Warm Sub-Saturns Orbiting Single Stars Are Spin-Orbit Aligned

In this work, we show that warm sub-Saturns orbiting single stars are predominantly aligned, in contrast to hot sub-Saturns, which are frequently misaligned, with the two populations differing at the 3.2$\sigma$ level. Because both populations are observed around cool stars, they are free from the ambiguity introduced by the $T_{\rm eff}$-$\lambda$ dependence. Together with the established alignment of warm Jupiters, this demonstrates, among single-star systems, that spin-orbit misalignment arises specifically in the close-in ``hot-Jupiter-analog'' regime, where tidal circularization is efficient ($\tau_e<\tau_{\rm age}$) and high-eccentricity migration is expected to operate. We further find that the transition between aligned and misaligned sub-Saturns occurs at wider orbital separations ($a_{\rm final}/R_p = 338\pm27$) than for Jupiters ($a_{\rm final}/R_p = 117\pm9$), consistent with the expectation that the lower masses (smaller $M_p/M_*$) and stronger tidal dissipation (lower $Q_p$) of sub-Saturns allow them to be circularized into wider final orbits within their lifetimes. Taken together, these results provide the clearest direct evidence to date that, in single-star systems, spin-orbit misalignments are produced by high-eccentricity migration. If this framework is correct, spin-orbit misalignments may also emerge among hot-Jupiter analogs in other mass regimes, including hot brown dwarfs around hot stars at $a_{\rm final}/R_p\lesssim100$ and isolated hot super-Earths at $a_{\rm final}/R_p\lesssim1000$, with the corresponding transition locations shifted by the dependence of the orbital-circularization timescale on $M_p/M_*$ and $Q_p$.