THE ELM SURVEY. I. A COMPLETE SAMPLE OF EXTREMELY LOW-MASS WHITE DWARFS

Abstract

We analyze radial velocity observations of the 12 extremely low-mass <0.25\nMsol white dwarfs (WDs) in the MMT Hypervelocity Star Survey. Eleven of the 12\nWDs are binaries with orbital periods shorter than 14 hours; the one\nnon-variable WD is possibly a pole-on system among our non-kinematically\nselected targets. Our sample is unique: it is complete in a well-defined range\nof apparent magnitude and color. The orbital mass functions imply that the\nunseen companions are most likely other WDs, although neutron star companions\ncannot be excluded. Six of the 11 systems with orbital solutions will merge\nwithin a Hubble time due to the loss of angular momentum through gravitational\nwave radiation. The quickest merger is J0923+3028, a g=15.7 ELM WD binary with\na 1.08 hr orbital period and a <130 Myr merger time. The chance of a supernova\nIa event among our ELM WDs is only 1%-7%, however. Three binary systems\n(J0755+4906, J1233+1602, and J2119-0018) have extreme mass ratios and will most\nlikely form stable mass-transfer AM CVn systems. Two of these objects, SDSS\nJ1233+1602 and J2119-0018, are the lowest surface gravity WDs ever found; both\nshow Ca II absorption likely from accretion of circumbinary material. We\npredict that at least one of our WDs is an eclipsing detached double WD system,\nimportant for constraining helium core WD models.\n