The late stages of gas giant planet formation are controlled by the inflow of gas from the surrounding protoplanetary disk. Although we have confidently detected accreting planets PDS 70 bc and AB Aur b so far, we still lack a detailed understanding of how proto-giant planets accumulate mass from their environment. Thus, further investigations of these known planets are crucial to gain insights into the formation of gas giant planets. Previous observations with VLT/MUSE have suggested that the apparent brightness of AB Aur b in the Paβ line is greater than that in Hα due to extinction. Consequently, observing the Paβ line presents a promising opportunity to characterize AB Aur b. Here we propose Subaru/IRCS echelle spectroscopy focusing on the Paβ line, with a spectral resolution of R=20,000 (15 km/s). According to the planetary shock model, the width of the Paβ line corresponds to the free-fall velocity (vff=√2GMP/RP), which translates to 109 km/s when gas predominantly accretes directly onto the planet. In contrast, when gas mainly accretes onto a circumplanetary disk at a greater radius, e.g., r>10RJup, the line width becomes less than 60 km/s. By measuring the width of the Paβ line, we can gain insights into the mechanism of accretion.
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