HST/STIS coronagraphic imaging of pre-main sequence stars

We present the results of a coronagraphic imaging survey of millimeter-selected classical T Tauri stars, and optically bright Herbig Ae stars. We find similar frequency of bipolar microjet detections, and similar evolutionary timescales for the optical brightness of disks exterior to 0.5 '' (50-70 AU at d = 100-140 pc). We find an increasing fraction of stars with known large disks which are optically not detected beginning at approximate to 5 Myr. Many of these same stars have previously been identified as having substantial reservoirs of millimeter-sized grains, suggesting that we are detecting the decoupling of the dust from the gas, which is expected to be a necessary precursor for large-scale planetesimal formation.