Ultraviolet accretion continua reveal a transition in T Tauri magnetic topology
Magnetic fields regulate accretion in T Tauri stars (TTSs) and evolve as radiative cores emerge during pre-main-sequence contraction. Stars above ~0.35-0.5 Msun transition from fully convective to partially convective interiors, a change associated with increasingly complex magnetic topologies and evolving dynamo behaviour. Because accretion flows are magnetically channelled, this evolution is expected to modify ultraviolet (UV) accretion diagnostics, although observational evidence has remained limited. We identify a transition in the UV continuum properties of TTSs using archival International Ultraviolet Explorer observations, complemented by recent spectra from the Hubble Space Telescope ULLYSES survey. An empirical UV photospheric sequence constructed from G-M main-sequence stars shows that late K-M TTSs broadly follow standard photosphere-plus-shock model predictions, whereas G-K5 stars display systematically lower F270/F303 continuum flux ratios (flux at 270 nm relative to 303 nm). Statistical analysis shows that transition models describe the observations significantly better than a single continuous trend, locating the transition near spectral type K5-K6, with its precise location limited by spectral-classification uncertainties, and consistent with the fully convective boundary. Comparison with photosphere-plus-shock models further shows that, while the late-type population is broadly encompassed by the parameter space sampled by the adopted models, the warmer stars extend systematically beyond it in this UV diagnostic. We interpret this transition as evidence for a change in magnetospheric accretion associated with the evolution of magnetic topology across the radiative-core boundary, resulting in a corresponding change in the emergent UV continuum.