Kernel phase imaging with VLT/NACO: high-contrast detection of new candidate low-mass stellar companions at the diffraction limit

Abstract

Directly imaging exoplanets is challenging because quasi-static phase aberrations in the pupil plane (speckles) can mimic the signal of a companion at small angular separations. Kernel phase, which is a generalization of closure phase (known from sparse aperture masking), is independent of pupil plane phase noise to second order and allows for a robust calibration of full pupil, extreme adaptive optics observations. We applied kernel phase combined with a principal component based calibration process to a suitable but not optimal, high cadence, pupil stabilized L' band ($3.8\mu \text{m}$) data set from the ESO archive. We detect eight low-mass companions, five of which were previously unknown, and two have angular separations of $\sim 0.8$-$1.2\lambda /D$ (i.e. $\sim 80$-$110\text{mas}$), demonstrating that kernel phase achieves a resolution below the classical diffraction limit of a telescope. While we reach a $5\sigma$ contrast limit of $\sim 1/100$ at such angular separations, we demonstrate that an optimized observing strategy with more diversity of PSF references (e.g. star-hopping sequences) would have led to a better calibration and even better performance. As such, kernel phase is a promising technique for achieving the best possible resolution with future space-based telescopes (e.g. JWST), which are limited by the mirror size rather than atmospheric turbulence, and with a dedicated calibration process also for extreme adaptive optics facilities from the ground.

17 pages, 13 figures, accepted by MNRAS

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• The Near Infrared Imager and Slitless Spectrograph for the James Webb Space Telescope -- IV. Aperture Masking Interferometry
• New binaries from the SHINE survey
• Large Adaptive Optics Survey for Substellar Objects (LASSO) Around Young, Nearby, Low-mass Stars with Robo-AO
• The Near Infrared Imager and Slitless Spectrograph for JWST -- V. Kernel Phase Imaging and Data Analysis
• Kernel Phase and Coronagraphy with Automatic Differentiation
• Dynamical masses for two M1 + mid-M dwarf binaries monitored during the SPHERE-SHINE survey
• Non-thermal radio continuum emission from young nearby stars
• Increasing the achievable contrast of infrared interferometry with an error correlation model
• High-resolution survey for planetary companions to young stars in the Taurus Molecular Cloud
• Differentiable Optics with dLux I: Deep calibration of Flat Field and Phase Retrieval with Automatic Differentiation
• NICMOS Kernel-Phase Interferometry I: Catalogue of Brown Dwarfs Observed in F110W and F170M
• The ExoGRAVITY survey: A K-band spectral library of giant exoplanet and brown dwarf companions
• Mid-infrared photometry of the T Tauri triple system with kernel phase interferometry
• Searching for Protoplanets around MWC 758 and MWC 480 in Br-$\gamma$ using Kernel Phase and SCExAO/CHARIS