(Proposal ID) S16B-009 (PI) Tamura, Motohide (Proposal Title) A definitive census of 4 young associations: Taurus, IC348, NGC1333 & alphaPer (Abstract) We propose to obtain new epoch wide field images of 4 young nearby associations with HSC. The time difference with archival images (up to 15 years) will allow us to compute accurate proper motions down to 0.3 mas/yr accuracy, or <1 km/s at the distance of these associations. These measurements will complement the Gaia mission i) down to the planetary mass regime; ii) in young deeply embedded regions where Gaia will be blind. These observations will allow us: a) to derive a complete census based on a kinematic selection of members; 2) perform a statistically significant study of the IMF and probe the fragmentation limit; 3) study the internal dynamics of each group separately, and perform a comparison of various groups representative of distinct initial conditions.