First detection and modeling of spatially resolved Lyαin TW Hya

Abstract

Abstract Lyman-α (Lyα) is the strongest emission line in the accretion-generated UV spectra from T-Tauri stars and, as such, plays a critical role in regulating chemistry within the surrounding protoplanetary disks. Due to its resonant nature, the scattering of Lyαphotons 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 Lyαemission across a protoplanetary disk in the iconic face-on T-Tauri star TW Hya, observed with HST-STIS at spatial offsets 0″, ±0.2″, and ±0.4″. To comprehensively interpret these Lyαspectra, we utilize a 3D Monte-Carlo Lyαradiative transfer simulation considering the H iwind and protoplanetary disk. From the simulation, we constrain the wind’s properties: the H icolumn density ∼1020 cm−2 and the outflow velocity ∼200 km s−1. Our findings indicate that successfully interpreting the observed spectra necessitates scattering contributions in the H ilayer within the disk. Furthermore, to explore the effect of Lyαradiative transfer on protoplanetary disk chemistry, we compute the radiation field within the scattering medium and reveal that the wind reflection causes more Lyαphotons to penetrate the disk. Our results show the necessity of spatially resolved Lyαobservations of a broad range of targets, which will decode the complex interactions between the winds, protoplanetary disks, and surrounding environments.