20 research outputs found
Fomalhaut b: Independent Analysis of the Hubble Space Telescope Public Archive Data
The nature and even the existence of a putative planet-mass companion
("Fomalhaut b") to Fomalhaut has been debated since 2008. In the present paper
we reanalyze the multi-epoch ACS/STIS/WFC3 Hubble Space Telescope (HST)
optical/near infrared images on which the discovery and some other claims were
based. We confirm that the HST images do reveal an object in orbit around
Fomalhaut but the detailed results from our analysis differ in some ways from
previous discussions. In particular, we do not confirm flux variability over a
two-year interval at 0.6 microns wavelength and we detect Fomalhaut b for the
first time at the short wavelength of 0.43 microns. We find that the HST image
of Fomalhaut b at m may be extended beyond the PSF. We cannot
determine from our astrometry if Fomalhaut b will cross or not the dust ring.
The optical through mid-infrared spectral energy distribution (SED) of
Fomalhaut b cannot be explained as due to direct or scattered radiation from a
massive planet. We consider two models to explain the SED: (1) a large
circumplanetary disk around an unseen planet and (2) the aftermath of a
collision during the past 50-150 years of two Kuiper Belt-like objects of radii
50 km.Comment: 24 pages, 9 figures, 5 tables, accepted on April, 3rd, 201
Self-coherent camera as a focal plane wavefront sensor: simulations
Direct detection of exoplanets requires high dynamic range imaging.
Coronagraphs could be the solution, but their performance in space is limited
by wavefront errors (manufacturing errors on optics, temperature variations,
etc.), which create quasi-static stellar speckles in the final image. Several
solutions have been suggested for tackling this speckle noise. Differential
imaging techniques substract a reference image to the coronagraphic residue in
a post-processing imaging. Other techniques attempt to actively correct
wavefront errors using a deformable mirror. In that case, wavefront aberrations
have to be measured in the science image to extremely high accuracy. We propose
the self-coherent camera sequentially used as a focal-plane wavefront sensor
for active correction and differential imaging. For both uses, stellar speckles
are spatially encoded in the science image so that differential aberrations are
strongly minimized. The encoding is based on the principle of light incoherence
between the hosting star and its environment. In this paper, we first discuss
one intrinsic limitation of deformable mirrors. Then, several parameters of the
self-coherent camera are studied in detail. We also propose an easy and robust
design to associate the self-coherent camera with a coronagraph that uses a
Lyot stop. Finally, we discuss the case of the association with a four-quadrant
phase mask and numerically demonstrate that such a device enables the detection
of Earth-like planets under realistic conditions. The parametric study of the
technique lets us believe it can be implemented quite easily in future
instruments dedicated to direct imaging of exoplanets.Comment: 15 pages, 14 figures, accepted in A&A (here is the final version
GPI PSF subtraction with TLOCI: the next evolution in exoplanet/disk high-contrast imaging
To directly image exoplanets and faint circumstellar disks, the noisy stellar
halo must be suppressed to a high level. To achieve this feat, the angular
differential imaging observing technique and the least-squares Locally
Optimized Combination of Images (LOCI) algorithm have now become the standard
in single band direct imaging observations and data reduction. With the
development and commissioning of new high-order high-contrast adaptive optics
equipped with integral field units, the image subtraction algorithm needs to be
modified to allow the optimal use of polychromatic images, field-rotated images
and archival data. A new algorithm, TLOCI (for Template LOCI), is designed to
achieve this task by maximizing a companion signal-to-noise ratio instead of
simply minimizing the noise as in the original LOCI algorithm. The TLOCI
technique uses an input spectrum and template Point Spread Functions (PSFs,
generated from unocculted and unsaturated stellar images) to optimize the
reference image least-squares coefficients to minimize the planet
self-subtraction, thus maximizing its throughput per wavelength, while
simultaneously providing a maximum suppression of the speckle noise. The new
algorithm has been developed using on-sky GPI data and has achieved impressive
contrast. This paper presents the TLOCI algorithm, on-sky performance, and will
discuss the challenges in recovering the planet spectrum with high fidelity.Comment: 13 pages, 8 figures, to appear in Proceedings of SPIE 914
Exoplanets imaging with a Phase-Induced Amplitude Apodization Coronagraph - I. Principle
Using 2 aspheric mirrors, it is possible to apodize a telescope beam without
