The path towards high-contrast imaging with the VLTI: the Hi-5 project

Abstract

The development of high-contrast capabilities has long been recognized as one of the top priorities for the VLTI. As of today, the VLTI routinely achieves contrasts of a few 10 −3 in the near-infrared with PIONIER (H band) and GRAVITY (K band). Nulling interferometers in the northern hemisphere and non-redundant aperture masking experiments have, however, demonstrated that contrasts of at least a few 10 −4 are within reach using specific beam combination and data acquisition techniques. In this paper, we explore the possibility to reach similar or higher contrasts on the VLTI. After reviewing the state-of-the-art in high-contrast infrared interferometry, we discuss key features that made the success of other high-contrast interferometric instruments (e.g., integrated optics, nulling, closure phase, and statistical data reduction) and address possible avenues to improve the contrast of the VLTI by at least one order of magnitude. In particular, we discuss the possibility to use integrated optics, proven in the near-infrared, in the thermal near-infrared (L and M bands, 3-5 µm), a sweet spot to image and characterize young extra-solar planetary systems. Finally, we address the science cases of a high-contrast VLTI imaging instrument and focus particularly on exoplanet science (young exoplanets, planet formation, and exozodiacal disks), stellar physics (fundamental parameters and multiplicity), and extragalactic astrophysics (active galactic nuclei and fundamental constants). Synergies and scientific preparation for other potential future instruments such as the Planet Formation Imager are also briefly discussed.

The HOSTS Survey for Exozodiacal Dust: Observational Results from the Complete Survey

“…It may also allow for the detection of such systems around Sun-like stars. The detailed study of the detected systems with the LBTI (Ertel et al 2018c) and current and future instruments on the Very Large Telescope Interferometer (Ertel et al 2018a;Kirchschlager et al 2018;Defrère et al 2018) may also allow us to determine the origin of the dust in these individual cases and the connection between the HZ dust and hotter dust even closer to the stars, thus helping to understand the origins and dynamics of the various dust species in the HZs of the stars and closer in.…”

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)…”

“…Reducing fringe tracking residuals below a radian of fringe phase is often seen as a way to achieve long integrations in the presence of detector noise, 1 but measuring fringe phase with a sub-radian accuracy has a much more fundamental application in enabling coherent integration 2 of differential quantities such as closure-phase, wavelength-differential phase 3 and dual fringe packet astrometry. 4,5 Several future interferometer concepts, such as high-contrast imaging in the mid-infrared with VLTI, 6 or the PFI 7 require high accuracy fringe tracking in order to operate. In fact, recent work on the concept of kernel nulling 8 has shown that nulling interferometry contrasts are often proportional to the cube of the fringe tracking residual, and require residuals smaller than 50 nm in order to achieve key high-contrast exoplanet detection goals.…”

Future of optical-infrared interferometry in Europe

“…While the E-ELT and its first suite of scientific instruments are being constructed, a 3rd generation of instruments for the VLTI could emphasize particular aspects (angular and/or spectral resolution, extended wavelength coverage, multi-object), and benefit of a mature and improved telescope infrastructure, including adaptive optics 3 and pistonstabilized beam trains. 4 In Defrère et al [7] on Hi-5, such a focused instrument concept is presented, which could be realized as a visitor-instrument, similar to the successful PIONIER. 5 This high-dynamic range imager for the thermal infrared relies on a combination of new integrated optics technology which is currently developed for thermal infrared wavelengths [8], and statistically robust data processing techniques, emphasizing the importance of technological progress.…”