# Direct Measurement of Interstellar Extinction toward Young Stars Using Atomic Hydrogen Ly{$\alpha$} Absorption

 Title Direct Measurement of Interstellar Extinction toward Young Stars Using Atomic Hydrogen Ly{$\alpha$} Absorption Publication Type Journal Article Year of Publication 2014 Authors McJunkin, M, France, K, Schneider, P C, Herczeg, G J, Brown, A, Hillenbrand, L, Schindhelm, E, Edwards, S Journal \apj Volume 780 Pagination 150 Keywords dust, extinction, Herbig Ae/Be, ISM: atoms, stars: pre-main sequence, stars: variables: T Tauri, ultraviolet: planetary systems Abstract {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 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)) {\ap}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 \~{}0.6 mag. We discuss possible explanations for this discrepancy in the context of a protoplanetary disk geometry. } DOI 10.1088/0004-637X/780/2/150