EMPIRICALLY ESTIMATED FAR-UV EXTINCTION CURVES FOR CLASSICAL T TAURI STARS

Measurements of extinction curves toward young stars are essential for calculating the intrinsic stellar spectrophotometric radiation. This flux determines the chemical properties and evolution of the circumstellar region, including the environment in which planets form. We develop a new technique using H2 emission lines pumped by stellar Lyα photons to characterize the extinction curve by comparing the measured far-ultraviolet H2 line fluxes with model H2 line fluxes. The difference between model and observed fluxes can be attributed to the dust attenuation along the line of sight through both the interstellar and circumstellar material. The extinction curves are fit by a Cardelli et al. (1989) model and the A V (H2) for the 10 targets studied with good extinction fits range from 0.5 to 1.5 mag, with R V values ranging from 2.0 to 4.7. A V and R V are found to be highly degenerate, suggesting that one or the other needs to be calculated independently. Column densities and temperatures for the fluorescent H2 populations are also determined, with averages of log10(N(H2)) = 19.0 and T = 1500 K. This paper explores the strengths and limitations of the newly developed extinction curve technique in order to assess the reliability of the results and improve the method in the future.