losing light or angular resolution: the output beam is produced by
``remapping'' the entrance beam to produce the desired light intensity
distribution in a new pupil. We present the Phase-Induced Amplitude Apodization
Coronagraph (PIAAC) concept, which uses this technique, and we show that it
allows efficient direct imaging of extrasolar terrestrial planets with a
small-size telescope in space. The suitability of the PIAAC for exoplanet
imaging is due to a unique combination of achromaticity, small inner working
angle (about 1.5 ), high throughput, high angular resolution and
large field of view. 3D geometrical raytracing is used to investigate the
off-axis aberrations of PIAAC configurations, and show that a field of view of
more than 100 in radius is available thanks to the correcting
optics of the PIAAC. Angular diameter of the star and tip-tilt errors can be
compensated for by slightly increasing the size of the occulting mask in the
focal plane, with minimal impact on the system performance. Earth-size planets
at 10 pc can be detected in less than 30s with a 4m telescope. Wavefront
quality requirements are similar to classical techniques.Comment: 35 pages, 16 figures, Accepted for publication in Ap
Status and performance of the THD2 bench in multi-deformable mirror configuration
The architecture of exoplanetary systems is relatively well known inward to 1
AU thanks to indirect techniques, which have allowed characterization of
thousands of exoplanet orbits, masses and sometimes radii. The next step is the
characterization of exoplanet atmospheres at long period, which requires direct
imaging capability. While the characterization of a handful of young giant
planets is feasible with dedicated instruments like SPHERE/VLT, GPI/Gemini,
SCExAO/Subaru and soon with the coronagraphic capabilities aboard JWST, the
spectroscopic study of mature giant planets and lower mass planets
(Neptune-like, Super Earths) requires the achievement of better coronagraphic
performance. While space-based coronagraph on WFIRST-AFTA might start this
study at low spectroscopic resolution, dedicated projects on large space
telescope and on the ELT will be required for a more complete spectroscopic
study of these faint planets. To prepare these future instruments, we developed
a high contrast imaging bench called THD, then THD2 for the upgraded version
using multi-DM configuration. The THD2 bench is designed to test and compare
coronagraphs as well as focal plane wavefront sensors and wavefront control
techniques. It can simulate the beam provided by a space telescope and soon the
first stage of adaptive optics behind a ground-based telescope. In this
article, we describe in details the THD2 bench and give the results of a recent
comparison study of the chromatic behavior for several coronagraph on the THD2.Comment: 9 pages, 5 figures, 1 Table, AO4ELT 2017 conference proceedin
M-band Imaging of the HR 8799 Planetary System Using an Innovative LOCI-based Background Subtraction Technique
Multi-wavelength observations/spectroscopy of exoplanetary atmospheres are
the basis of the emerging exciting field of comparative exoplanetology. The HR
8799 planetary system is an ideal laboratory to study our current knowledge gap
between massive field brown dwarfs and the cold 5-Gyr old Solar system planets.
The HR 8799 planets have so far been imaged at J- to L-band, with only upper
limits available at M-band. We present here deep high-contrast Keck II adaptive
optics M-band observations that show the imaging detection of 3 of the 4
currently known HR 8799 planets. Such detections were made possible due to the
development of an innovative LOCI-based background subtraction scheme that is 3
times more efficient than a classical median background subtraction for Keck II
AO data, representing a gain in telescope time of up to a factor of 9. These
M-band detections extend the broad band photometric coverage out to about 5
microns and provide access to the strong CO fundamental absorption band at
4.5microns. The new M-band photometry shows that the HR 8799 planets are
located near the L/T-type dwarf transition, similar to what was found by other
studies. We also confirm that the best atmospheric fits are consistent with low
surface gravity, dusty and non-equilibrium CO/CH4 chemistry models.Comment: 10 pages, 4 figures, 1 table, accepted in ApJ
Near-Infrared Detection and Characterization of the Exoplanet HD 95086 b with the Gemini Planet Imager
HD 95086 is an intermediate-mass debris-disk-bearing star. VLT/NaCo observations revealed it hosts a companion (HD
95086 b) at AU. Follow-up observations at 1.66 and 2.18
yielded a null detection, suggesting extremely red colors for the planet and
the need for deeper direct-imaging data. In this Letter, we report H- () and - () band detections of HD 95086 b from
Gemini Planet Imager (GPI) commissioning observations taken by the GPI team.
