The impact of unresolved binaries on searches for white dwarfs in open clusters

Many open clusters have a deficit of observed white dwarfs (WDs) compared with predictions of the number of stars that evolve into WDs. We evaluate the number of WDs produced in open clusters and the number of those WDs that are detectable using photometric selection techniques. This calculation includes the effects of varying the initial mass function (IMF), the maximum progenitor masses of WDs, and the binary fraction. Differences between the calculated number of observable WDs and the actual number of WDs observed in a specific cluster then indicate the true deficit of WDs that must be explained through effects such as dynamical evolution of the cluster or close binary evolution. Observations of WDs in three open clusters, the Hyades, Pleiades, and Praesepe, are compared to the calculated observable populations in those clusters. The results suggest that a large portion of the white dwarf deficit may be explained by the presence of WDs in unresolved binary systems. However, the calculated WD populations still overpredict the number of observable WDs in each cluster. While these calculations cannot determine the cause of this residual white dwarf deficit, potential explanations include a steep high-mass IMF, dynamical evolution of the cluster, or an increased likelihood of equal-mass components in a binary system. Observations of complete WD samples in open clusters covering a range of ages and masses can help to distinguish between these possibilities.