DIRECT MEASUREMENT OF INTERSTELLAR EXTINCTION TOWARD YOUNG STARS USING ATOMIC HYDROGEN Lyα ABSORPTION

Interstellar reddening corrections are necessary to reconstruct the intrinsic spectral energy distributions (SEDs) of accreting protostellar systems. The stellar SED determines the heating and chemical processes that can occur in circumstellar disks. Measurement of neutral hydrogen absorption against broad Ly alpha emission profiles in young stars can be used to obtain the total Hi column density (N(H I)) along the line of sight. We measure N(H I) with new and archival ultraviolet observations from the Hubble Space Telescope (HST) of 31 classical T Tauri and Herbig Ae/Be stars. The Hi column densities range from log(10)(N(H I)) approximate to 19.6-21.1, with corresponding visual extinctions of A(V) = 0.02-0.72 mag, assuming an R-V of 3.1. We find that the majority of the Hi absorption along the line of sight likely comes from interstellar rather than circumstellar material. Extinctions derived from new HST blue-optical spectral analyses, previous IR and optical measurements, and new X-ray column densities on average overestimate the interstellar extinction toward young stars compared to the N(H I) values by similar to 0.6 mag. We discuss possible explanations for this discrepancy in the context of a protoplanetary disk geometry.