2020ApJ...897...60P

The Sun likely formed as part of a group of stars. A close stellar flyby by one of the solar siblings is probably responsible for the sharp outer edge in the solar system's mass distribution. The frequency of such close flybys can be used to determine the likely type of birth environment of the solar system. Young stellar groups develop very quickly, expanding significantly within just a few million years. Here we model this strong dynamical development of young stellar groups and determine the resulting close flyby history. We find that solar system equivalents are predominantly produced in areas with stellar densities in the range 5 x 10⁴ pc^–3 < n_local < 2 x 10⁵ pc^–3. Remarkably, we find that only two very distinct types of stellar groups can be considered as serious contestants as the cradle of the Sun-high-mass, extended associations (M_c > 20,000 M_☉) and intermediate-mass, compact clusters (M_c < 3000 M_☉). Present-day counterparts would be the association NGC 2244 and the M44 cluster, respectively. In these two types of stellar groups, close flybys take place at a sufficiently high rate, while not being too destructive either. A final decision between these two remaining options will require the incorporation of constraints from cosmochemical studies.