Combining Fourier plane observables for high-contrast imaging of young giant exoplanets
Creators: name: Kammerer, Jens
Publisher: name: The Australian National University
Year: 2021

description: The vast majority of the ~4000 exoplanets known of today have only been detected through indirect techniques, providing a limited amount of information on their physical properties and dynamical environment. On the other hand, direct techniques can provide astrometric, photometric, and spectroscopic information required to study the formation and evolution of exoplanets. However, such direct observations are challenging due to the high contrast between an exoplanet and its parent star, as well as their small apparent separation. Interferometric techniques at infrared wavelengths are able to overcome the limitation in terms of angular resolution, but are still limited in contrast at small angular separations. A further increase in contrast is necessary to make the bulk of young giant exoplanets, orbiting their parent star at Solar System scales, accessible to interferometry in general. Here, we focus on improving our understanding of and mitigating the systematic errors which limit the sensitivity of interferometric observations. With the kernel phase technique, we survey nearby and young stars for sub-stellar companions. We develop a data reduction pipeline capable of reconstructing saturated PSFs, centering them with sub-pixel accuracy and extracting their Fourier plane observables including correlations. These correlations are then used, together with a calibration strategy based on principal component analysis, to improve the sensitivity to faint companions. In archival VLT/NACO data, we detect eight low-mass stellar companions, five of which were previously unknown, and two have angular separations of ~0.8-1.2 lambda/D (i.e., ~80-110 mas). Furthermore, we achieve typical 5-sigma contrast limits of ~6 mag at separations of 0.2 arcsec and ~8 mag at separations of 0.5 arcsec for a Keck/NIRC2 survey of 55 single class I and class II stars in Taurus. These results clearly demonstrate that the kernel phase technique is now capable of detecting young giant exoplanets in the nearest star-forming regions. We further utilize this technique to obtain mid-infrared photometry of the famous T Tauri triple system, including its southern binary T Tau Sa/Sb at an apparent separation of only ~0.2 lambda/D. Our observations reveal a recent decrease in the mid-infrared brightness of T Tau Sb of ~2 mag. We suspect that it has moved along its orbit behind the southern circumbinary disk and now suffers from increased dust extinction. With the demonstration of the improved contrast and the unprecedented angular resolution in the mid-infrared, the kernel phase technique is a promising method for exoplanet imaging with the James Webb Space Telescope and the Extremely Large Telescopes. We finally extend our study to long-baseline interferometry by extracting the correlations present in VLTI/GRAVITY data. The GRAVITY instrument has recently been used to spectroscopically characterize exoplanets in the near-infrared. We develop an analytical model to describe the correlations and show that the faint source detection limits of GRAVITY improve by a factor of ~2 when accounting for them in the model fitting process. Exoplanet science with GRAVITY is still in its infancy and our technical improvements will help to increase its scientific return. Moreover, future instruments such as GRAVITY+, SCIFY, or LIFE will greatly benefit from a complete treatment of the systematic errors.