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

## TL;DR

This paper demonstrates that kernel phase imaging can detect low-mass stellar companions at the diffraction limit, achieving resolutions below classical limits, and identifies five new companions using VLT/NACO data.

## Contribution

The study applies kernel phase combined with principal component calibration to high-cadence L' band data, revealing new companions and showcasing super-resolution capabilities.

## Key findings

- Detected eight low-mass companions, five previously unknown.
- Achieved resolution below the classical diffraction limit (~80-110 mas).
- Reached a 5σ contrast limit of approximately 1/100 at small angular separations.

## 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 $\sim0.8$-$1.2~\lambda/D$ (i.e. $\sim80$-$110~\text{mas}$), demonstrating that kernel phase achieves a resolution below the classical diffraction limit of a telescope. While we reach a $5\sigma$ contrast limit of $\sim1/100$ 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.