While pre-main-sequence stars (PMS) of ages less than a few Myr are surrounded by optically thick circumstellar discs, older stars of ages 30 - 100 Myr, found in nearby young open clusters, show no strong signatures of disc emission. This suggests that disc material vanish into larger bodies, possibly planets, and/or through mass accretion onto the central star on time scales less than 30 Myr. To gain observational progress in the understanding of the time-scales and processes involved in the disc-clearing there is a need for samples of 10 Myr old stars. A successful identification of a 10 Myr old cluster will open up unprecedented possibilities to study the full history of PMS stars and their surrounding discs. For this reason, we are presently undertaking a search for 3 - 10 Myr young open clusters and have collected multi-color ccd-photometry of candidate clusters both in the northern and southern hemisphere. This search has provided us with three promising cluster candidates, likely to host a population of 10 Myr old stars. We have collected and are proposing for multi-object spectroscopy of the PMS-candidates in these clusters to reject non-members of the clusters and then to accurately determine the age of the cluster.
On the theoretical side, this project aims to model time-evolution of accretion discs and the processes responsible for converting the energy-flux of the accretion flow into radiation. The standard scenario used in the analysis of optical and UV emission from young stars is the magnetospheric model where the disc material is elevated from the disc plane and channeled onto the stellar surface. An essential prediction from this model is the formation of a shock at the impact region. We have developed a new numerical model to calculate the emission from this region. To further test this model we have been awarded two observing runs (23 orbits) with the HST using the STIS instrument during Cycle 8. These observations will provide tests of the geometrically distribution of the emitting gas and theories of the mass-loading of magnetic field lines at the magnetosphere-disc interface. Analysis of the UV emission lines will also provide improved calibrations between ultraviolet continuum emission and accretion luminosities, and thus improve estimates of mass accretion rates for T Tauri stars.