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Published April 20, 2013 | Published
Journal Article Open

Accretion Rates for T Tauri Stars Using Nearly Simultaneous Ultraviolet and Optical Spectra


We analyze the accretion properties of 21 low-mass T Tauri stars using a data set of contemporaneous near-UV (NUV) through optical observations obtained with the Hubble Space Telescope Imaging Spectrograph and the ground-based Small and Medium Aperture Research Telescope System, a unique data set because of the nearly simultaneous broad wavelength coverage. Our data set includes accreting T Tauri stars in Taurus, Chamaeleon I, η Chamaeleon, and the TW Hydra Association. For each source we calculate the accretion rate (Ṁ) by fitting the NUV and optical excesses above the photosphere, produced in the accretion shock, introducing multiple accretion components characterized by a range in energy flux (or density) for the first time. This treatment is motivated by models of the magnetospheric geometry and accretion footprints, which predict that high-density, low filling factor accretion spots coexist with low-density, high filling factor spots. By fitting the UV and optical spectra with multiple accretion components, we can explain excesses which have been observed in the near-IR. Comparing our estimates of Ṁ to previous estimates, we find some discrepancies; however, they may be accounted for when considering assumptions for the amount of extinction and variability in optical spectra. Therefore, we confirm many previous estimates of the accretion rate. Finally, we measure emission line luminosities from the same spectra used for the Ṁ estimates, to produce correlations between accretion indicators (Hβ, Ca II K, C II], and Mg II) and accretion properties obtained simultaneously.

Additional Information

© 2013 American Astronomical Society. Received 2012 December 18; accepted 2013 March 3; published 2013 April 2. We thank Melissa McClure for valuable discussions regarding extinction estimates and veiling in the NIR and providing results prior to publication. We also thank Ted Bergin, Lee Hartmann, JonMiller, and Fred Adams for providing comments on an early version of this paper as part of L. Ingleby's thesis. This work was supported by NASA grants for Guest Observer program 11616 to the University of Michigan, Caltech, 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 a Sagan Exoplanet Fellowship from NASA and administered by the NASA Exoplanet Science Institute. R.D.A. acknowledges support from the Science and Technology Facilities Council (STFC) through an Advanced Fellowship (ST/G00711X/1). S.G.G. acknowledges support from STFC via a Ernest Rutherford Fellowship (ST/J003255/1).

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