument design and its polarimetric performance. The first HARPSpol
observations show that it can attain a polarimetric sensitivity of
~10^-5 (after addition of many lines) and that no significant
instrumental polarization effects are present.
by Snik, F., de Wijn, A. G., Ichimoto, K., Fischer, C. E., Keller, C. U., Lites, B. W., is now available here.
Abstract: Context. The weak, turbulent magnetic fields that supposedly permeate
most of the solar photosphere are difficult to observe, because the
Zeeman effect is virtually blind to them. The Hanle effect, acting on
the scattering polarization in suitable lines, can in principle be used
as a diagnostic for these fields. However, the prediction that the
majority of the weak, turbulent field resides in intergranular lanes
also poses significant challenges to scattering polarization
observations because high spatial resolution is usually difficult to
attain. Aims: We aim to measure the difference in scattering
polarization between granules and intergranules. We present the
respective center-to-limb variations, which may serve as input for
future models. Methods: We perform full Stokes filter polarimetry
at different solar limb positions with the CN band filter of the
Hinode-SOT Broadband Filter Imager, which represents the first
scattering polarization observations with sufficient spatial resolution
to discern the granulation. Hinode-SOT offers unprecedented spatial
resolution in combination with high polarimetric sensitivity. The CN
band is known to have a significant scattering polarization signal, and
is sensitive to the Hanle effect. We extend the instrumental
polarization calibration routine to the observing wavelength, and
correct for various systematic effects. Results: The scattering
polarization for granules (i.e., regions brighter than the median
intensity of non-magnetic pixels) is significantly larger than for
intergranules. We derive that the intergranules (i.e., the remaining
non-magnetic pixels) exhibit (9.8±3.0)% less scattering
polarization for 0.2 < μ ≤ 0.3, although systematic effects
cannot be completely excluded. Conclusions: These observations
constrain MHD models in combination with (polarized) radiative transfer
in terms of CN band line formation, radiation anisotropy, and magnetic
fields.
by Bettonvil, F. C. M., Collados, M., Feller, A., Gelly, B. F., Keller, C. U., Kentischer, T. J., López Ariste, A., Pleier, O., Snik, F., Socas-Navarro, H., is now available here.
Abstract: EST (European Solar Telescope) is a 4-m class solar telescope, which is
currently in the conceptual design phase. EST will be located at the
Canary Islands and aims at observations with the best possible spectral,
spatial and temporal resolution and best polarimetric performance, of
the solar photosphere and chromosphere, using a suite of instruments
that can efficiently produce two-dimensional spectropolarimetric
information of the thermal, dynamic and magnetic properties of the
plasma over many scale heights, and ranging from λ=350 until 2300
nm. In order to be able to fulfill the stringent requirements for
polarimetric sensitivity and accuracy, from the very beginning the
polarimetry has been included in the design work. The overall philosophy
has been to use a combination of techniques, which includes a telescope
with low (and stable) instrumental polarization, optimal full Stokes
polarimeters, differential measurement schemes, fast modulation and
demodulation, and accurate calibration. The current baseline optical
layout consists of a 14-mirror layout, which is polarimetrically
compensated and nonvarying in time. In the polarization free F2 focus
ample space is reserved for calibration and modulators and a
polarimetric switch. At instrument level the s-, and p-planes of
individual components are aligned, resulting in a system in which
eigenvectors can travel undisturbed through the system.
by Keller, Christoph U., Schmid, Hans Martin, Venema, Lars B., Hanenburg, Hiddo, Jager, Rieks, Kasper, Markus, Martinez, Patrice, Rigal, Florence, Rodenhuis, Michiel, Roelfsema, Ronald, Snik, Frans, Verinaud, Christophe, Yaitskova, Natalia, is now available here.
Abstract: EPOL is the imaging polarimeter part of EPICS (Exoplanet Imaging Camera
and Spectrograph) for the 42-m E-ELT. It is based on sensitive imaging
polarimetry to differentiate between linearly polarized light from
exoplanets and unpolarized, scattered starlight and to characterize
properties of exoplanet atmospheres and surfaces that cannot be
determined from intensity observations alone. EPOL consists of a
coronagraph and a dual-beam polarimeter with a liquid-crystal retarder
to exchange the polarization of the two beams. The polarimetry thereby
increases the contrast between star and exoplanet by 3 to 5 orders of
magnitude over what the extreme adaptive optics and the EPOL coronagraph
alone can achieve. EPOL operates between 600 and 900 nm, can select more
specific wavelength bands with filters and aims at having an integral
field unit to obtain linearly polarized spectra of known exoplanets. We
present the conceptual design of EPOL along with an analysis of its
performance.
