Warm or massive gas giant planets, brown dwarfs, and debris disks around nearby stars are now routinely observed by dedicated high-contrast imaging instruments that are mounted 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 their spectral characterization. However, low spatial frequency differential aberrations between the ExAO sensing path and the science path represent critical limitations for the detection of giant planets with a contrast lower than a few 10 −6 at very small separations (<0.3 ) from their host star. In our previous work, we proposed a wavefront sensor based on Zernike phase-contrast methods to circumvent this problem 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 first performed measurements in the presence of Zernike or Fourier modes introduced with the deformable mirror. Our experimental results are consistent with the results in simulations, confirming the ability of our sensor to measure small aberrations (<50 nm rms) with nanometric accuracy. Following these results, we corrected the long-lived non-common path aberrations in SPHERE based on ZELDA measurements and estimated a contrast gain of 10 in the coronagraphic image at 0.2 , reaching the raw contrast limit set by the coronagraph in the instrument. In addition to this encouraging result, the simplicity of the design and its phase reconstruction algorithm makes ZELDA an excellent candidate for the online 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 facilitate the observation of cold gaseous or massive rocky planets around nearby stars.

Context. Several exoplanet direct-imaging instruments (VLT-SPHERE, Gemini Planet Imager, etc.) will soon be in operation, providing original data for comparative exoplanetary science to the community. To this end, exoplanet imagers use an extreme adaptive optics (XAO) system to correct the atmospheric turbulence and provide a highly corrected beam to a near-infrared (NIR) coronagraph for suppressing diffracted stellar light. The performance of the coronagraph is, however, limited by the non-common path aberrations (NCPA) due to the differential wavefront errors existing between the visible XAO sensing path and the NIR science path and leading to residual speckles that hide the faintest exoplanets in the coronagraphic image. Aims. Accurate calibration of the NCPA in exoplanet imagers is mandatory to correct the residual, quasi-static speckles remaining in the coronagraphic images after XAO correction in order to allow the observation of exoplanets that are at least 10 6 fainter than their host star. Several approaches have been developed during these past few years to reach this goal. We propose an approach based on the Zernike phase-contrast method operating in the same wavelength as the coronagraph for the measurements of the NCPA between the optical path seen by the visible XAO wavefront sensor and that seen by the NIR coronagraph. Methods. This approach uses a focal plane phase mask of size ∼λ/D, where λ and D denote the wavelength and the telescope aperture diameter, respectively, to measure the quasi-static aberrations in the upstream pupil plane by encoding them into intensity variations in the downstream pupil image. The principle of this approach as described in several classical optical textbooks is simplified by the omission of the spatial variability of the amplitude diffracted by the phase mask. We develop a more rigorous formalism, leading to highly accurate measurement of the NCPA, in a quasi-linear way during the observation. Results. With prospects of achieving subnanometric measurement accuracy with this approach for a static phase map of standard deviation 44 nm rms at λ = 1.625 μm (0.026 λ), we estimate a possible reduction of the NCPA due to chromatic differential optics by a factor ranging from 3 to 10 in the presence of adaptive optics (AO) residuals compared with the expected performance of a typical current-generation system. This would allow a reduction of the level of quasi-static speckles in the detected images by a factor 10 to 100, thus correspondingly improving the capacity to observe exoplanets.

We present the first images of four debris disks observed in scattered light around the young (4-250 Myr old) M dwarfs TWA 7 and TWA 25, the K6 star HD 35650, and the G2 star HD 377. We obtained these images by reprocessing archival Hubble Space Telescope NICMOS coronagraph data with modern post-processing techniques as part of the Archival Legacy Investigation of Circumstellar Environments (ALICE) program. All four disks appear faint and compact compared with other debris disks resolved in scattered light. The disks around TWA 25, HD 35650, and HD 377 appear very inclined, while TWA 7's disk is viewed nearly face-on. The surface brightness of HD 35650's disk is strongly asymmetric. These new detections raise the number of disks resolved in scattered light around M and late-K stars from one (the AU Mic system) to four. This new sample of resolved disks enables comparative studies of heretofore scarce debris disks around low-mass stars relative to solar-type stars.

The Apodized Pupil Lyot Coronagraph (APLC) is a diffraction suppression system installed in the recently deployed instruments Palomar/P1640, Gemini/GPI, and VLT/SPHERE to allow direct imaging and spectroscopy of circumstellar environments. Using a prolate apodization, the current implementations offer raw contrasts down to 10 −7 at 0.2 arcsec from a star over a wide bandpass (20%), in the presence of central obstruction and struts, enabling the study of young or massive gaseous planets. Observations of older or lighter companions at smaller separations would require improvements in terms of the inner working angle (IWA) and contrast, but the methods originally used for these designs were not able to fully explore the parameter space. We propose a novel approach to improve the APLC performance. Our method relies on the linear properties of the coronagraphic electric field with the apodization at any wavelength to develop numerical solutions producing coronagraphic star images with high-contrast region in broadband light. We explore the parameter space by considering different aperture geometries, contrast levels, dark-zone sizes, bandpasses, and focal plane mask sizes. We present an application of these solutions to the case of Gemini/GPI with a design delivering a 10 −8 raw contrast at 0.19 arcsec and offering a significantly reduced sensitivity to low-order aberrations compared to the current implementation. Optimal solutions have also been found to reach 10 −10 contrast in broadband light regardless of the aperture shape, with effective IWA in the 2-3.5 λ/D range, therefore making the APLC a suitable option for the future exoplanet direct imagers on the ground or in space.

