HST/COS Spectra of DF Tau and V4046 SGR: First Detection of Molecular Hydrogen Absorption Against the Ly$\alpha$ Emission Line

Abstract

We report the first detection of molecular hydrogen (H${}_2$) absorption in the Lyman-$\alpha$ emission line profiles of two classical T Tauri stars (CTTSs), DF Tau and V4046 Sgr, observed by \emph{HST}/COS. This absorption is the energy source for many of the Lyman-band H${}_2$ fluorescent lines commonly seen in the far-ultraviolet spectra of CTTSs. We find that the absorbed energy in the H${}_2$ pumping transitions from a portion of the Lyman-$\alpha$ line significantly differ from the amount of energy in the resulting fluorescent emission. By assuming additional absorption in the H I Lyman-$\alpha$ profile along our light of sight, we can correct the H${}_2$ 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.

13 pages, 4 figures, accepted by ApJL in February 2011

References in corpus (8)

• The Magnetic Fields of Classical T Tauri Stars
• UV excess measures of accretion onto young very low-mass stars and brown dwarfs
• The Ultraviolet Radiation Environment Around M dwarf Exoplanet Host Stars
• The Origins of Fluorescent H2 Emission From T Tauri~Stars
• Magnetic-field measurements of T Tauri stars in the Orion Nebula cluster
• Dynamical Mass Estimates for Incomplete Orbits: Young Multiple Stars in Taurus and Ophiuchus
• Molecules in the disk orbiting the twin young suns of V4046 Sgr
• The Far-Ultraviolet "Continuum" in Protoplanetary Disk Systems I: Electron-Impact H2 and Accretion Shocks

Cited by in corpus (14)

• The Cosmic Origins Spectrograph
• A Far-ultraviolet Atlas of Low-resolution Hubble Space Telescope Spectra of T Tauri Stars
• High-resolution Ultraviolet Radiation Fields of Classical T Tauri Stars
• NUV Excess in Slowly Accreting T Tauri Stars: Limits Imposed by Chromospheric Emission
• Lyman alpha dominance of the Classical T Tauri FUV Radiation Field
• The Distribution and Chemistry of H${}_2$CO in the DM Tau Protoplanetary Disk
• CO/H2 Abundance Ratio ~ 10^{-4} in a Protoplanetary Disk
• The Far-Ultraviolet "Continuum" in Protoplanetary Disk Systems II: CO Fourth Positive Emission and Absorption
• Characterizing CO Fourth Positive Emission in Young Circumstellar Disks
• The Evolution of Inner Disk Gas in Transition Disks
• CO and H2 Absorption in the AA Tauri Circumstellar Disk
• Binarity and Accretion in AGB Stars: HST/STIS Observations of UV Flickering in Y Gem
• Probing the Inner Regions of Protoplanetary Disks with CO Absorption Line Spectroscopy
• Signatures of Hot Molecular Hydrogen Absorption from Protoplanetary Disks: I. Non-thermal Populations