by Roelfsema, Ronald, Schmid, Hans Martin, Pragt, Johannes, Gisler, Daniel, Waters, Rens, Bazzon, Andreas, Baruffolo, Andrea, Beuzit, Jean-Luc, Boccaletti, Anthony, Charton, Julien, Cumani, Claudio, Dohlen, Kjetil, Downing, Mark, Elswijk, Eddy, Feldt, Markus, Groothuis, Charlotte, de Haan, Menno, Hanenburg, Hiddo, Hubin, Norbert, Joos, Franco, Kasper, Markus, Keller, Christoph, Kragt, Jan, Lizon, Jean-Louis, Mouillet, David, Pavlov, Aleksej, Rigal, Florence, Rochat, Sylvain, Salasnich, Bernardo, Steiner, Peter, Thalmann, Christian, Venema, Lars, Wildi, François, is now available here.
Abstract: 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.
by Kasper, Markus, Beuzit, Jean-Luc, Verinaud, Christophe, Gratton, Raffaele G., Kerber, Florian, Yaitskova, Natalia, Boccaletti, Anthony, Thatte, Niranjan, Schmid, Hans Martin, Keller, Christoph, Baudoz, Pierre, Abe, Lyu, Aller-Carpentier, Emmanuel, Antichi, Jacopo, Bonavita, Mariangela, Dohlen, Kjetil, Fedrigo, Enrico, Hanenburg, Hiddo, Hubin, Norbert, Jager, Rieks, Korkiakoski, Visa, Martinez, Patrice, Mesa, Dino, Preis, Olivier, Rabou, Patrick, Roelfsema, Ronald, Salter, Graeme, Tecza, Mathias, Venema, Lars, is now available here.
Abstract: Presently, dedicated instruments at large telescopes (SPHERE for the
VLT, GPI for Gemini) are about to discover and explore self-luminous
giant planets by direct imaging and spectroscopy. The next generation of
30m-40m ground-based telescopes, the Extremely Large Telescopes (ELTs),
have the potential to dramatically enlarge the discovery space towards
older giant planets seen in reflected light and ultimately even a small
number of rocky planets. EPICS is a proposed instrument for the European
ELT, dedicated to the detection and characterization of Exoplanets by
direct imaging, spectroscopy and polarimetry. ESO completed a phase-A
study for EPICS with a large European consortium which - by simulations
and demonstration experiments - investigated state-of-the-art
diffraction and speckle suppression techniques to deliver highest
contrasts. The paper presents the instrument concept and analysis as
well as its main innovations and science capabilities. EPICS is capable
of discovering hundreds of giant planets, and dozens of lower mass
planets down to the rocky planets domain.
by Snik, Frans, Rietjens, Jeroen H. H., van Harten, Gerard, Stam, Daphne M., Keller, Christoph U., Smit, J. Martijn, Laan, Erik C., Verlaan, Ad L., Ter Horst, Rik, Navarro, Ramón, Wielinga, Klaas, Moon, Scott G., Voors, Robert, is now available here.
Abstract: SPEX (Spectropolarimeter for Planetary EXploration) is an innovative,
compact instrument for spectropolarimetry, and in particular for
detecting and characterizing aerosols in planetary atmospheres. With its
~1-liter volume it is capable of full linear spectropolarimetry, without
moving parts. The degree and angle of linear polarization of the
incoming light is encoded in a sinusoidal modulation of the intensity
spectrum by an achromatic quarter-wave retarder, an athermal
multiple-order retarder and a polarizing beam-splitter in the entrance
pupil. A single intensity spectrum thus provides the spectral dependence
of the degree and angle of linear polarization. Polarimetry has proven
to be an excellent tool to study microphysical properties (size, shape,
composition) of atmospheric particles. Such information is essential to
better understand the weather and climate of a planet. The current
design of SPEX is tailored to study Martian dust and ice clouds from an
orbiting platform: a compact module with 9 entrance pupils to
simultaneously measure intensity spectra from 400 to 800 nm, in
different directions along the flight direction (including two limb
viewing directions). This way, both the intensity and polarization
scattering phase functions of dust and cloud particles within a ground
pixel are sampled while flying over it. We describe the optical and
mechanical design of SPEX, and present performance simulations and
initial breadboard measurements. Several flight opportunities exist for
SPEX throughout the solar system: in orbit around Mars, Jupiter and its
moons, Saturn and Titan, and the Earth.