The ODYSSEUS Survey. Characterizing Magnetospheric Geometries and Hotspot Structures in T Tauri Stars

Abstract Magnetospheric accretion is a key process that shapes the inner disks of T Tauri stars, controlling mass and angular momentum evolution. It produces strong ultraviolet and optical emission that irradiates the planet-forming environment. In this work, we characterize the magnetospheric geometries, accretion rates, extinction properties, and hotspot structures of 67 T Tauri stars in the largest and most consistent study of ultraviolet and optical accretion signatures to date. To do so, we apply an accretion flow model to velocity-resolved H α profiles for T Tauri stars from the Hubble Space Telescope (HST) ULLYSES program with consistently derived stellar parameters. We find typical magnetospheric truncation radii to be almost half of the usually assumed value of 5 stellar radii. We then model the same stars’ HST/STIS spectra with an accretion shock model, finding a diverse range of hotspot structures. Phase-folding multiepoch shock models reveals rotational modulation of observed hotspot energy flux densities, indicative of hotspots that persist for at least three stellar rotation periods. For the first time, we perform a large-scale, self-consistent comparison of accretion rates measured using accretion flow and shock models, finding them to be consistent within ∼0.16 dex for contemporaneous observations. Finally, we find that up to 50% of the total accretion luminosity is at short wavelengths accessible only from space, highlighting the crucial role of ultraviolet spectra in constraining accretion spectral energy distributions, hotspot structure, and extinction.