Direct Measurement of Interstellar Extinction Toward Young Stars Using
  Atomic Hydrogen Lyman-$\alpha$ 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
Lyman-$\alpha$ emission profiles in young stars can be used to obtain the total
H I 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 H I column densities range
from log$_{10}$(N(H I)) $\approx 19.6 - 21.1$, 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 H I 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 $\sim 0.6$ mag.
We discuss possible explanations for this discrepancy in the context of a
protoplanetary disk geometry.
