HST/COS spectra of DF Tau and V4046 Sgr: first detection of molecular hydrogen absorption against the Lyα emission line.
YANG H., LINSKY J.L. and FRANCE K.
Abstract (from CDS):
We report the first detection of molecular hydrogen (H2) absorption in the Lyα emission line profiles of two classical T Tauri stars (CTTSs), DF Tau and V4046 Sgr, observed by the Hubble Space Telescope/Cosmic Origins Spectrograph. 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 Lyα line significantly differ from the amount of energy in the resulting fluorescent emission. By assuming additional absorption in the H I Lyα profile along our light of sight, we can correct the H2absorption/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.