Early Evolution of Stellar Groups and Clusters: Environmental Effects on Forming Planetary Systems

This paper studies the dynamical evolution of young groups/clusters, with N = 100-1000 members, from their embedded stage out to ages of ~10 Myr. We use N-body simulations to explore how their evolution depends on the system size N and the initial conditions. Motivated by recent observations suggesting that stellar groups begin their evolution with subvirial speeds, this study compares subvirial starting states with virial starting states. Multiple realizations of equivalent cases (100 simulations per initial condition) are used to build up a robust statistical description of these systems, e.g., the probability distribution of closest approaches, the mass profiles, and the probability distribution for the radial location of cluster members. These results provide a framework from which to assess the effects of groups/clusters on the processes of star and planet formation and to study cluster evolution. The distributions of radial positions are used in conjunction with the probability distributions of the expected far-ultraviolet (FUV) luminosities (calculated here as a function of cluster size N) to determine the radiation exposure of circumstellar disks. The distributions of closest approaches are used in conjunction with scattering cross sections (calculated here as a function of stellar mass using ~105 Monte Carlo scattering experiments) to determine the probability of disruption for newly formed solar systems. We use the nearby cluster NGC 1333 as a test case in this investigation. The main conclusion of this study is that clusters in this size range have only a modest effect on forming planetary systems. The interaction rates are low, so that the typical solar system experiences a single encounter with closest approach distance b ~ 1000 AU. The radiation exposure is also low, with median FUV flux G0 ~ 900 (1.4 ergs s-1 cm-2), so that photoevaporation of circumstellar disks is only important beyond 30 AU. Given the low interaction rates and modest radiation levels, we suggest that solar system disruption is a rare event in these clusters.