We have spatially resolved five debris disks (HD 30447, HD 35841, HD 141943, HD 191089, and HD 202917) for the first time in near-infrared scattered light by reanalyzing archival Hubble Space Telescope (HST)/NICMOS coronagraphic images obtained between 1999 and 2006. One of these disks (HD 202917) was previously resolved at visible wavelengths using HST/Advanced Camera for Surveys. To obtain these new disk images, we performed advanced point-spread function subtraction based on the Karhunen-Loève Image Projection (KLIP) algorithm on recently reprocessed NICMOS data with improved detector artifact removal (Legacy Archive PSF Library And Circumstellar Environments Legacy program). Three of the disks (HD 30447, HD 35841, and HD 141943) appear edge-on, while the other two (HD 191089 and HD 202917) appear inclined. The inclined disks have been sculpted into rings; in particular, the disk around HD 202917 exhibits strong asymmetries. All five host stars are young (8-40 Myr), nearby (40-100 pc) F and G stars, and one (HD 141943) is a close analog to the young sun during the epoch of terrestrial planet formation. Our discoveries increase the number of debris disks resolved in scattered light from 19 to 23 (a 21% increase). Given their youth, proximity, and brightness (V = 7.2 to 8.5), these targets are excellent candidates for follow-up investigations of planet formation at visible wavelengths using the HST /STIS coronagraph, at near-infrared wavelengths with the Gemini Planet Imager (GPI) and Very Large Telescope (VLT)/SPHERE, and at thermal infrared wavelengths with the James Webb Space Telescope NIRCam and MIRI coronagraphs.

We present the first scattered-light images of the debris disk around 49 Ceti, a ∼40 Myr A1 main-sequence star at 59 pc, famous for hosting two massive dust belts as well as large quantities of atomic and molecular gas. The outer disk is revealed in reprocessed archival Hubble Space Telescope NICMOS-F110W images, as well as new coronagraphic H-band images from the Very Large Telescope SPHERE instrument. The disk extends from 1 1 (65 au) to 4 6 (250 au) and is seen at an inclination of 73°, which refines previous measurements at lower angular resolution. We also report no companion detection larger than 3M Jup at projected separations beyond 20 au from the star (0 34). Comparison between the F110W and H-band images is consistent with a gray color of 49 Ceti's dust, indicating grains larger than 2 μm. Our photometric measurements indicate a scattering efficiency/infrared excess ratio of 0.2-0.4, relatively low compared to other characterized debris disks. We find that 49 Ceti presents morphological and scattering properties very similar to the gas-rich HD 131835 system. From our constraint on the disk inclination we find that the atomic gas previously detected in absorption must extend to the inner disk, and that the latter must be depleted of CO gas. Building on previous studies, we propose a schematic view of the system describing the dust and gas structure around 49 Ceti and hypothetical scenarios for the gas nature and origin.

We introduce a new class of solutions for Apodized Pupil Lyot Coronagraphs (APLC) with segmented aperture telescopes to remove broadband diffracted light from a star with a contrast level of 10 10 . These new coronagraphs provide a key advance to enabling direct imaging and spectroscopy of Earth twins with future large space missions. Building on shaped pupil (SP) apodization optimizations, our approach enables twodimensional optimizations of the system to address any aperture features such as central obstruction, support structures or segment gaps. We illustrate the technique with a design that could reach 10 10 contrast level at 34 mas for a 12 m segmented telescope over a 10% bandpass centered at a wavelength λ 0 =500 nm. These designs can be optimized specifically for the presence of a resolved star, and in our example, for stellar angular size up to 1.1 mas. This would allow probing the vicinity of Sun-like stars located beyond 4.4 pc, therefore fully retiring this concern. If the fraction of stars with Earth-like planets is η ⊕ = 0.1, with 18% throughput, assuming a perfect, stable wavefront and considering photon noise only, 12.5 exo-Earth candidates could be detected around nearby stars with this design and a 12 m space telescope during a five-year mission with two years dedicated to exo-Earth detection (one total year of exposure time and another year of overheads). Our new hybrid APLC/SP solutions represent the first numerical solution of a coronagraph based on existing mask technologies and compatible with segmented apertures, and that can provide contrast compatible with detecting and studying Earth-like planets around nearby stars. They represent an important step forward towards enabling these science goals with future large space missions.

Giant exoplanets on wide orbits have been directly imaged around young stars. If the thermal background in the mid-infrared can be mitigated, then exoplanets with lower masses can also be imaged. Here we present a ground-based mid-infrared observing approach that enables imaging low-mass temperate exoplanets around nearby stars, and in particular within the closest stellar system, α Centauri. Based on 75–80% of the best quality images from 100 h of cumulative observations, we demonstrate sensitivity to warm sub-Neptune-sized planets throughout much of the habitable zone of α Centauri A. This is an order of magnitude more sensitive than state-of-the-art exoplanet imaging mass detection limits. We also discuss a possible exoplanet or exozodiacal disk detection around α Centauri A. However, an instrumental artifact of unknown origin cannot be ruled out. These results demonstrate the feasibility of imaging rocky habitable-zone exoplanets with current and upcoming telescopes.