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 UV spectra from
T-Tauri stars. Due to its resonant nature, Ly$alpha$ emission carries
information about the physical properties of the H I medium via the scattering
process. This work presents spatially resolved 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
radiative transfer simulation in a wind-disk geometry. Successfully reproducing
the observed spectra requires scattering contributions from both the wind and
the H I disk. We constrain the properties of the wind, the H I column density
($sim 10^{20} rm cm^{-2}$) and the outflow velocity ($sim 200 rm km
s^{-1}$). To reproduce the observed spatial distribution of Ly$alpha$, we find
that the wind must cover the H I disk when viewed face-on. Furthermore, to
explore the effect of Ly$alpha$ radiative transfer in T-Tauri stars, we
compute the radiation field within the scattering medium and reveal that the
wind reflection causes more Ly$alpha$ photons to penetrate the disk. We also
show the dependence between the disk inclination angle and the spatially
resolved Ly$alpha$ spectra. Understanding the role of Ly$alpha$ emission in
T-Tauri stars is pivotal for decoding the complex interactions between the
winds, protoplanetary disks, and surrounding environments, which can
significantly impact the chemistry in the protoplanetary disk. Our observation
and modeling of spatially resolved Ly$alpha$ show the necessity of spatially
resolved Ly$alpha$ observation of a broad range of targets.
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