A. Boccaletti

A Giant Planet Imaged in the Disk of the Young Star β Pictoris

Planet Is Born The 10-million-year-old star β Pictoris, has long been suspected to host a planet. Through images obtained with the Very Large Telescope, an array of four telescopes located in Chile, Lagrange et al. (p. 57, published online 10 June) now confirm the presence of a young, giant planet, β Pictoris b, orbiting within the dusty disk that surrounds the star. β Pictoris b orbits closer to its star than Uranus and Neptune do to the Sun in our solar system. This orbital separation is consistent with the in situ formation of the planet via a core accretion mechanism. Thus, giant planets can form within a stellar dust disk in only a few million years. The Very Large Telescope reveals that a huge planet formed within a star’s dusty disk in a few million years. Here, we show that the ~10-million-year-old β Pictoris system hosts a massive giant planet, β Pictoris b, located 8 to 15 astronomical units from the star. This result confirms that gas giant planets form rapidly within disks and validates the use of disk structures as fingerprints of embedded planets. Among the few planets already imaged, β Pictoris b is the closest to its parent star. Its short period could allow for recording of the full orbit within 17 years.

The Four-Quadrant Phase-Mask Coronagraph. I. Principle

We describe a new type of coronagraph, based on the principle of a phase mask as proposed by Roddier and Roddier a few years ago but using an original mask design found by one of us (D. R.), a four-quadrant binary phase mask (0, n) covering the full Ðeld of view at the focal plane. The mutually destructive interferences of the coherent light from the main source produce a very efficient nulling. The computed rejection rate of this coronagraph appears to be very high since, when perfectly aligned and phase-error free, it could in principle reduce the total amount of light from the bright source by a factor of 108, corresponding to a gain of 20 mag in brightness at the location of the Ðrst Airy ring, relative to the Airy peak. In the real world the gain is of course reduced by a strong factor, but nulling is still performing quite well, provided that the perturbation of the phase, for instance, due to the EarthIs atmosphere, is efficiently corrected by adaptive optics. We show from simulations that a detection at a contrast of 10 mag between a star and a faint companion is achievable in excellent conditions, while 8 mag appears routinely feasible. This coronagraph appears less sensitive to atmospheric turbulence and has a larger dynamic range than other recently proposed nulling techniques : the phase-mask coronagraph (by Roddier and Roddier) or the Achro- matic Interfero-Coronagraph (by Gay and Rabbia). We present the principle of the four-quadrant corona- graph and results of a Ðrst series of simulations. We compare those results with theoretical performances of other devices. We brieNy analyze the di†erent limitations in space or ground-based observations, as well as the issue of manufacturing the device. We also discuss several ways to improve the detection of a faint companion around a bright object. We conclude that, with respect to previous techniques, an instrument equipped with this coronagraph should have better performance and even enable the imaging of extrasolar giant planets at a young stage, when coupled with additional cleaning techniques.

Photometric characterization of exoplanets using angular and spectral differential imaging

In recent years, there has been intensive research into the direct detection of exoplanets. Data obtained in the future with high-contrast imaging instruments, optimized for the direct detection of giant planets, may be strongly limited by speckle noise. Specific observing strategies and data analysis methods, such as angular and spectral differential imaging, are required to attenuate the noise level and possibly to detect the flux of faint planets. Even though these methods are very efficient at suppressing the speckles, the photometry of faint planets is dominated by the speckle residuals. The determination of the effective temperature and surface gravity of the detected planets from photometric measurements in different bands is then limited by the photometric error on the planet flux. In this paper, we investigate this photometric error and the consequences on the determination of the physical parameters of the detected planets. We perform detailed end-to-end simulation with the CAOS-based software package for spectro-polarimetric high-contrast exoplanet research (SPHERE) to obtain realistic data representing typical observing sequences in the Y, J, H and K s bands with a high-contrast imager. The simulated data are used to measure the photometric accuracy as a function of contrast for planets detected with angular and spectral+angular differential methods. We apply this empirical accuracy to study the characterization capabilities of a high-contrast differential imager. We show that the expected photometric performances will allow the detection and characterization of exoplanets down to a Jupiter mass at angular separations of 1.0 and 0.2 arcsec, respectively, around high-mass and low-mass stars with two observations in different filter pairs. We also show that the determination of the physical parameters of the planets from photometric measurements in different filter pairs is essentially limited by the error on the determination of the surface gravity.

