2024MNRAS.529.2656C

Lyman-α (Lyα) is the strongest emission line in the accretion-generated ultraviolet spectra from T Tauri stars and, as such, plays a critical role in regulating chemistry within the surrounding protoplanetary discs. Due to its resonant nature, the scattering of Lyα photons along the line of sight encodes information about the physical properties of the intervening H I medium. In this work, we present the first spatially resolved spectral images of Lyα emission across a protoplanetary disc in the iconic face-on T Tauri star TW Hya, observed with HST-STIS at spatial offsets 0, ±0.2, and ±0.4 arcsec. To comprehensively interpret these Lyα spectra, we utilize a 3D Monte Carlo Lyα radiative transfer simulation considering the H I wind and protoplanetary disc. From the simulation, we constrain the wind's properties: the H I column density $\sim 10^{20} {\rm \, cm^{-2}}$ and the outflow velocity $\sim 200 {\rm \, km\, s^{-1}}$. Our findings indicate that successfully interpreting the observed spectra necessitates scattering contributions in the H I layer within the disc. Furthermore, to explore the effect of Lyα radiative transfer on protoplanetary disc chemistry, we compute the radiation field within the scattering medium and reveal that the wind reflection causes more Lyα photons to penetrate the disc. Our results show the necessity of spatially resolved Lyα observations of a broad range of targets, which will decode the complex interactions between the winds, protoplanetary discs, and surrounding environments.