The Evolution of Protoplanetary Disks: Probing the Inner Disk of Very Low Accretors

Abstract

We report FUV, optical, and NIR observations of three T Tauri stars in the Orion OB1b subassociation with H$\hat{I}\pm$ equivalent widths consistent with low or absent accretion and various degrees of excess flux in the mid-infrared. We aim to search for evidence of gas in the inner disk in HST ACS/SBC spectra, and to probe the accretion flows onto the star using H$\hat{I}\pm$ and He I $\hat{I}»$10830 in spectra obtained at the Magellan and SOAR telescopes. At the critical age of 5 Myr, the targets are at different stages of disk evolution. One of our targets is clearly accreting, as shown by redshifted absorption at free-fall velocities in the He I line and wide wings in H$\hat{I}\pm$; however, a marginal detection of FUV H${}_2$ suggests that little gas is present in the inner disk, although the spectral energy distribution indicates that small dust still remains close to the star. Another target is surrounded by a transitional disk, with an inner cavity in which little sub-micron dust remains. Still, the inner disk shows substantial amounts of gas, accreting onto the star at a probably low, but uncertain rate. The third target lacks both a He I line or FUV emission, consistent with no accretion or inner gas disk; its very weak IR excess is consistent with a debris disk. Different processes occurring in targets with ages close to the disk dispersal time suggest that the end of accretion phase is reached in diverse ways.

13 pages, 3 figures. Accepted for publication in The Astrophysical Journal