Measurement of interstellar extinction for classical T Tauri stars using far-UV H

Understanding the interstellar and potentially circumstellar extinction in the sight lines of classical T Tauri stars is an important ingredient for constructing reliable spectral energy distributions, which catalyze protoplanetary disk chemistry, for example. Therefore, some attempts of measuring AV toward individual stars have been made using partly different wavelength regimes and different underlying assumptions. We used strong lines of Lyα fluorescent H2 and derived the extinction based on the assumption of optically thin transitions. We investigated a sample of 72 classical T Tauri stars observed with the Hubble Space Telescope in the framework of the ULLYSES program. We computed AV and RV values for the 34 objects with sufficient data quality and an additionally AV value for the canonical RV = 3.1 value. Our results agree largely with values obtained from optical data. Moreover, we confirm the degeneracy between AV and RV and present possibilities to break this. Finally, we discuss whether the assumption of optical thin lines is valid.