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 µm) data set from the ESO archive. We detect eight low-mass companions, five of which were previously unknown, and two have angular separations of ∼ 0.8-1.2 λ/D (i.e. ∼ 80-110 mas), demonstrating that kernel phase achieves a resolution below the classical diffraction limit of a telescope. While we reach a 5σ contrast limit of ∼ 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.

Kernel-phase detection limits

“…3.2 make it possible to estimate the magnitude of this bias a priori. Proper treatment of the calibration would require the subtraction of an estimate of the calibration term, using either the POISE algorithm of Ireland (2013) or the KL decomposition approach described by Kammerer et al (2019) that relies on the observation of multiple calibration sources. Here we estimate the impact of an unaccounted-for calibration error on the contrast detection limits by adding the residual determined after analysis of the simulation that included the OPD maps to the diagonal of the covariance.…”

Mid-infrared photometry of the T Tauri triple system with kernel phase interferometry

“…Later, Pope et al (2013) used the technique to detect brown dwarf companions, also with HST/NICMOS, and Pope et al (2016) and Sallum & Skemer (2019) showed that kernel phase should outperform sparse aperture masking under appropriate seeing conditions longward of ∼ 3 µm from the ground given the reduced sensitivity to sky background noise. Recently, Kammerer et al (2019) used the kernel phase technique to detect eight stellar companions in an archival VLT/Nasmyth Adaptive Optics System (NAOS) nearinfrared imager and spectrograph (NACO) high-contrast imaging survey, two of which are below the classical diffraction limit at angular separations of ∼ 0.8 and ∼ 1.2 λ/D.…”

“…and Σ d is the variance frame (see Kammerer et al 2019). Here, we obtain the variance frame directly from the frame itself, namely,…”

“…If we are in the high-contrast regime (ξ n>1 0.5) and only fit for two components, the model can be simplified by linearization (Kammerer et al 2019) and by putting the brighter component into the center of the frame (∆ RA,1 = ∆ DEC,1 = 0). Then, we can solve analytically for the relative companion flux, Rejected frame threshold Fig.…”

“…on an RA-DEC grid, where θ ref is the binary model (Equation 8for N = 2) evaluated on an RA-DEC grid and for a small relative reference flux ξ 2,ref 0.5 (see Kammerer et al 2019). We note that with the T Tau system, we are not in the high-contrast regime where this simplification holds.…”