The Subaru telescope is currently performing a strategic program (SSP) using Subaru/IRD, a high-dispersion near-infrared (NIR) spectroscopy, to search for exoplanets around nearby M dwarfs via radial velocity (RV) monitoring. The SSP targets with signatures of massive companions such as RV trends have been deprioritized, but this RV information remains useful for studying the stellar multiplicity of nearby M dwarfs. To search for companions around such M dwarfs, we observed 14 IRD-SSP targets using Keck/NIRC2 observations with pyramid NIR wavefront sensing, leading to high sensitivity to substellar-mass companions within a few arcseconds.
Including Subaru/AO188-corrected images at the IRD fiber injection module CCD, we detected seven companions (four new companions) in total at projected separations between ~2-20 au from the primary stars. A comparison of the colors with the spectral library suggests that two new companions are located at the boundary between late-M and early-L spectral types.
We also utilized angular differential imaging and reference-star differential imaging for targets where no bright companions were resolved but did not reveal any additional companion candidates. The NIRC2 detection limits could constrain potential substellar-mass companions at 10 au or further. The failure to detect companions with Keck/NIRC2 around the IRD-SSP stars with significant RV trends makes these objects promising targets for further RV monitoring or deeper imaging with JWST to search for smaller-mass companions below the NIRC2 detection limits.