From HR 8799 to Y-dwarf binaries: JWST interferometry across the stellar IMF

Thomas Vandal ( Université de Montréal )

High-contrast imaging is crucial for studying giant exoplanets, enabling astrometric follow-ups, luminosity measurements, and atmospheric characterization. However, imaging has so far been limited to planets more massive than Jupiter at large separations (3 AU and above). The James Webb Space Telescope (JWST) marks the beginning of a new era for direct imaging, opening the parameter space to Saturn-mass planets, but coronagraphs on board JWST are restricted to separations above 400 mas. To overcome this, the Aperture Masking Interferometry (AMI) mode of the Near Infrared Imager and Slitless Spectrograph (NIRISS) transforms JWST into an interferometer. This is ideal to search for Jupiter-mass planets as close as 100 mas from a bright star. We present the first results from a NIRISS AMI program probing the inner regions around HR 8799 and HD 95086. The structure of these two emblematic systems hints at the presence of close-in planets that have so far eluded direct confirmation. Our AMI observations mark a step further towards a complete view of these systems, which is crucial to derive mass estimates from dynamical simulations and to enable follow-up observations, both of which are required to calibrate planet formation models. For fainter hosts, clear-pupil imaging can attain similar performance to AMI thanks to Kernel Phase Imaging (KPI), a generalization of interferometric techniques to regular images. KPI, combined with the exquisite stability of JWST at 4.8 um, presents a unique opportunity to search for planetary-mass companions around the faintest and coolest brown dwarfs, with spectral type Y. We conducted a survey of 20 Y dwarfs with JWST’s Near Infrared Camera (NIRCam) to probe mass ratios down to 0.1 and separations as low as 0.5 AU. This program recently unveiled the first Y+Y binary, WISE-0336, whose two components, separated by 1 AU, could both have planetary masses, depending on their age. We present a first complete analysis of this survey, including an in-depth companion search. These new constraints on Y dwarf multiplicity provide a unique insight on planet formation, extending previous studies around M dwarfs and brown dwarfs to the lowest substellar primaries. Altogether, our AMI and KPI programs outline how JWST interferometry’s high-contrast capabilities at short separations can extend our knowledge of planet formation across the entire stellar initial mass function.