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

Abstract

Many open clusters have a deficit of observed white dwarfs (WDs) compared with predictions of the number of stars to have evolved into WDs. We evaluate the number of WDs produced in open clusters and the number of those WDS 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 over-predict 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 mass can help to distinguish between these possibilities.

9 pages, 3 figures, accepted for publication in the Astrophysical Journal

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• On the origin of the ultramassive white dwarf GD50
• Further investigation of white dwarfs in the open clusters NGC2287 and NGC3532
• A Photometric and Spectroscopic Search for White Dwarfs in the Open Clusters NGC 6633 and NGC 7063
• Hubble Space Telescope observations of the Kepler-field cluster NGC 6819. I. The bottom of the white dwarf cooling sequence
• Spectroscopic and photometric studies of white dwarfs in the Hyades
• Spectroscopic Identification of Faint White Dwarf Candidates in the Praesepe Open Star Cluster