Probing the inner AU geometry of T Tauri and Herbig stars 
          Jorick Vink, Imperial College London
          Janet Drew, Imperial College London
          Tim Harries, University of Exeter
          Rene Oudmaijer, School of Physics and Astronomy, Leeds
          Yvonne Unruh, Imperial College London

          We present Halpha spectropolarimetry observations on a large sample of  T Tauri and Herbig Ae/Be stars. A change in the linear polarisation across Halpha is detected in most of the T~Tauri (9/10) and Herbig Ae (9/11) objects, which we interpret in terms of a compact source of line photons that is scattered off a rotating accretion disk. We find consistency between the PA of the polarisation and those of imaged disk PAs from infrared and millimetre imaging and interferometry studies, which probe the much larger scales. For the Herbig Ae stars AB Aur, MWC 480 and CQ Tau, as well as the borderline object RY Tau, we find the polarisation PA to be perpendicular to the imaged disk, which is expected for single scattering. On the other hand, the polarisation PA aligns with the outer disk PA for the T Tauri stars DR Tau and SU Aur as well as FU Ori, conform the expectation of multiple scatterings. This difference can be explained if the inner (sub AU) gas disks that we find for Herbig Ae stars are optically thin, whilst those around our T Tauri objects and FU~Ori are optically thick. Finally, we present a novel technique that combines known inclination angles and our recent Monte Carlo scattering models to constrain the inner rim sizes of  SU Aur, GW Ori, AB Aur, and CQ Tau.