Asymmetric features in the protoplanetary disk MWC 758

Context. The study of dynamical processes in protoplanetary disks is essential to understand planet formation. In this context, transition disks are prime targets because they are at an advanced stage of disk clearing and may harbor direct signatures of disk evolution. Aims. We aim to derive new constraints on the structure of the transition disk MWC 758, to detect non-axisymmetric features and understand their origin. Methods. We obtained infrared polarized intensity observations of the protoplanetary disk MWC 758 with SPHERE/VLT at 1.04 m to resolve scattered light at a smaller inner working angle (0.093 00 ) and a higher angular resolution (0.027 00 ) than previously achieved. Results. We observe polarized scattered light within 0.53 00 (148 au) down to the inner working angle (26 au) and detect distinct nonaxisymmetric features but no fully depleted cavity. The two small-scale spiral features that were previously detected with HiCIAO are resolved more clearly, and new features are identified, including two that are located at previously inaccessible radii close to the star. We present a model based on the spiral density wave theory with two planetary companions in circular orbits. The best model requires a high disk aspect ratio (H=r 0.20 at the planet locations) to account for the large pitch angles which implies a very warm disk. Conclusions. Our observations reveal the complex morphology of the disk MWC 758. To understand the origin of the detected features, the combination of high-resolution observations in the submillimeter with ALMA and detailed modeling is needed.

Fundamental Limitations of High Contrast Imaging Set by Small Sample Statistics

In this paper, we review the impact of small sample statistics on detection thresholds and corresponding confidence levels (CLs) in high-contrast imaging at small angles. When looking close to the star, the number of resolution elements decreases rapidly toward small angles. This reduction of the number of degrees of freedom dramatically affects CLs and false alarm probabilities. Naively using the same ideal hypothesis and methods as for larger separations, which are well understood and commonly assume Gaussian noise, can yield up to one order of magnitude error in contrast estimations at fixed CL. The statistical penalty exponentially increases toward very small inner working angles. Even at 5-10 resolution elements from the star, false alarm probabilities can be significantly higher than expected. Here we present a rigorous statistical analysis that ensures robustness of the CL, but also imposes a substantial limitation on corresponding achievable detection limits (thus contrast) at small angles. This unavoidable fundamental statistical effect has a significant impact on current coronagraphic and future high-contrast imagers. Finally, the paper concludes with practical recommendations to account for small number statistics when computing the sensitivity to companions at small angles and when exploiting the results of direct imaging planet surveys.

The infra-red dual imaging and spectrograph for SPHERE: design and performance

The SPHERE (Spectro-Polarimetric High-contrast Exoplanet Research) planet finder instrument for ESOs VLT telescope, scheduled for first light in 2011, aims to detect giant extra-solar planets in the vicinity of bright stars by the aid of an extreme-AO turbulence compensation system and to characterize the objects found through spectroscopic and polarimetric observations. Dual imaging observations within the Y, J, H and Ks atmospheric windows (~0.95 - 2.32μm) will be done by the aid of the IRDIS cryogenic camera. We describe briefly the science goals of IRDIS and present its system architecture. Current status of the instrument design is presented, and expected performance is described in terms of end-to-end simulations.

The position of β Pictoris b position relative to the debris disk

Context. We detected in 2009 a giant, close-by planet orbiting β Pic, a young star surrounded by a disk that has been extensively studied for more than 20 years. We showed that if the planet were located on an inclined orbit, this could account for several peculiarities of the β Pictoris system. However, the available data did not permit us to measure the inclination of β Pic b with respect to the disk, and in particular to establish in which component of the disk – either the main, extended disk or the inner inclined component/disk – the planet was located. Comparison between the observed planet position and the disk orientation measured using previous imaging data was not an option because of potential biases in the measurements. Aims. Our aim is to measure precisely the planet location with respect to the dust disk using a single high-resolution image, and correcting for systematics or errors that degrade the precision of the disk and planet relative-position measurements. Methods. We gathered new NaCo data in the Ks band, with a set-up optimized to derive simultaneously the orientation(s) of the disk(s) and the planet projected position. Results. We show that the projected position of β Pic b is above the midplane of the main disk. With the current data and knowledge of the system, this implies that β Pic b cannot be located in the main disk. The data instead suggest that the planet is located in the inclined component.

First light of the VLT planet finder SPHERE III. New spectrophotometry and astrometry of the HR 8799 exoplanetary system

