Because of its imaging capability, the UV spectrometer would allow us to study in detail the variability of the environment of T Tauri stars, where winds and collimated jets encounter interstellar material, producing shocks (Herbig-Haro objects). These phenomena can be studied by the monitoring of the semi-forbidden lines of CII and CIII they produce.
The Herbig Ae/Be stars do not possess outer convection zones, and yet present many signs of magnetic activity, in particular rotational modulation of lines formed in their winds and chromospheres. The origin of this activity is unknown, but various models involving either a deuterium-burning shell or a turbulent region below the stellar surface have been proposed. STARS would be a perfect tool to tackle this problem: the monitoring of UV lines probing the wind and chromosphere (MgII, FeII) would allow us to infer the structure of the magnetic field that is presumably responsible for their modulation, while the seismic data would provide decisive tests of the hypotheses about the stratification of their interiors.