Christoph U. Keller
Professor of Experimental Astrophysics
Leiden Observatory
Leiden University, The Netherlands
by van Holstein, R. G., Stolker, T., Jensen-Clem, R., Ginski, C., Milli, J., de Boer, J., Girard, J. H., Wahhaj, Z., Bohn, A. J., Millar-Blanchaer, M. A., Benisty, M., Bonnefoy, M., Chauvin, G., Dominik, C., Hinkley, S., Keller, C. U., Keppler, M., Langlois, M., Marino, S., Ménard, F., Perrot, C., Schmidt, T. O. B., Vigan, A., Zurlo, A., Snik, F., is now available here.
Abstract: Young giant planets and brown dwarf companions emit near-infrared
radiation that can be linearly polarized up to several percent. This
polarization can reveal the presence of a circumsubstellar accretion
disk, rotation-induced oblateness of the atmosphere, or an inhomogeneous
distribution of atmospheric dust clouds. We measured the near-infrared
linear polarization of 20 known directly imaged exoplanets and brown
dwarf companions with the high-contrast imager SPHERE-IRDIS at the VLT.
We reduced the data using the IRDAP pipeline to correct for the
instrumental polarization and crosstalk with an absolute polarimetric
accuracy <0.1% in the degree of polarization. We report the first
detection of polarization originating from substellar companions, with a
polarization of several tenths of a percent for DH Tau B and GSC
6214-210 B in H-band. By comparing the measured polarization with that
of nearby stars, we find that the polarization is unlikely to be caused
by interstellar dust. Because the companions have previously measured
hydrogen emission lines and red colors, the polarization most likely
originates from circumsubstellar disks. Through radiative transfer
modeling, we constrain the position angles of the disks and find that
the disks must have high inclinations. The presence of these disks as
well as the misalignment of the disk of DH Tau B with the disk around
its primary star suggest in situ formation of the companions. For the 18
other companions, we do not detect significant polarization and place
subpercent upper limits on their degree of polarization. These
non-detections may indicate the absence of circumsubstellar disks, a
slow rotation rate of young companions, the upper atmospheres containing
primarily submicron-sized dust grains, and/or limited cloud
inhomogeneity. Finally, we present images of the circumstellar disks of
DH Tau, GQ Lup, PDS 70, Beta Pic, and HD 106906.
by Haffert, S. Y., van Holstein, R. G., Ginski, C., Brinchmann, J., Snellen, I. A. G., Milli, J., Stolker, T., Keller, C. U., Girard, J., is now available here.
Abstract: Context. Direct imaging provides a steady flow of newly discovered giant
planets and brown dwarf companions. These multi-object systems can
provide information about the formation of low-mass companions in wide
orbits and/or help us to speculate about possible migration scenarios.
Accurate classification of companions is crucial for testing formation
pathways. Aims: In this work we further characterise the recently
discovered candidate for a planetary-mass companion CS Cha b and
determine if it is still accreting. Methods: MUSE is a
four-laser-adaptive-optics-assisted medium-resolution integral-field
spectrograph in the optical part of the spectrum. We observed the CS Cha
system to obtain the first spectrum of CS Cha b. The companion is
characterised by modelling both the spectrum from 6300 Å to 9300
Å and the photometry using archival data from the visible to the
near-infrared (NIR). Results: We find evidence of accretion and
outflow signatures in Hα and OI emission. The atmospheric models
with the highest likelihood indicate an effective temperature of 3450
± 50 K with a log g of 3.6 ± 0.5 dex. Based on
evolutionary models, we find that the majority of the object is
obscured. We determine the mass of the faint companion with several
methods to be between 0.07 M⊙ and 0.71 M⊙
with an accretion rate of Ṁ = 4 × 10-11±0.4
M⊙ yr-1. Conclusions: Our results show
that CS Cha B is most likely a mid-M-type star that is obscured by a
highly inclined disc, which has led to its previous classification using
broadband NIR photometry as a planetary-mass companion. This shows that
it is important and necessary to observe over a broad spectral range to
constrain the nature of faint companions.
