2025ApJ...993..181P

Classical T Tauri stars (CTTSs) exhibit strong variability over timescales of minutes to decades. However, much theoretical work assumes that CTTSs are in stable spin states. Here we test expectations for CTTS angular momentum regulation by comparing star and disk rotation. We measure stellar rotation periods and disk corotation radii (R_co) for 47 CTTSs from the Hubble Space Telescope ULLYSES sample. We compare R_co to the magnetospheric truncation radii (R_i) and show that most CTTSs are in sub-equilibrium spin states based on model predictions. This may indicate efficient angular momentum loss processes that maintain slow rotation rates throughout the CTTS phase. We find evidence of such processes in the form of magnetospheric outflows and episodic accretion, and our observations are consistent with the presence of accretion-powered stellar winds. We confirm predictions that R_i is variable over timescales of days, causing some CTTSs to cross accretion stability regime boundaries. We characterize light-curve morphologies and confirm that our inclined CTTSs with R_i ∼ R_co show dipper light curves, consistent with expectations from disk warp models. However, dippers occur at all values of R_i/R_co, suggesting that they do not need to be near the propeller regime. Finally, we show that our measured R_i locations are consistent with observed ultra-short-period planet (USP) semimajor axes. If USPs are stable against tidal dissipation, as has been suggested in the literature, then our work provides a plausible USP formation channel. These results show that the star-disk connection produces a large variety of accretion and stellar spin configurations, most of which are likely not in equilibrium.