SIMBAD references

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 A_V 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 A_V and R_V values for the 34 objects with sufficient data quality and an additionally A_V value for the canonical R_V = 3.1 value. Our results agree largely with values obtained from optical data. Moreover, we confirm the degeneracy between A_V and R_V and present possibilities to break this. Finally, we discuss whether the assumption of optical thin lines is valid.