Subaru/SCExAO Direct Imaging Survey for Substellar Companions around Accelerating Nearby Stars

Several large direct imaging surveys based on blind target selections have led to the discovery of planets and brown dwarfs around nearby stars. However, they also revealed the low occurrence rate (< 10%; Nielsen et al. 2019) of those substellar companions that are detectable with current high-contrast instruments. This low yield from the blind surveys has limited the opportunity to characterize the diversity of substellar companions. To overcome this problem, we started a direct imaging survey that adopts new concept in target selection, which uses the measurement of proper motions from the Hipparcos and Gaia Catalog of Accelerations (HGCA). The proper motion of a star accompanied by a companion is accelerated because they orbit around the common center of mass. A measurement of proper motion acceleration from HGCA is precise enough to detect the reflex motion caused by even a giant planet; It is thus available as the sign of a substellar companion. In addition, the HGCA data can be combined with RV and direct imaging data in orbit modeling to determine the dynamical mass of a companion, which is model-independent and helps test evolutionary/atmosphere theories of cool objects. We have observed nearby accelerating young stars selected from the HGCA using the extreme adaptive optics system SCExAO and the integral field spectrograph CHARIS on the Subaru 8.2m Telescope. As a result, we identified a brown dwarf companion with a dynamical mass of about 50 Jupiter masses (M_Jup) around the Sun-like star HD 33632 Ab (Currie et al. 2020), which is the first direct imaging discovery of a brown dwarf around an HGCA-selected target. In addition, we provided the first direct imaging discovery of a substellar companion orbiting a main-sequence star in the Hyades open cluster (Kuzuhara et al. 2022). The companion, HIP 21152 B, has an estimated dynamical mass of 28 (-5.4, +8.4) M_Jup and semi-major axis of 17.5 (-3.8, +7.2) au. We also uncovered the A-type star HIP 99770 hosts a massive giant planet, whose dynamical mass was estimated to be 13.9–16.1 M_Jup (Currie et al. 2023). Furthermore, a substellar companion around another A-type star was discovered (Tobin et al. in prep). Thus, our success represents the higher discovery efficiency than conventional blind surveys. Recently, our extensive survey program (34 nights over 2.5 yrs on Subaru and 8 nights over 2 yrs on Keck) was accepted for this survey; that will accelerate the discovery of substellar companions, including those suitable for the Roman CGI target. In this contribution, we present our survey progress, highlight the above interesting discoveries, and introduce our plan.