The planet position in both spectral channels is consistent with the NaCo
measurements and we confirm it to be comoving. Our photometry yields colors of
H-L'= mag and K-L'= mag, consistent with
previously reported 5- upper limits in H and Ks. The photometry of HD
95086 b best matches that of 2M 1207 b and HR 8799 cde. Comparing its spectral
energy distribution with the BT-SETTL and LESIA planet atmospheric models
yields T600-1500 K and log g2.1-4.5. Hot-start
evolutionary models yield M= M. Warm-start models reproduce the
combined absolute fluxes of the object for M=4-14 M for a wide range of
plausible initial conditions (S=8-13 k/baryon). The
color-magnitude diagram location of HD 95086 b and its estimated
T and log g suggest that the planet is a peculiar L-T
transition object with an enhanced amount of photospheric dust.Comment: 4 pages, 4 figures, 3 tables, accepted on April, 15th, 201
The International Deep Planet Survey II: The frequency of directly imaged giant exoplanets with stellar mass
Radial velocity and transit methods are effective for the study of short
orbital period exoplanets but they hardly probe objects at large separations
for which direct imaging can be used. We carried out the international deep
planet survey of 292 young nearby stars to search for giant exoplanets and
determine their frequency. We developed a pipeline for a uniform processing of
all the data that we have recorded with NIRC2/Keck II, NIRI/Gemini North,
NICI/Gemini South, and NACO/VLT for 14 years. The pipeline first applies
cosmetic corrections and then reduces the speckle intensity to enhance the
contrast in the images. The main result of the international deep planet survey
is the discovery of the HR 8799 exoplanets. We also detected 59 visual multiple
systems including 16 new binary stars and 2 new triple stellar systems, as well
as 2,279 point-like sources. We used Monte Carlo simulations and the Bayesian
theorem to determine that 1.05[+2.80-0.70]% of stars harbor at least one giant
planet between 0.5 and 14M_J and between 20 and 300 AU. This result is obtained
assuming uniform distributions of planet masses and semi-major axes. If we
consider power law distributions as measured for close-in planets instead, the
derived frequency is 2.30[+5.95-1.55]%, recalling the strong impact of
assumptions on Monte Carlo output distributions. We also find no evidence that
the derived frequency depends on the mass of the hosting star, whereas it does
for close-in planets. The international deep planet survey provides a database
of confirmed background sources that may be useful for other exoplanet direct
imaging surveys. It also puts new constraints on the number of stars with at
least one giant planet reducing by a factor of two the frequencies derived by
almost all previous works.Comment: 83 pages, 13 figures, 15 Tables, accepted in A&
Increasing the raw contrast of VLT/SPHERE with the dark-hole technique. II. On-sky wavefront correction and coherent differential imaging
Context. Direct imaging of exoplanets takes advantage of state-of-the-art
adaptive optics (AO) systems, coronagraphy, and post-processing techniques.
Coronagraphs attenuate starlight to mitigate the unfavorable flux ratio between
an exoplanet and its host star. AO systems provide diffraction-limited images
of point sources and minimize optical aberrations that would cause starlight to
leak through coronagraphs. Post-processing techniques then estimate and remove
residual stellar speckles such as noncommon path aberrations (NCPAs) and
diffraction from telescope obscurations. Aims. We aim to demonstrate an
efficient method to minimize the speckle intensity due to NCPAs during an
observing night on VLT/SPHERE. Methods. We implement an iterative dark-hole
(DH) algorithm to remove stellar speckles on-sky before a science observation.
It uses a pair-wise probing estimator and a controller based on electric field
conjugation. This work presents the first such on-sky minimization of speckles
with a DH technique on SPHERE. Results. We show the standard deviation of the
normalized intensity in the raw images is reduced by a factor of up to 5 in the
corrected region with respect to the current calibration strategy under median
conditions for VLT. This level of contrast performance obtained with only 1 min
of exposure time reaches median performances on SPHERE that use post-processing
methods requiring 1h-long sequences of observations. We also present an
alternative calibration method that takes advantage of the starlight coherence
and improves the post-processed contrast levels rms by a factor of about 3.
Conclusions. This on-sky demonstration represents a decisive milestone for the
future design, development, and observing strategy of the next generation of
ground-based exoplanet imagers for 10m to 40m telescope.Comment: Accepted in Astronomy & Astrophysic