Measurement of interstellar extinction for classical T Tauri stars using far-UV H_2 line fluxes

Abstract

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 Lyalpha fluorescent H$_ 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.