The White Dwarf Deficit in Open Clusters: Dynamical Processes

In Galactic open clusters, there is an apparent paucity of white dwarfs compared with the number expected assuming a reasonable initial mass function and assuming that main-sequence stars with an initial mass of less than ~8 M_☉ become white dwarfs. We suggest that this lack of white dwarfs is due at least in part to dynamical processes. Nonspherically symmetric mass loss during the post-main-sequence evolution would lead to an isotropic recoil speed of a few kilometers per second for the white dwarf remnant. This recoil speed can cause a substantial fraction of the white dwarfs formed in a cluster to leave the system. We investigate this dynamical process by carrying out high-precision N-body simulations of intermediate-mass open clusters, where we apply an isotropic recoil speed to the white dwarf remnants. Our models suggest that almost all white dwarfs would be lost from the cluster if the average recoil speed were to exceed twice the velocity dispersion of the cluster.