Far-Ultraviolet Spectroscopy of the Circumstellar and Interstellar Environment of Young Stars

I have analyzed absorption from the CO Fourth Positive band system in the ultraviolet spectra of 6 Classical T Tauri stars, tripling the measurements in the literature. CO traces the molecular gas in the inner disk, providing constraints on the material in the planet-forming environment. I fit an absorption model in order to determine the column density and temperature of the gas in the disk. My CO rotational temperatures agree well with CO fluorescence measurements in the ultraviolet, but are in between infrared CO absorption and emission measurements.;I also fit absorption profiles of HI against the Lyman-alpha emission from a large sample of young stars (Classical T Tauri and Herbig Ae/Be) in order to determine the amount of interstellar extinction along the line of sight. Knowing the extinction value will allow us to reconstruct the intrinsic emission from the stars, which is the radiation impacting the protoplanetary disk. This radiation determines the thermal and chemical structure of the material that may form planets. I find lower visual extinction values than those in the literature using optical, infrared, and X-ray measurement techniques.;In addition, I have created a new technique using H2 fluorescence to empirically estimate the full ultraviolet extinction curve of young stars. I compare predicted line fluxes from my created H2 fluorescence model to observed fluxes from 7 strong progressions in order to determine the extinction over the 1100 - 1700 angstrom range. I then fit my extinction curves with models from the literature to determine best-fit Av and Rv values. I find that this technique is limited by the degeneracy between the Av and Rv values, needing one or the other to be determined independently. I hope to improve the technique and mitigate the limitations in future work.