HST/COS Spectra of DF Tau and V4046 SGR: First Detection of Molecular Hydrogen Absorption Against the Lyα Emission Line
Abstract
We report the first detection of molecular hydrogen (H2) absorption in the Lyman-α emission line profiles of two classical T Tauri stars (CTTSs), DF Tau and V4046 Sgr, observed by HST/COS. This absorption is the energy source for many of the Lyman-band H2 fluorescent lines commonly seen in the far-ultraviolet spectra of CTTSs. We find that the absorbed energy in the H2 pumping transitions from a portion of the Lyman-α line significantly differ from the amount of energy in the resulting fluorescent emission. By assuming additional absorption in the H I Lyman-α profile along our light of sight, we can correct the H2 absorption/emission ratios so that they are close to unity. The required H I absorption for DF Tau is at a velocity close to the radial velocity of the star, consistent with H I absorption in the edge-on disk and interstellar medium. For V4046 Sgr, a nearly face-on system, the required absorption is between +100 km/s and +290 km/s, most likely resulting from H I gas in the accretion columns falling onto the star.