Spectral Biases, Starspot Morphology, and Dynamo Transitions on the Pre-main Sequence: Insights from the X-Shooter WTTS Library

Starspots are ubiquitous in young, low-mass stars, yet their impact on the spectral classification and fundamental parameter inference of pre-main-sequence stars (PMS) has been largely overlooked. In this study, we demonstrate that cool starspots systematically distort spectral morphology and bias the effective temperatures, surface gravities, and luminosities derived for nonaccreting weak-lined T Tauri stars (WTTSs). Using a sample of 56 WTTSs with high-resolution, broadband X-Shooter spectra, we perform two-temperature spectral fits that explicitly account for spot coverages and temperature contrasts. These composite models consistently outperform traditional single-temperature fits, particularly in the 3350–4000 K regime, where spot contributions dominate the red-optical and near-IR flux. Moreover, we propose that surface gravity discrepancies between optical and IR measurements are a natural consequence of spot-dominated emission in PMS stars. We find that single-temperature models can overestimate effective temperatures by up to 700 K and underestimate log g by 1–2 dex. Using spot-corrected effective temperatures, we derive masses and ages from traditional, magnetic, and spotted evolutionary models, finding that spot corrections systematically raise inferred masses by up to 80% and stellar ages by up to 0.5 dex. These discrepancies are strongest for stars in the 0.3–0.8 M_⊙ range. Using starspots as a proxy for magnetic topology, we find evidence that a shift from largely axisymmetric to nonaxisymmetric magnetic fields dominated by small-scale structures coincides with the formation of a radiative core during PMS evolution, effectively distinguishing between the convective and interface dynamo regimes.