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Published December 20, 2011 | Published
Journal Article Open

Near-ultraviolet Excess in Slowly Accreting T Tauri Stars: Limits Imposed by Chromospheric Emission


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.

Additional Information

© 2011 American Astronomical Society. Received 2011 April 22; accepted 2011 October 27; published 2011 November 28. This paper includes data gathered with the 6.5 m Magellan Telescopes located at Las Campanas Observatory, Chile. We thank Will Fischer for help reducing the Phoenix data to obtain the He i line profile. We thank the SMARTS service for obtaining the SMARTS spectra. Stony Brook University is a member of the SMARTS partnership. This work was supported by NASA grants for Guest Observer program 11616 to the University of Michigan, Caltech, Stony Brook University, and the University of Colorado. Based on observations made with the NASA/ESA Hubble Space Telescope, obtained from the Space Telescope Science Institute data archive. STScI is operated by the Association of Universities for Research in Astronomy, Inc. under NASA contract NAS 5-26555. C.E. was supported by the National Science Foundation under award no. 0901947. R. A. acknowledges support from the Science & Technology Facilities Council (STFC) through an Advanced Fellowship (ST/G00711X/1).

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