The extracted spectrum of CS Cha B is only available at the CDS via
anonymous ftp to http://cdsarc.u-strasbg.fr
(ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/640/L12
by Bohn, Alexander J., Kenworthy, Matthew A., Ginski, Christian, Rieder, Steven, Mamajek, Eric E., Meshkat, Tiffany, Pecaut, Mark J., Reggiani, Maddalena, de Boer, Jozua, Keller, Christoph U., Snik, Frans, Southworth, John, is now available here.
Abstract: Even though tens of directly imaged companions have been discovered in
the past decades, the number of directly confirmed multiplanet systems
is still small. Dynamical analysis of these systems imposes important
constraints on formation mechanisms of these wide-orbit companions. As
part of the Young Suns Exoplanet Survey we report the detection of a
second planetary-mass companion around the 17 Myr-old, solar-type star
TYC 8998-760-1 that is located in the Lower Centaurus Crux subgroup of
the Scorpius-Centaurus association. The companion has a projected
physical separation of 320 au and several individual photometric
measurements from 1.1 to 3.8 microns constrain a companion mass of 6
± 1 MJup, which is equivalent to a mass ratio of q =
0.57 ± 0.10% with respect to the primary. With the previously
detected 14 ± 3 MJup companion that is orbiting the
primary at 160 au, TYC 8998-760-1 is the first directly imaged
multiplanet system that is detected around a young, solar analog. We
show that circular orbits are stable, but that mildly eccentric orbits
for either/both components (e > 0.1) are chaotic on gigayear
timescales, implying in situ formation or a very specific ejection by an
unseen third companion. Due to the wide separations of the companions
TYC 8998-760-1 is an excellent system for spectroscopic and photometric
follow-up with space-based observatories such as the James Webb Space
Telescope. * Based on observations collected at the European
Organisation for Astronomical Research in the Southern Hemisphere under
ESO programs 099.C-0698(A), 0101.C-0341(A), 2103.C-5012(B), and
0104.C-0265(A).
by Bos, S. P., Vievard, S., Wilby, M. J., Snik, F., Lozi, J., Guyon, O., Norris, B. R. M., Jovanovic, N., Martinache, F., Sauvage, J.-F., Keller, C. U., is now available here.
Abstract: Context. High-contrast imaging (HCI) observations of exoplanets can be
limited by the island effect (IE). The IE occurs when the main wavefront
sensor (WFS) cannot measure sharp phase discontinuities across the
telescope's secondary mirror support structures (also known as spiders).
On the current generation of telescopes, the IE becomes a severe problem
when the ground wind speed is below a few meters per second. During
these conditions, the air that is in close contact with the spiders
cools down and is not blown away. This can create a sharp optical path
length difference between light passing on opposite sides of the
spiders. Such an IE aberration is not measured by the WFS and is
therefore left uncorrected. This is referred to as the low-wind effect
(LWE). The LWE severely distorts the point spread function (PSF),
significantly lowering the Strehl ratio and degrading the contrast. Aims: In this article, we aim to show that the focal-plane wavefront
sensing (FPWFS) algorithm, Fast and Furious (F&F), can be used to
measure and correct the IE/LWE. The F&F algorithm is a sequential
phase diversity algorithm and a software-only solution to FPWFS that
only requires access to images of non-coronagraphic PSFs and control of
the deformable mirror. Methods: We deployed the algorithm on the
SCExAO HCI instrument at the Subaru Telescope using the internal
near-infrared camera in H-band. We tested with the internal source to
verify that F&F can correct a wide variety of LWE phase screens.
Subsequently, F&F was deployed on-sky to test its performance with
the full end-to-end system and atmospheric turbulence. The performance
of the algorithm was evaluated by two metrics based on the PSF quality:
(1) the Strehl ratio approximation (SRA), and (2) variance of the
normalized first Airy ring (VAR). The VAR measures the distortion of the
first Airy ring, and is used to quantify PSF improvements that do not or
barely affect the PSF core (e.g., during challenging atmospheric
conditions). Results: The internal source results show that
F&F can correct a wide range of LWE phase screens. Random LWE phase
screens with a peak-to-valley wavefront error between 0.4 μm and 2
μm were all corrected to a SRA > 90% and an VAR ⪅ 0.05.