Context. The planetary system discovered around the young A-type HR 8799 provides a unique laboratory to: a) test planet formation theories; b) probe the diversity of system architectures at these separations, and c) perform comparative (exo)planetology. Aims. We present and exploit new near-infrared images and integral-field spectra of the four gas giants surrounding HR 8799 obtained with SPHERE, the new planet finder instrument at the Very Large Telescope, during the commissioning and science verification phase of the instrument (July–December 2014). With these new data, we contribute to completing the spectral energy distribution (SED) of these bodies in the 1.0–2.5 μm range. We also provide new astrometric data, in particular for planet e, to further constrain the orbits. Methods. We used the infrared dual-band imager and spectrograph (IRDIS) subsystem to obtain pupil-stabilized, dual-band H2H3 (1.593 μm, 1.667 μm), K1K2 (2.110 μm, 2.251 μm), and broadband J (1.245 μm) images of the four planets. IRDIS was operated in parallel with the integral field spectrograph (IFS) of SPHERE to collect low-resolution (R ~ 30), near-infrared (0.94–1.64 μm) spectra of the two innermost planets HR 8799 d and e. The data were reduced with dedicated algorithms, such as the Karhunen-Loeve image projection (KLIP), to reveal the planets. We used the so-called negative planets injection technique to extract their photometry, spectra, and measure their positions. We illustrate the astrometric performance of SPHERE through sample orbital fits compatible with SPHERE and literature data. Results. We demonstrated the ability of SPHERE to detect and characterize planets in this kind of systems, providing spectra and photometry of its components. The spectra improve upon the signal-to-noise ratio of previously obtained data and increase the spectral coverage down to the Y band. In addition, we provide the first detection of planet e in the J band. Astrometric positions for planets HR 8799 bcde are reported for the epochs of July, August, and December 2014. We measured the photometric values in J, H2H3, K1K2 bands for the four planets with a mean accuracy of 0.13 mag. We found upper limit constraints on the mass of a possible planet f of 3–7 MJup . Our new measurements are more consistent with the two inner planets d and e being in a 2d:1e or 3d:2e resonance. The spectra of HR 8799 d and e are well matched by those of L6-8 field dwarfs. However, the SEDs of these objects are redder than field L dwarfs longward of 1.6 μm.

Impact of angular differential imaging on circumstellar disk images

Context. Direct imaging of circumstellar disks requires high-contrast and high-resolution techniques. The angular differential imaging (ADI) technique is one of them, initially developed for point-like sources but now increasingly applied to extended objects such as disks. This new field of application raises many questions because the disk images reduced with ADI depend strongly on the amplitude of field rotation and the ADI data reduction strategy. Both of them directly affect the disk observable properties. Aims. Our aim is to characterize the applicability and biases of some ADI data reduction strategies for different disk morphologies. A particular emphasis is placed on parameters mostly used for disks such as their surface brightness distribution, their width if the disk is a ring, and local features such as gaps or asymmetries. We first present a general method for predicting and quantifying those biases. In a second step we illustrate them for some widely used ADI algorithms applied to typical debris disk morphologies: inclined rings with various inner/outer slopes and width. Last, our aim is also to propose improvements of classical ADI to limit the biases on extended objects. Methods. Simulated fake disks seen under various observing conditions were used to reduce ADI data and quantify the resulting biases. These conclusions are complemented by previous results from NaCo L’ real-disk images of HR 4796A. Results. As expected, ADI induces flux losses on disks. This makes this technique appropriate only for low- to medium-inclination disks. A theoretical criterion is derived to predict the amount of flux loss for a given disk morphology, and quantitative estimates of the biases are given in some specific configurations. These biases alter the disk observable properties, such as the slopes of the disk surface brightness or the radial/azimuthal extent of the disk. Additionally, this work demonstrates that ADI can very easily create artificial features without involving astrophysical processes. For example, a particularly striking feature appears for a ring when the amplitude of field rotation is too small. The two ring ansae are surrounded by two flux-depleted regions, which makes them appear as bright blobs. This observation does not require any astrophysical process such as dust blown by radiation pressure, as previously proposed in H-band images of HR 4796A. Conclusions. The ADI techniques behave as spatial filtering algorithms and can bias disk observables. Therefore, the filtering process needs to be properly calibrated when deriving disk parameters from processed images.

Dissecting the Moth: discovery of an off-centered ring in the HD 61005 debris disk with high-resolution imaging

The debris disk known as “The Moth” is named after its unusually asymmetric surface brightness distribution. It is locat ed around the � 90 Myr old G8V star HD 61005 at 34.5 pc and has previously been imaged by the HST at 1.1 and 0.6� m. Polarimetric observations suggested that the circumstellar material consists of two d istinct components, a nearly edge-on disk or ring, and a swept-back feature, the result of interaction with the interstellar medium. We r esolve both components at unprecedented resolution with VLT/NACO Hband imaging. Using optimized angular differential imaging techniques to remove the light of the star, we reveal the disk component as a distinct narrow ring at inclination i = 84: 3�1: 0 � . We determine a semi-major axis of a = 61: 25�0: 85 AU and an eccentricity of e = 0: 045�0: 015, assuming that periastron is located along the apparent disk major axis. Therefore, the ring center is offset from the star by at least 2: 75�0: 85 AU. The offset, together with a relatively steep inner rim, could indic ate a planetary companion that perturbs the remnant planetesimal belt. From our imaging data we set upper mass limits for companions that exclude any object above the deuterium-burning limit for separations down to 0: 00 3. The ring shows a strong brightness asymmetry along both the major and minor axis. A brighter front side could indicate forward-scattering grains, while the brightness difference between the NE and SW components can be only partly explained by the ring center offset, suggesting additional density enhancements on one side of the ring. The swept-back component appears as two streamers originating near the NE and SW edges of the debris ring.