Lyα Scattering Models Trace Accretion and Outflow Kinematics in T Tauri Systems*

Abstract

T Tauri stars produce broad Lyα emission lines that contribute ∼88% of the total UV flux incident on the inner circumstellar disks. Lyα photons are generated at the accretion shocks and in the protostellar chromospheres and must travel through accretion flows, winds, and jets, the protoplanetary disks, and the interstellar medium before reaching the observer. This trajectory produces asymmetric, double-peaked features that carry kinematic and opacity signatures of the disk environments. To understand the link between the evolution of Lyα emission lines and the disks themselves, we model HST-COS spectra from targets included in Data Release 3 of the Hubble UV Legacy Library of Young Stars as Essential Standards program. We find that resonant scattering in a simple spherical expanding shell is able to reproduce the high-velocity emission line wings, providing estimates of the average velocities within the bulk intervening H i. The model velocities are significantly correlated with the K-band veiling, indicating a turnover from Lyα profiles absorbed by outflowing winds to emission lines suppressed by accretion flows as the hot inner disk is depleted. Just 30% of targets in our sample have profiles with redshifted absorption from accretion flows, many of which have resolved dust gaps. At this stage, Lyα photons may no longer intersect with disk winds along the path to the observer. Our results point to a significant evolution of Lyα irradiation within the gas disks over time, which may lead to chemical differences that are observable with ALMA and JWST.

The Radial Distribution and Excitation of H2 around Young Stars in the HST-ULLYSES Survey

“…The plot demonstrates the correlation between Lyα (as derived in Section 3.1) and accretion-dominated emission from C IV. This is consistent with the broad, bright Lyα profiles of CTTSs(Schindhelm et al 2012;Arulanantham et al 2023) being powered by mass accretion (e.g.,France et al 2014b).…”

“…For the H 2 radial distribution analysis, we consider four progressions: the [1, 7] and [1,4] progressions that are pumped within 100 km s −1 of the Lyα line center and two progressions ([0, 1] and [0, 2]) that are pumped from the wing of the Lyα emission profile (379 and 487 km s −1 from line center, respectively). We analyze multiple progressions because selfabsorption can selectively impact the lower-energy level gas (Wood et al 2002;McJunkin et al 2016) and that offstar Lyα emission and the local scattering geometry can impact the flux distributions of the different progressions (Walter et al 2003;Arulanantham et al 2023). We will return to a discussion of these results in Section 4.…”

Twenty-five Years of Accretion onto the Classical T Tauri Star TW Hya

“…The Lyα emission from TW Hya has weak wings that extend to even larger velocities, as noted by France et al (2014). The fluxes on the prominent line wings are well reproduced by models with superimposed Gaussian profiles to represent the stellar and accretion emission and interstellar medium and outflow absorption (e.g., Herczeg et al 2004;Schindhelm et al 2012) or resonant scattering through an outflowing H I shell (see analysis of DM Tau by Arulanantham et al 2023), but those models do not capture the extremely high-velocity wings out to 2000 km s −1 . The Hβ line wings extend to ∼1200 km s −1 , while higher H lines do not have such broad wings (see, e.g., Wilson et al 2022).…”

“…obscured by circumstellar and interstellar H I. Corrections lead to total Lyα fluxes that are roughly 2 times higher than the observed flux (e.g., Herczeg et al 2004;Schindhelm et al 2012;Arulanantham et al 2023).…”

“…The exclusion of lines is even more important toward very low-mass stars and brown dwarfs, since the optical line luminosity to UV continuum luminosity approaches unity (Alcalá et al 2014;Zhou et al 2014). The Lyα luminosity for most of those sources is unmeasured and may be higher than Hα and other lines in the Balmer series (Arulanantham et al 2023).…”