The True Stellar Obliquity of a Sub-Saturn Planet from the Tierras Observatory and the Keck Planet Finder

We measure the true obliquity of TOI-2364, a K dwarf with a sub-Saturn-mass (M_p = 0.18 M_J) transiting planet on the upper edge of the hot-Neptune desert. We used new Rossiter-McLaughlin observations gathered with the Keck Planet Finder to measure the sky-projected obliquity λ = 7° + 10°-11°. Combined with a stellar rotation period of 23.47 ± 0.29 days measured with photometry from the Tierras Observatory, this yields a stellar inclination of 90° ± 13° and a true obliquity ψ = 15.°6+7.°7-7.°3, indicating that the planet’s orbit is well aligned with the rotation axis of its host star. The determination of ψ is important for investigating a potential bimodality in the orbits of short-period sub-Saturns around cool stars, which tend to be either aligned with or perpendicular to their host stars’ spin axes.