Testing Stellar Evolution Theory with the Binary Fraction of Low-Mass White Dwarfs

We describe spectroscopic observations of 21 low-mass (< 0.45M⊙) white dwarfs from the PalomarGreen Survey obtained over a period of four years. We use both radial velocity analysis and infrared photometry to identify binary systems, and find that the fraction of single, low-mass white dwarfs is 30 with an uncertainty of 18%. We discuss the inverse relationship between the mass and the binary fraction of white dwarfs and compare this relationship to existing theoretical models. We also compare the period distribution of 0.2 and 0.4 solar mass white dwarfs and find that lower mass white dwarfs have systematically shorter orbital periods. Our results support the idea that single low-mass white dwarfs can be formed through enhanced mass-loss from a metal-rich progenitor star. This work is supported in part by the NSF REU and DOD ASSURE programs under NSF grant no. 0754568 and by the Smithsonian Institution.