The Possible Astrometric Signature of a Planetary-mass Companion to the Nearby Young Star TW Piscis Austrini (Fomalhaut B): Constraints from Astrometry, Radial Velocities, and Direct Imaging

Abstract

Abstract We present constraints on the presence of substellar companions to the nearby (d ∼ 7.6 pc) young (440 ± 40 Myr) K4Ve star TW Piscis Austrini, the wide (∼0.3 pc) companion to the A4V star Fomalhaut. We combined absolute astrometry from Hipparcos and Gaia with literature radial velocity measurements and dedicated high-contrast imaging observations obtained with Keck/NIRC2 to achieve sensitivity to brown dwarf and planetary-mass companions (≳2 M Jup) over many decades of orbital period (≲103 yr). The significant astrometric acceleration measured between the Hipparcos and Gaia catalogs, reported previously in the literature, cannot be explained by the orbital motion of TW PsA around the barycenter of the Fomalhaut triple system. Instead, we find that it is consistent with the reflex motion induced by an orbiting substellar companion. The combination of astrometry, radial velocities, and a deep L′ imaging data set leads to a constraint on the companion mass of M Jup. However, the period of the companion is poorly constrained, with a highly multimodal period posterior distribution due to aliasing with the 24.25 yr baseline between Hipparcos and Gaia. If confirmed through continued astrometric or spectroscopic monitoring or via direct detection, the companion to TW PsA would represent a choice target for detailed atmospheric characterization with high-contrast instruments on the upcoming James Webb Space Telescope and Wide Field Infrared Survey Telescope.