2025PASJ...77.1101I

Sub-Neptunes are small planets between the size of the Earth and Neptune. The orbital and bulk properties of transiting sub-Neptunes can provide clues for their formation and evolution of small planets. In this paper, we report on follow-up observations of a planetary system around the mid-M dwarf TOI-654, whose transiting sub-Neptune TOI-654 b ($P=1.53\,$d) is validated as a suitable target for the atmospheric observation. We measure the planetary mass and stellar properties with the InfraRed Doppler instrument (IRD) mounted on the Subaru telescope and obtain the stellar and planetary properties from additional transit observations by the Transit Exoplanetary Survey Satellite (TESS) and a series of the Multicolor Simultaneous Camera for studying Atmospheres of Transiting exoplanets (MuSCAT). As a result, the planetary mass of TOI-654 b is determined to be $M_{{\rm p}} = 8.71 \pm 1.25\, M_{\oplus}$, and the radius is updated to be $R_{\rm p} = 2.378 \pm 0.089\, R_{\oplus}$. The bulk density suggests that the planet is composed of a rocky and volatile-rich core or a rocky core surrounded by a small amount of H/He envelope. TOI-654 b is one of the unique planets located around the radius valley and also on the edge of the Neptune desert. The precise mass determination enables us to constrain the atmospheric properties by future spectroscopic observations especially for the emission with the James Webb Space Telescope and Ariel.