Furthermore, the on-sky results show that F&F is able to improve the
PSF quality during very challenging atmospheric conditions
(1.3-1.4″seeing at 500 nm). Closed-loop tests show that F&F is
able to improve the VAR from 0.27-0.03 and therefore significantly
improve the symmetry of the PSF. Simultaneous observations of the PSF in
the optical (λ = 750 nm, Δλ = 50 nm) show that
during these tests we were correcting aberrations common to the optical
and NIR paths within SCExAO. We could not conclusively determine if we
were correcting the LWE and/or (quasi-)static aberrations upstream of
SCExAO. Conclusions: The F&F algorithm is a promising
focal-plane wavefront sensing technique that has now been successfully
tested on-sky. Going forward, the algorithm is suitable for
incorporation into observing modes, which will enable PSFs of higher
quality and stability during science observations.
by Burggraaff, Olivier, Perduijn, Armand B., van Hek, Robert F., Schmidt, Norbert, Keller, Christoph U., Snik, Frans, is now available here.
Abstract: Spectropolarimetry is a powerful technique for remote sensing of the
environment. It enables the retrieval of particle shape and size
distributions in air and water to an extent that traditional
spectroscopy cannot. SPEX is an instrument concept for
spectropolarimetry through spectral modulation, providing snapshot, and
hence accurate, hyperspectral intensity and degree and angle of linear
polarization. Successful SPEX instruments have included groundSPEX and
SPEX airborne, which both measure aerosol optical thickness with high
precision, and soon SPEXone, which will fly on PACE. Here, we present a
low-cost variant for consumer cameras, iSPEX 2, with universal
smartphone support. Smartphones enable citizen science measurements
which are significantly more scaleable, in space and time, than
professional instruments. Universal smartphone support is achieved
through a modular hardware design and SPECTACLE data processing. iSPEX 2
will be manufactured through injection molding and 3D printing. A
smartphone app for data acquisition and processing is in active
development. Production, calibration, and validation will commence in
the summer of 2020. Scientific applications will include citizen science
measurements of aerosol optical thickness and surface water reflectance,
as well as low-cost laboratory and portable spectroscopy.
by Hunziker, S., Schmid, H. M., Mouillet, D., Milli, J., Zurlo, A., Delorme, P., Abe, L., Avenhaus, H., Baruffolo, A., Bazzon, A., Boccaletti, A., Baudoz, P., Beuzit, J. L., Carbillet, M., Chauvin, G., Claudi, R., Costille, A., Daban, J.-B., Desidera, S., Dohlen, K., Dominik, C., Downing, M., Engler, N., Feldt, M., Fusco, T., Ginski, C., Gisler, D., Girard, J. H., Gratton, R., Henning, Th., Hubin, N., Kasper, M., Keller, C. U., Langlois, M., Lagadec, E., Martinez, P., Maire, A. L., Menard, F., Meyer, M. R., Pavlov, A., Pragt, J., Puget, P., Quanz, S. P., Rickman, E., Roelfsema, R., Salasnich, B., Sauvage, J.-F., Siebenmorgen, R., Sissa, E., Snik, F., Suarez, M., Szulágyi, J., Thalmann, Ch., Turatto, M., Udry, S., van Holstein, R. G., Vigan, A., Wildi, F., is now available here.
Abstract: Aims: RefPlanets is a guaranteed time observation programme that
uses the Zurich IMaging POLarimeter (ZIMPOL) of Spectro-Polarimetric
High-contrast Exoplanet REsearch instrument at the Very Large Telescope
to perform a blind search for exoplanets in wavelengths from 600 to 900
nm. The goals of this study are the characterisation of the
unprecedented high polarimetic contrast and polarimetric precision
capabilities of ZIMPOL for bright targets, the search for polarised
reflected light around some of the closest bright stars to the Sun, and
potentially the direct detection of an evolved cold exoplanet for the
first time. Methods: For our observations of α Cen A and B,
Sirius A, Altair, ɛ Eri and τ Ceti we used the
polarimetricdifferential imaging (PDI) mode of ZIMPOL which removes the
speckle noise down to the photon noise limit for angular separations
≿0.6''. We describe some of the instrumental effects that dominate
the noise for smaller separations and explain how to remove these
additional noise effects in post-processing. We then combine PDI with
angular differential imaging as a final layer of post-processing to
further improve the contrast limits of our data at these separations.
