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Published October 1, 2009 | Published
Journal Article Open

Far-Ultraviolet H_2 Emission from Circumstellar Disks


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 1, 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 H_2 to study the evolution of the gas in the inner disk. We find that the H_2 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 H_2 in the disk, the fact that the non-accreting sources show no H_2 emission implies that the H_2 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 Å, we estimate that the column density of H_2 left in the inner regions of the debris disks in our sample is less than ~3 × 10^(–6) g cm^(-2), 9 orders of magnitude below the surface density of the minimum mass solar nebula at 1 AU.

Additional Information

© 2009 American Astronomical Society. Print publication: Issue 2 (2009 October 1); received 2009 August 12; accepted for publication 2009 August 27; published 2009 September 14. We thank Al Glassgold for discussions clarifying the ionization mechanisms in the disk. This work was supported by NASA through grants GO-08317, GO-09081, GO-9374, GO- 10810, and GO-10840 from the Space Telescope Science Institute. This material is also based upon work supported by the National Science Foundation under Grant No. 0707777 to E.A.B.

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Published - Ingleby2009p6001Astrophys_J_Lett.pdf


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