Name/Affiliation: Matthew McJunkin (University of Colorado at Boulder)
Measuring Extinction Toward Young Stars With Interstellar Lyman-alpha Absorption and Molecular Hydrogen Fluorescence
Interstellar reddening corrections are essential to reconstruct the intrinsic spectral energy distributions (SEDs) of protostellar systems. The stellar SED determines the heating and chemical evolution in circumstellar disks. Measuring interstellar neutral hydrogen absorption against the Lyman-alpha emission of young stars can be used to calculate a visual extinction, assuming a grain size distribution and an interstellar reddening curve. We find visual extinctions for 31 classical T Tauri and Herbig Ae/Be stars of A_v = 0.02 - 0.72 mag, which are on average ~0.6 mag lower than previous measurements in the IR and optical. High molecular fractions or large dust-to-gas ratios could explain this discrepancy. We also present preliminary work on a molecular hydrogen (H2) fluorescence model to characterize the full extinction curve along the line of sight towards a subset of these young stars. Using the Lyman-alpha radiation of the stars to pump H2 electronic transitions from the ground state, we model the flux that is re-emitted through the subsequent fluorescent cascade. By comparing the observed H2 line flux to the model of the emitted H2 line flux, we can extract the extinction over the 1100-1700 Angstrom wavelength region. The shape of the extinction curve allows us to characterize the dust grain distribution in the intervening material as well as to recover the intrinsic SED of the stars over a wide wavelength range.