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 towards 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 towards 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 which 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.

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 VLT/SPHERE at 1.04 µm to resolve scattered light at a smaller inner working angle (0.093 ) and a higher angular resolution (0.027 ) than previously achieved. Results. We observe polarized scattered light within 0.53 (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.

Context. Young circumstellar disks are the birthplaces of planets. Their study is of prime interest to understand the physical and chemical conditions under which planet formation takes place. Only very few detections of planet candidates within these disks exist, and most of them are currently suspected to be disk features. Aims. In this context, the transition disk around the young star PDS 70 is of particular interest, due to its large gap identified in previous observations, indicative of ongoing planet formation. We aim to search for the presence of an embedded young planet and search for disk structures that may be the result of disk-planet interactions and other evolutionary processes. Methods. We analyse new and archival near-infrared (NIR) images of the transition disk PDS 70 obtained with the VLT/SPHERE, VLT/NaCo and Gemini/NICI instruments in polarimetric differential imaging (PDI) and angular differential imaging (ADI) modes. Results. We detect a point source within the gap of the disk at about 195 mas (∼22 au) projected separation. The detection is confirmed at five different epochs, in three filter bands and using different instruments. The astrometry results in an object of bound nature, with high significance.The comparison of the measured magnitudes and colours to evolutionary tracks suggests that the detection is a companion of planetary mass. The luminosity of the detected object is consistent with that of an L-type dwarf, but its IR colours are redder, possibly indicating the presence of warm surrounding material. Further, we confirm the detection of a large gap of ∼54 au in size within the disk in our scattered light images, and detect a signal from an inner disk component. We find that its spatial extent is very likely smaller than ∼17 au in radius, and its position angle is consistent with that of the outer disk. The images of the outer disk show evidence of a complex azimuthal brightness distribution which is different at different wavelengths and may in part be explained by Rayleigh scattering from very small grains. Conclusions. The detection of a young protoplanet within the gap of the transition disk around PDS 70 opens the door to a so far observationally unexplored parameter space of planetary formation and evolution. Future observations of this system at different wavelengths and continuing astrometry will allow us to test theoretical predictions regarding planet-disk interactions, planetary atmospheres and evolutionary models.Based on observations performed with ESO Telescopes at the Paranal Observatory under programmes 095.C-0298, 095.C-0404, 096.C-0333, 097.C-0206, 097.C-1001, 099.C-0891. are imprinted by the initial conditions of the disks and which develop through a variety of dynamical interactions is crucial for understanding the planet population. It is therefore of high importance to study planets and their environments at the stage during which these objects are formed. Transition disks (TDs) are of key interest in this context, as many of them are believed to bear direct ...

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.