Results: For good observing conditions we achieve polarimetric
contrast limits of 15.0-16.3 mag at the effective inner working angle of
~0.13'', 16.3-18.3 mag at 0.5'', and 18.8-20.4 mag at 1.5''. The
contrast limits closer in (≾0.6'') display a significant
dependence on observing conditions, while in the photon-noise-dominated
regime (≿0.6'') the limits mainly depend on the brightness of the
star and the total integration time. We compare our results with
contrast limits from other surveys and review the exoplanet detection
limits obtained with different detection methods. For all our targets we
achieve unprecedented contrast limits. Despite the high polarimetric
contrasts we are not able to find any additional companions or extended
polarised light sources in the data obtained so far.
Based on observations made with ESO Telescopes at the La Silla Paranal
Observatory under programme IDs: 095.C-0312(B), 096.C-0326(A),
097.C-0524(A), 097.C-0524(B), 098.C-0197(A), 099.C-0127(A),
099.C-0127(B), 0102.C-0435(A).
by van Holstein, R. G., Girard, J. H., de Boer, J., Snik, F., Milli, J., Stam, D. M., Ginski, C., Mouillet, D., Wahhaj, Z., Schmid, H. M., Keller, C. U., Langlois, M., Dohlen, K., Vigan, A., Pohl, A., Carbillet, M., Fantinel, D., Maurel, D., Origné, A., Petit, C., Ramos, J., Rigal, F., Sevin, A., Boccaletti, A., Le Coroller, H., Dominik, C., Henning, T., Lagadec, E., Ménard, F., Turatto, M., Udry, S., Chauvin, G., Feldt, M., Beuzit, J.-L., is now available here.
Abstract: Context. Circumstellar disks and self-luminous giant exoplanets or
companion brown dwarfs can be characterized through direct-imaging
polarimetry at near-infrared wavelengths. SPHERE/IRDIS at the Very Large
Telescope has the capabilities to perform such measurements, but
uncalibrated instrumental polarization effects limit the attainable
polarimetric accuracy. Aims: We aim to characterize and correct
the instrumental polarization effects of the complete optical system,
that is, the telescope and SPHERE/IRDIS. Methods: We created a
detailed Mueller matrix model in the broadband filters Y, J, H, and
Ks and calibrated the model using measurements with SPHERE's
internal light source and observations of two unpolarized stars. We
developed a data-reduction method that uses the model to correct for the
instrumental polarization effects, and applied it to observations of the
circumstellar disk of T Cha. Results: The instrumental
polarization is almost exclusively produced by the telescope and
SPHERE's first mirror and varies with telescope altitude angle. The
crosstalk primarily originates from the image derotator (K-mirror). At
some orientations, the derotator causes severe loss of signal (> 90%
loss in the H- and Ks-band) and strongly offsets the angle of
linear polarization. With our correction method we reach, in all
filters, a total polarimetric accuracy of ≲0.1% in the degree of
linear polarization and an accuracy of a few degrees in angle of linear
polarization. Conclusions: The correction method enables us to
accurately measure the polarized intensity and angle of linear
polarization of circumstellar disks, and is a vital tool for detecting
spatially unresolved (inner) disks and measuring the polarization of
substellar companions. We have incorporated the correction method in a
highly-automated end-to-end data-reduction pipeline called IRDAP, which
we made publicly available online.
Based on observations made with ESO telescopes at the La Silla Paranal
Observatory under program ID 60.A-9800(S), 60.A-9801(S) and
096.C-0248(C).
