Pupil stabilization for SPHERE's extreme AO and high performance coronagraph system

We propose a new concept of pupil motion sensor for astronomical adaptive optics systems and present experimental results obtained during the first laboratory validation of this concept. Pupil motion is an important issue in the case of extreme adaptive optics, high contrast systems, such as the proposed Planet Finder instruments for the ESO and Gemini 8-meter telescopes. Such high contrast imaging instruments will definitively require pupil stabilization to minimize the effect of quasi-static aberrations. The concept for pupil stabilization we propose uses the flux information from the AO system wave-front sensor to drive in closed loop a pupil tip-tilt mirror located in a focal plane. A laboratory experiment validates this concept and demonstrates its interest for high contrast imaging instrument.Comment: This paper was published in Optics Express and is made available as an electronic reprint with the permission of OSA. The paper can be found at http://www.opticsexpress.org/abstract.cfm?id=144687 on the OSA websit

Analysis of ground-based differential imager performance

In the context of extrasolar planet direct detection, we evaluated the performance of differential imaging with ground-based telescopes. This study was carried out in the framework of the VLT-Planet Finder project and is further extended to the case of Extremely Large Telescopes. Our analysis is providing critical specifications for future instruments mostly in terms of phase aberrations but also regarding alignments of the instrument optics or offset pointing on the coronagraph. It is found that Planet Finder projects on 8m class telescopes can be successful at detecting Extrasolar Giant Planets providing phase aberrations, alignments and pointing are accurately controlled. The situation is more pessimistic for the detection of terrestrial planets with Extremely Large Telescopes for which phase aberrations must be lowered at a very challenging level

Direct exoplanet detection and characterization using the ANDROMEDA method: Performance on VLT/NaCo data

Context. The direct detection of exoplanets with high-contrast imaging requires advanced data processing methods to disentangle potential planetary signals from bright quasi-static speckles. Among them, angular differential imaging (ADI) permits potential planetary signals with a known rotation rate to be separated from instrumental speckles that are either statics or slowly variable. The method presented in this paper, called ANDROMEDA for ANgular Differential OptiMal Exoplanet Detection Algorithm is based on a maximum likelihood approach to ADI and is used to estimate the position and the flux of any point source present in the field of view. Aims. In order to optimize and experimentally validate this previously proposed method, we applied ANDROMEDA to real VLT/NaCo data. In addition to its pure detection capability, we investigated the possibility of defining simple and efficient criteria for automatic point source extraction able to support the processing of large surveys. Methods. To assess the performance of the method, we applied ANDROMEDA on VLT/NaCo data of TYC-8979-1683-1 which is surrounded by numerous bright stars and on which we added synthetic planets of known position and flux in the field. In order to accommodate the real data properties, it was necessary to develop additional pre-processing and post-processing steps to the initially proposed algorithm. We then investigated its skill in the challenging case of a well-known target, $\beta$ Pictoris, whose companion is close to the detection limit and we compared our results to those obtained by another method based on principal component analysis (PCA). Results. Application on VLT/NaCo data demonstrates the ability of ANDROMEDA to automatically detect and characterize point sources present in the image field. We end up with a robust method bringing consistent results with a sensitivity similar to the recently published algorithms, with only two parameters to be fine tuned. Moreover, the companion flux estimates are not biased by the algorithm parameters and do not require a posteriori corrections. Conclusions. ANDROMEDA is an attractive alternative to current standard image processing methods that can be readily applied to on-sky data

Search for cool giant exoplanets around young and nearby stars - VLT/NaCo near-infrared phase-coronagraphic and differential imaging

[Abridged] Context. Spectral differential imaging (SDI) is part of the observing strategy of current and future high-contrast imaging instruments. It aims to reduce the stellar speckles that prevent the detection of cool planets by using in/out methane-band images. It attenuates the signature of off-axis companions to the star, such as angular differential imaging (ADI). However, this attenuation depends on the spectral properties of the low-mass companions we are searching for. The implications of this particularity on estimating the detection limits have been poorly explored so far. Aims. We perform an imaging survey to search for cool (Teff<1000-1300 K) giant planets at separations as close as 5-10 AU. We also aim to assess the sensitivity limits in SDI data taking the photometric bias into account. This will lead to a better view of the SDI performance. Methods. We observed a selected sample of 16 stars (age < 200 Myr, d < 25 pc) with the phase-mask coronagraph, SDI, and ADI modes of VLT/NaCo. Results. We do not detect any companions. As for the sensitivity limits, we argue that the SDI residual noise cannot be converted into mass limits because it represents a differential flux, unlike the case of single-band images. This results in degeneracies for the mass limits, which may be removed with the use of single-band constraints. We instead employ a method of directly determining the mass limits. The survey is sensitive to cool giant planets beyond 10 AU for 65% and 30 AU for 100% of the sample. Conclusions. For close-in separations, the optimal regime for SDI corresponds to SDI flux ratios >2. According to the BT-Settl model, this translates into Teff<800 K. The methods described here can be applied to the data interpretation of SPHERE. We expect better performance with the dual-band imager IRDIS, thanks to more suitable filter characteristics and better image quality.Comment: 19 pages, 16 figures, accepted for publication in A&A, version including language editin

