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

Abstract

Abstract We report FUV, optical, and NIR observations of three T Tauri stars in the Orion OB1b subassociation with Hα 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 Advanced Camera for Surveys/Solar Blind Channel spectra, and to probe the accretion flows onto the star using Hα and He i λ10830 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 freefall velocities in the He i line and wide wings in Hα; however, a marginal detection of FUV H2 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 the accretion phase is reached in diverse ways.