2019ApJ...871..163L

We analyze high-quality stellar catalogs for 24 young and nearby (within 1 kpc) embedded clusters and present a catalog of 32 groups which have a high concentration of protostars. The median effective radius of these groups is 0.17 pc. The median protostellar and pre-main-sequence star surface densities are 46 M_☉ pc^–2 and 11 M_☉ pc^–2, respectively. We estimate the age of these groups using a model of constant birthrate and random accretion stopping and find a median value of 0.25 Myr. Some groups in Aquila, Serpens, Corona Australia, and Ophichus L1688 show high protostellar surface density and high molecular gas surface density, and seem to be undergoing vigorous star formation. These groups provide an excellent opportunity to study the initial conditions of clustered star formation. Comparisons of protostellar and pre-main-sequence stellar surface densities reveal continuous low-mass star formation of these groups over several Myr in some clouds. For groups with typical protostellar separations of less than 0.4 pc, we find that these separations agree well with the thermal Jeans fragmentation scale. On the other hand, for groups with typical protostellar separations larger than 0.4 pc, these separations are always larger than the associated Jeans length.