Search for cool extrasolar giant planets combining coronagraphy, spectral and angular differential imaging

Spectral differential imaging (SDI) is part of the observing strategy of current and on-going high-contrast imaging instruments on ground-based telescopes. Although it improves the star light rejection, SDI attenuates the signature of off-axis companions to the star, just like angular differential imaging (ADI). However, the attenuation due to SDI has the peculiarity of being dependent on the spectral properties of the companions. To date, no study has investigated these effects. Our team is addressing this problem based on data from a direct imaging survey of 16 stars combining the phase-mask coronagraph, the SDI and the ADI modes of VLT/NaCo. The objective of the survey is to search for cool (Teff<1000-1300 K) giant planets at separations of 5-10 AU orbiting young, nearby stars (<200 Myr, <25 pc). The data analysis did not yield any detections. As for the estimation of the sensitivity limits of SDI-processed images, we show that it requires a different analysis than that used in ADI-based surveys. Based on a method using the flux predictions of evolutionary models and avoiding the estimation of contrast, we determine directly the mass sensitivity limits of the survey for the ADI processing alone and with the combination of SDI and ADI. We show that SDI does not systematically improve the sensitivity due to the spectral properties and self-subtraction of point sources.Comment: 5 pages, 2 figure

Deep imaging survey of young, nearby austral stars: VLT/NACO near-infrared Lyot-coronographic observations

Context. High contrast and high angular resolution imaging is the optimal search technique for substellar companions to nearby stars at physical separations larger than typically 10 AU. Two distinct populations of substellar companions, brown dwarfs and planets, can be probed and characterized. As a result, fossile traces of processes of formation and evolution can be revealed by physical and orbital properties, both for individual systems and as an ensemble. Aims. Since November 2002, we have conducted a large, deep imaging, survey of young, nearby associations of the southern hemisphere. Our goal is detection and characterization of substellar companions with projected separations in the range 10–500 AU. We have observed a sample of 88 stars, primarily G to M dwarfs, younger than 100 Myr, and within 100 pc of Earth. Methods. The VLT/NACO adaptive optics instrument of the ESO Paranal Observatory was used to explore the faint circumstellar environment between typically 0.1 and 10". Diffraction-limited observations in H and K_s-band combined with Lyot-coronagraphy enabled us to reach primary star-companion brightness ratios as small as 10^(-6). The existence of planetary mass companions could therefore be probed. We used a standardized observing sequence to precisely measure the position and flux of all detected sources relative to their visual primary star. Repeated observations at several epochs enabled us to discriminate comoving companions from background objects. Results. We report the discovery of 17 new close (0.1–5.0") multiple systems. HIP 108195 AB and C (F1 III-M6), HIP 84642 AB (a~14 AU, K0-M5) and TWA22 AB (a~1.8 AU; M6-M6) are confirmed comoving systems. TWA22 AB is likely to be a rare astrometric calibrator that can be used to test evolutionary model predictions. Among our complete sample, a total of 65 targets were observed with deep coronagraphic imaging. About 240 faint companion candidates were detected around 36 stars. Follow-up observations with VLT or HST for 83% of these stars enabled us to identify a large fraction of background contaminants. Our latest results that pertain to the substellar companions to GSC 08047-00232, AB Pic and 2M1207 (confirmed during this survey and published earlier), are reviewed. Finally, a statistical analysis of our complete set of coronagraphic detection limits enables us to place constraints on the physical and orbital properties of giant planets between typically 20 and 150 AU

