Kernel-nulling for a robust direct interferometric detection of extrasolar planets

Abstract

Context. Combining the resolving power of long-baseline interferometry with the high-dynamic range capability of nulling still remains the only technique that can directly sense the presence of structures in the innermost regions of extrasolar planetary systems. Aims. Ultimately, the performance of any nuller architecture is constrained by the partial resolution of the on-axis star whose light it attempts to cancel out. However from the ground, the effective performance of nulling is dominated by residual time-varying instrumental phase and background errors that keep the instrument off the null. Our work investigates robustness against instrumental phase. Methods. We introduce a modified nuller architecture that enables the extraction of information that is robust against piston excursions. Our method generalizes the concept of kernel, now applied to the outputs of the modified nuller so as to make them robust to second order pupil phase error. We present the general method to determine these kernel-outputs and highlight the benefits of this novel approach. Results. We present the properties of VIKiNG: the VLTI Infrared Kernel NullinG, an instrument concept within the Hi-5 framework for the 4-UT VLTI infrastructure that takes advantage of the proposed architecture, to produce three self-calibrating nulled outputs. Conclusions. Stabilized by a fringe-tracker that would bring piston-excursions down to 50 nm, this instrument would be able to directly detect more than a dozen extrasolar planets so-far detected by radial velocity only, as well as many hot transiting planets and a significant number of very young exoplanets.

Image-plane fringe tracker for adaptive-optics assisted long baseline interferometry

“…The total intensity centroids I x and I y are also found. The telescope tilts are then found by a maximum likelihood combination of these centroids, for example for telescope 1 with uniform visibility on all baselines, we arrive at: [8,6,6,6, −5, −5, −5] (10)…”

Hi-5: a potential high-contrast thermal near-infrared imager for the VLTI

“…42 More recently, the idea of "kernel nulling" has been proposed as a more generalized approach. 43 The idea behind these concepts is to combine the outputs of a first nulling stage in a second mixing stage, with the goal of creating an output signal more robust against imperfect cophasing of the incoming stellar light. With such an architecture, contrasts of at least 10 −5 are in theory possible for reasonable fringe tracking performance of 150 nm RMS as shown in Figure 3.…”

“…45 Applied to the VLTI with the NAOMI adaptive optics system functioning on the ATs, this approach could in theory enable L-band contrasts of a few 10 −6 for bright stars (m L ∼3 and fringe tracking sensitivity down to H 12 at the level of ∼300 nm RMS in most seeing conditions. 43 Currently, the GRAVITY fringe tracker achieves a limiting magnitude of K 7.0 with the ATs and K 10.0 with the UTs in single-field mode, with performance levels of 200-300 nm RMS, depending on conditions. Under very good conditions, 1 to 2 magnitudes deeper can be achieved but generally at the expense of stability.…”

Planet formation imager: project update

“…• Nuller design for exoplanet detections at K, L bands. This will impact arrangement of telescopes within 1.2 km baselines to optimize clever nulling schemes (e.g., Hsiao et al, 33 Lacour et al, 34 Martinache & Ireland 35 ). Table 3 contains information on two reference facility architectures for PFI, one is a 12×3 m array and the other a 12 × 8 m array.…”

“…New promising beam combination strategies are also under investigations, such as "kernel nulling, which combines the advantages of nulling and closure phase and provides a metric robust against imperfect cophasing of the incoming stellar light. 35 Heimdallr is a proposed fringe tracker (PI: Michael Ireland) to enable high precision and high sensitivity fringe tracking at VLTI. Heimdallr could be used with Hi-5, using the kernel-nulling (VIKiNG) architecture 35 to obtain very well-calibrated nulling interferometry.…”