| dc.description.abstract | High contrast imaging of nearby circumstellar environments is the only technique that provides sensitivity to planetary mass companions at wide orbital separations and hence will extensively map out the outer architectures of planetary systems through the coming years with the advent of next generation telescopes. Since this technique preferentially observes companions around young stars, it is also exceptionally well-positioned to place valuable constraints on competing models of planet formation and migration that describe the early dynamical and thermal evolution of planets. Previous efforts have led to successful detection of wide-separation companions and numerous scattered light images of disks. However these detections have been quite uncommon and surveys in the last $10{-}15$ years have had a low rate of detection, {returning a number of companions that is insufficient to statistically place constraints on planetary formation models as well as models of the early entropy of planets}. This poor detection rate of directly imaged planets is probably because of the fact that the recent studies indicate that the peak of the extrasolar giant planet distribution lies at ${\sim}2{-}3\,\rm{au}$, near the water frost line, which is largely inaccessible with $8{-}10\,\rm{m}$ ground based telescopes due to their fundamental limiting resolution, at near-infrared wavelengths.
JWST promises to be the most versatile infrared observatory for the next two decades. The \textit{Near Infrared and Slitless Spectrograph (NIRISS)} instrument, when used in the Aperture Masking Interferometry (AMI) mode, provides an unparalleled combination of {angular resolution and} sensitivity compared to any existing observatory at mid-infrared wavelengths. This thesis extensively investigates the capability of this mode using simulated observations of the members of young, kinematic moving groups of $\beta$ Pictoris, TW Hydrae, and the Taurus-Auriga association, in conjunction with evolutionary models. In addition to this, it also explores the results of the AMI dataset of the Direct Imaging Early Release (ERS), which observed the star HIP\,65426, a member of the Scorpius–Centaurus association, at $3.8\,\rm{\mu m}$.
It was found that the on-sky performance of this mode is under-performing by ${\sim}1{-}2$ mag, in terms of detecting the faintest possible companion, relative to the host star. Although no additional planets were discovered around HIP\,65426, the observation ruled out any new companions ${\sim}7.5$ magnitudes fainter than the host star (or equivalently $10{-}12\,\rm{M\textsubscript{Jup}}$) beyond separations of ${\sim}0.07$". With this result, this mode is now confirmed to be capable of imaging ${\gtrsim}1\,\rm{M\textsubscript{Jup}}$ companions near the water frost line around the stars of young moving groups (like $\beta$ Pictoris and TW Hydrae) and more massive companions (${\gtrsim}10\,\rm{M\textsubscript{Jup}}$) around stars in the relatively further star-forming regions (like the Scorpius–Centaurus association). This will allow the planning and successful execution of future observations to probe inner regions of nearby stellar systems, which is an essentially unexplored parameter space. And, when
combined with dynamical masses from Gaia, such measurements will provide a much more robust characterization of the initial
entropies of these young planets, thereby placing powerful constraints on their early thermal histories. | en_GB |
