UV-driven Evolution and Chemistry of Protoplanetary Disks: Insights from HST's ULLYSES Program

Nicole Arulanantham

Beginning in December 2019, approximately 70 T Tauri stars were observed at UV wavelengths through the Hubble UV Legacy Library of Young Stars as Essential Standards Director's Discretionary program (ULLYSES). These publicly available spectra provide critical observational constraints on UV irradiation of protoplanetary disks, a key driver of the gas phase chemistry that sets the initial conditions for planet formation. We use the HST data and radiative transfer models to reproduce the LyA emission seen by surface layers of the gas disks, which typically makes up the largest component of the total integrated UV flux. The model LyA flux available to photodissociate molecules like H2O, HCN, and CH3CN within disks with dust cavities and gaps varies by two orders of magnitude, and we explore how the spread might translate to observable chemical differences. We also investigate whether ultraviolet emission lines from UV-fluorescent H2 can be used to break the degeneracy between disk flaring angle and UV irradiation of the gas in models of sub-mm CN emission. This work will be particularly useful for interpreting molecular features in spatially unresolved disks observed with ALMA and JWST.