The data-reduction pipeline IRDAP is available at http://https://irdap.readthedocs.io
by de Boer, J., Langlois, M., van Holstein, R. G., Girard, J. H., Mouillet, D., Vigan, A., Dohlen, K., Snik, F., Keller, C. U., Ginski, C., Stam, D. M., Milli, J., Wahhaj, Z., Kasper, M., Schmid, H. M., Rabou, P., Gluck, L., Hugot, E., Perret, D., Martinez, P., Weber, L., Pragt, J., Sauvage, J.-F., Boccaletti, A., Le Coroller, H., Dominik, C., Henning, T., Lagadec, E., Ménard, F., Turatto, M., Udry, S., Chauvin, G., Feldt, M., Beuzit, J.-L., is now available here.
Abstract: Context. Polarimetric imaging is one of the most effective techniques
for high-contrast imaging and for the characterization of protoplanetary
disks, and it has the potential of becoming instrumental in the
characterization of exoplanets. The Spectro-Polarimetric High-contrast
Exoplanet REsearch (SPHERE) instrument installed on the Very Large
Telescope (VLT) contains the InfraRed Dual-band Imager and Spectrograph
(IRDIS) with a dual-beam polarimetric imaging (DPI) mode, which offers
the capability of obtaining linear polarization images at high contrast
and resolution. Aims: We aim to provide an overview of the
polarimetric imaging mode of VLT/SPHERE/IRDIS and study its optical
design to improve observing strategies and data reduction.
Methods: For H-band observations of TW Hydrae, we compared two data
reduction methods that correct for instrumental polarization effects in
different ways: a minimization of the "noise" image
(Uϕ), and a correction method based on a polarimetric
model that we have developed, as presented in Paper II of this study.
Results: We use observations of TW Hydrae to illustrate the data
reduction. In the images of the protoplanetary disk around this star, we
detect variability in the polarized intensity and angle of linear
polarization that depend on the pointing-dependent instrument
configuration. We explain these variations as instrumental polarization
effects and correct for these effects using our model-based correction
method. Conclusions: The polarimetric imaging mode of IRDIS has
proven to be a very successful and productive high-contrast polarimetric
imaging system. However, the instrument performance is strongly
dependent on the specific instrument configuration. We suggest
adjustments to future observing strategies to optimize polarimetric
efficiency in field-tracking mode by avoiding unfavorable derotator
angles. We recommend reducing on-sky data with the pipeline called
IRDAP, which includes the model-based correction method (described in
Paper II) to optimally account for the remaining telescope and
instrumental polarization effects and to retrieve the true polarization
state of the incident light.
Based on observations made with ESO Telescopes at the La Silla Paranal
Observatory under programme ID 095.C-0273(D).
by Snik, Frans, Keller, Christoph U., Doelman, David S., Kühn, Jonas, Patty, C. H. Lucas, Hoeijmakers, H. Jens, Pallichadath, Vidhya, Stam, Daphne M., Pommerol, Antoine, Poch, Olivier, Demory, Brice-Olivier, is now available here.
Abstract: We present the design of a point-and-shoot non-imaging full-Stokes
spectropolarimeter dedicated to detecting life on Earth from an orbiting
platform like the ISS. We specifically aim to map circular polarization
in the spectral features of chorophyll and other biopigments for our
planet as a whole. These non-zero circular polarization signatures are
caused by homochirality of the molecular and supramolecular
configurations of organic matter, and are considered the most
unambiguous biomarker. To achieve a fully solid-state snapshot design,
we implement a novel spatial modulation that completely separates the
circular and linear polarization channels. The polarization modulator
consists of a patterned liquid-crystal quarter-wave plate inside the
spectrograph slit, which also constitutes the first optical element of
the instrument. This configuration eliminates cross-talk between linear
and circular polarization, which is crucial because linear polarization
signals are generally much stronger than the circular polarization
signals. This leads to a quite unorthodox optical concept for the
spectrograph, in which the object and the pupil are switched. We discuss
the general design requirements and trade-offs of LSDpol (Life Signature
Detection polarimeter), a prototype instrument that is currently under
development.