A probable giant planet imaged in the Beta Pictoris disk

Since the discovery of its dusty disk in 1984, Beta Pictoris has become the prototype of young early-type planetary systems, and there are now various indications that a massive Jovian planet is orbiting the star at ~ 10 AU. However, no planets have been detected around this star so far. Our goal was to investigate the close environment of Beta Pic, searching for planetary companion(s). Deep adaptive-optics L'-band images of Beta Pic were recorded using the NaCo instrument at the Very Large Telescope. A faint point-like signal is detected at a projected distance of ~ 8 AU from the star, within the North-East side of the dust disk. Various tests were made to rule out with a good confidence level possible instrumental or atmospheric artifacts. The probability of a foreground or background contaminant is extremely low, based in addition on the analysis of previous deep Hubble Space Telescope images. The object L'=11.2 apparent magnitude would indicate a typical temperature of ~1500 K and a mass of ~ 8 Jovian masses. If confirmed, it could explain the main morphological and dynamical peculiarities of the Beta Pic system. The present detection is unique among A-stars by the proximity of the resolved planet to its parent star. Its closeness and location inside the Beta Pic disk suggest a formation process by core accretion or disk instabilities rather than a binary-like formation process.Comment: 5 pages, 3 figures, 1 table. A&A Letters, in pres

Evidence for Retrogene Origins of the Prion Gene Family

The evolutionary origin of prion genes, only known to exist in the vertebrate lineage, had remained elusive until recently. Following a lead from interactome investigations of the murine prion protein, our previous bioinformatic analyses revealed the evolutionary descent of prion genes from an ancestral ZIP metal ion transporter. However, the molecular mechanism of evolution remained unexplored. Here we present a computational investigation of this question based on sequence, intron-exon, synteny and pseudogene analyses. Our data suggest that during the emergence of metazoa, a cysteine-flanked core domain was modularly inserted, or arose de novo, in a preexisting ZIP ancestor gene to generate a prion-like ectodomain in a subbranch of ZIP genes. Approximately a half-billion years later, a genomic insertion of a spliced transcript coding for such a prion-like ZIP ectodomain may have created the prion founder gene. We document that similar genomic insertions involving ZIP transcripts, and probably relying on retropositional elements, have indeed occurred more than once throughout evolution

Calibration of quasi-static aberrations in exoplanet direct-imaging instruments with a Zernike phase-mask sensor. II. Concept validation with ZELDA on VLT/SPHERE

Warm or massive gas giant planets, brown dwarfs, and debris disks around nearby stars are now routinely observed by dedicated high-contrast imaging instruments on large, ground-based observatories. These facilities include extreme adaptive optics (ExAO) and state-of-the-art coronagraphy to achieve unprecedented sensitivities for exoplanet detection and spectral characterization. However, differential aberrations between the ExAO sensing path and the science path represent a critical limitation for the detection of giant planets with a contrast lower than a few $10^{-6}$ at very small separations (<0.3\as) from their host star. In our previous work, we proposed a wavefront sensor based on Zernike phase contrast methods to circumvent this issue and measure these quasi-static aberrations at a nanometric level. We present the design, manufacturing and testing of ZELDA, a prototype that was installed on VLT/SPHERE during its reintegration in Chile. Using the internal light source of the instrument, we performed measurements in the presence of Zernike or Fourier modes introduced with the deformable mirror. Our experimental and simulation results are consistent, confirming the ability of our sensor to measure small aberrations (<50 nm rms) with nanometric accuracy. We then corrected the long-lived non-common path aberrations in SPHERE based on ZELDA measurements. We estimated a contrast gain of 10 in the coronagraphic image at 0.2\as, reaching the raw contrast limit set by the coronagraph in the instrument. The simplicity of the design and its phase reconstruction algorithm makes ZELDA an excellent candidate for the on-line measurements of quasi-static aberrations during the observations. The implementation of a ZELDA-based sensing path on the current and future facilities (ELTs, future space missions) could ease the observation of the cold gaseous or massive rocky planets around nearby stars.Comment: 13 pages, 12 figures, A&A accepted on June 3rd, 2016. v2 after language editin

The SPHERE data center: a reference for high contrast imaging processing

The objective of the SPHERE Data Center is to optimize the scientific return of SPHERE at the VLT, by providing optimized reduction procedures, services to users and publicly available reduced data. This paper describes our motivation, the implementation of the service (partners, infrastructure and developments), services, description of the on-line data, and future developments. The SPHERE Data Center is operational and has already provided reduced data with a good reactivity to many observers. The first public reduced data have been made available in 2017. The SPHERE Data Center is gathering a strong expertise on SPHERE data and is in a very good position to propose new reduced data in the future, as well as improved reduction procedures.Comment: SF2A proceeding