Data from Arulanantham et al. 2021a

Ultraviolet spectra of protoplanetary disks trace distributions of warm gas at radii where rocky planets are expected to form. We combine UV observations of H2 and CO emission to more extensively map inner disk surface layers, where gas temperature distributions allow radially stratified fluorescence from the two species. Our sample includes 12 classical T Tauri stars with archival data from the Cosmic Origins Spectrograph onboard the Hubble Space Telescope (HST-COS). The spectra for each target include two bands of CO emission (UV-CO) and a suite of H2 emission lines (UV-H2). We calculate an empirical emitting radius for each species from its Gaussian emission line widths, demonstrating that, on average, the UV-CO fluorescence originates further from the central star (r ~ 20 AU) than the UV-H2 (r ~ 0.8 AU). This result is supported by 2-D radiative transfer models, which show that the peak and outer radii of the UV-CO flux distributions generally extend further into the outer disk than the UV-H2. We confirm that the morphologies of the UV-CO bands and LyA radiation fields are significantly correlated and discover that both trace the degree of dust evolution in the disk. The UV tracers appear to follow the same sequence of disk evolution as optical forbidden line emission from jets and winds, as the observed LyA profiles transition between dominant red wing and dominant blue wing shapes when the high-velocity optical emission disappears. Our results suggest a scenario where UV radiation fields, disk winds and jets, and molecular gas evolve in harmony with the dust disks throughout their lifetimes.