Kavli Affiliate: Gregory J. Herczeg
| First 5 Authors: Seok-Jun Chang, Nicole Arulanantham, Max Gronke, Gregory J. Herczeg, Edwin A. Bergin
| Summary:
Lyman-$alpha$ (Ly$alpha$) 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 Ly$alpha$ 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 Ly$alpha$ emission across a protoplanetary disk in the
iconic face-on T-Tauri star TW Hya, observed with HST-STIS at spatial offsets
0$”$, $pm 0.2”$, and $pm 0.4”$. To comprehensively interpret these
Ly$alpha$ spectra, we utilize a 3D Monte-Carlo Ly$alpha$ radiative transfer
simulation considering the H I wind and protoplanetary disk. 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 disk.
Furthermore, to explore the effect of Ly$alpha$ radiative transfer on
protoplanetary disk chemistry, we compute the radiation field within the
scattering medium and reveal that the wind reflection causes more Ly$alpha$
photons to penetrate the disk. Our results show the necessity of spatially
resolved Ly$alpha$ observations of a broad range of targets, which will decode
the complex interactions between the winds, protoplanetary disks, and
surrounding environments.
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