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The far-ultraviolet (FUV; 912--1700 \AA) radiation field from accreting central stars in Classical T Tauri systems influences the disk chemistry during the period of giant planet formation. Previous efforts to measure the true stellar+accretion-generated FUV luminosity (both hot gas emission lines and continua) have been complicated by a combination of low-sensitivity and/or low-spectral resolution and did not include the contribution from the bright Ly$\alpha$ emission line. In this work, we present a high-resolution spectroscopic study of the FUV radiation fields of 16 T Tauri stars whose dust disks display a range of evolutionary states. We include reconstructed Ly$\alpha$ line profiles and remove atomic and molecular disk emission (from H$_{2}$ and CO fluorescence) to provide robust measurements of both the FUV continuum and hot gas lines (e.g., Ly$\alpha$, \ion{N}{5}, \ion{C}{4}, \ion{He}{2}) for an appreciable sample of T Tauri stars for the first time. We find that the flux of the typical Classical T Tauri Star FUV radiation field at 1 AU from the central star is $\sim$ 10$^{7}$ times the average interstellar radiation field. The Ly$\alpha$ emission line contributes an average of 88% of the total FUV flux, with the FUV continuum accounting for an average of 8%. Both the FUV continuum and Ly$\alpha$ flux are strongly correlated with \ion{C}{4} flux, suggesting that accretion processes dominate the production of both of these components. On average, only $\sim$ 0.5% of the total FUV flux is emitted between the Lyman limit (912 \AA) and the H$_{2}$ (0 -- 0) absorption band at 1110 \AA. The total and component-level high-resolution radiation fields are made publicly available in machine-readable format., Comment: 22 pages, 15 figures. Accepted to ApJ. On-line data at http://cos.colorado.edu/~kevinf/ctts_fuvfield.html

The formation timescale and final architecture of exoplanetary systems are closely related to the properties of the molecular disks from which they form. Observations of the spatial distribution and lifetime of the molecular gas at planet-forming radii (r < 10 AU) are important for understanding the formation and evolution of exoplanetary systems. Towards this end, we present the largest spectrally resolved survey of H2 emission around low-mass pre-main sequence stars compiled to date. We use a combination of new and archival far-ultraviolet spectra from the COS and STIS instruments on the Hubble Space Telescope to sample 34 T Tauri stars (27 actively accreting CTTSs and 7 non-accreting WTTSs) with ages ranging from roughly 1-10 Myr. We observe fluorescent H2 emission, excited by LyA photons, in 100 of the accreting sources, including all of the transitional disks in our sample (CS Cha, DM Tau, GM Aur, UX Tau A, LkCa15, HD 135344B and TW Hya). The spatial distribution of the emitting gas is inferred from spectrally resolved H2 line profiles. Some of the emitting gas is produced in outflowing material, but the majority of H2 emission appears to originate in a rotating disk. For the disk-dominated targets, the H2 emission originates predominately at r < 3 AU. The emission line-widths and inner molecular radii are found to be roughly consistent with those measured from mid-IR CO spectra., ApJ - accepted. 19 pages, 12 figures

We analyze the far-ultraviolet (FUV) spectra of 33 classical T Tauri stars (CTTS), including 20 new spectra obtained with the Advanced Camera for Surveys Solar Blind Channel (ACS/SBC) on the Hubble Space Telescope. Of the sources, 28 are in the ~1 Myr old Taurus-Auriga complex or Orion Molecular Cloud, 4 in the 8-10 Myr old Orion OB1a complex and one, TW Hya, in the 10 Myr old TW Hydrae Association. We also obtained FUV ACS/SBC spectra of 10 non-accreting sources surrounded by debris disks with ages between 10 and 125 Myr. We use a feature in the FUV spectra due mostly to electron impact excitation of \h2 to study the evolution of the gas in the inner disk. We find that the \h2 feature is absent in non-accreting sources, but is detected in the spectra of CTTS and correlates with accretion luminosity. Since all young stars have active chromospheres which produce strong X-ray and UV emission capable of exciting \h2 in the disk, the fact that the non-accreting sources show no \h2 emission implies that the \h2 gas in the inner disk has dissipated in the non-accreting sources, although dust (and possibly gas) remains at larger radii. Using the flux at 1600 {\AA}, we estimate that the column density of \h2 left in the inner regions of the debris disks in our sample is less than ~ 3x10^-6 g cm^-2, nine orders of magnitude below the surface density of the minimum mass solar nebula at 1 AU., Comment: 15 pages, 4 figures, accepted to ApJL

Young stars surrounded by disks with very low mass accretion rates are likely in the final stages of inner disk evolution and therefore particularly interesting to study. We present ultraviolet (UV) observations of the ~5-9 Myr old stars RECX-1 and RECX-11, obtained with the Cosmic Origins Spectrograph and Space Telescope Imaging Spectrograph on the Hubble Space Telescope, as well as optical and near-infrared spectroscopic observations. The two stars have similar levels of near-UV emission, although spectroscopic evidence indicates that RECX-11 is accreting and RECX-1 is not. The line profiles of Hα and He I λ10830 in RECX-11 show both broad and narrow redshifted absorption components that vary with time, revealing the complexity of the accretion flows. We show that accretion indicators commonly used to measure mass accretion rates, e.g., U-band excess luminosity or the Ca II triplet line luminosity, are unreliable for low accretors, at least in the middle K spectral range. Using RECX-1 as a template for the intrinsic level of photospheric and chromospheric emission, we determine an upper limit of 3 × 10^(–10) M_☉ yr–1 for RECX-11. At this low accretion rate, recent photoevaporation models predict that an inner hole should have developed in the disk. However, the spectral energy distribution of RECX-11 shows fluxes comparable to the median of Taurus in the near-infrared, indicating that substantial dust remains. Fluorescent H2 emission lines formed in the innermost disk are observed in RECX-11, showing that gas is present in the inner disk, along with the dust.

We exploit the high sensitivity and moderate spectral resolution of the Hubble Space Telescope Cosmic Origins Spectrograph to detect far-ultraviolet (UV) spectral features of carbon monoxide (CO) present in the inner regions of protoplanetary disks for the first time. We present spectra of the classical T Tauri stars HN Tau, RECX-11, and V4046 Sgr, representative of a range of CO radiative processes. HN Tau shows CO bands in absorption against the accretion continuum. The CO absorption most likely arises in warm inner disk gas. We measure a CO column density and rotational excitation temperature of N(CO) = (2 ± 1) × 1017 cm−2 and T rot(CO) 500 ± 200 K for the absorbing gas. We also detect CO A–X band emission in RECX-11 and V4046 Sgr, excited by UV line photons, predominantly H i Lyα. All three objects show emission from CO bands at λ > 1560 Å, which may be excited by a combination of UV photons and collisions with non-thermal electrons. In previous observations these emission processes were not accounted for due to blending with emission from the accretion shock, collisionally excited H2, and photo-excited H2, all of which appeared as a “continuum” whose components could not be separated. The CO emission spectrum is strongly dependent upon the shape of the incident stellar Lyα emission profile. We find CO parameters in the range: N(CO) ∼ 1018–1019 cm−2, T rot(CO) ≳ 300 K for the Lyα-pumped emission. We combine these results with recent work on photo-excited and collisionally excited H2 emission, concluding that the observations of UV-emitting CO and H2 are consistent with a common spatial origin. We suggest that the CO/H2 ratio (≡ N(CO)/N(H2)) in the inner disk is ∼1, a transition between the much lower interstellar value and the higher value observed in solar system comets today, a result that will require future observational and theoretical study to confirm.