High-contrast detection of exoplanets with a kernel-nuller at the VLTI

Abstract

Context. The conventional approach to direct imaging is to use a single aperture coronagraph with wavefront correction via extreme adaptive optics (AO). Such systems are limited to observing beyond an inner working angle (IWA) of a few λ/D. Nulling interferometry with two or more apertures will enable detections of companions at separations at and beyond the formal diffraction limit. Aims. In this paper, we evaluate the astrophysical potential of a kernel-nuller as the prime high-contrast imaging mode of the Very Large Telescope Interferometer (VLTI). Methods. By taking into account baseline projection effects induced by Earth rotation, we introduce some diversity in the response of the nuller as a function of time. This response is depicted by transmission maps. We also determine whether we can extract the astrometric parameters of a companion from the kernel outputs, which are the primary intended observable quantities of the kernel-nuller. This then leads us to comment on the characteristics of a possible observing program for the discovery of exoplanets. Results. We present transmission maps for both the raw nuller outputs and their subsequent kernel outputs. To further examine the properties of the kernel-nuller, we introduce maps of the absolute value of the kernel output. We also identify 38 targets for the direct detection of exoplanets with a kernel-nuller at the focus of the VLTI. Conclusions. With continued upgrades of the VLTI infrastructure, which will reduce fringe tracking residuals, a kernel-nuller would enable the detection of young giant exoplanets at separations <10 AU, where radial velocity and transit methods are more sensitive.