8-12 juin, 2014


On the Formation of the Hot, Bloated White Dwarfs Discovered using Kepler

Lorne Nelson (Bishop's University)

Saul Rappaport KCSR, MIT

More than 2000 binaries have been discovered among the 160,000 stars that Kepler monitored during its search for Earth-like planetary transits. Four of these have been identified as having hot, bloated white dwarf companions (e.g., KOI 1224; Breton et al. 2012) and we now claim to have discovered two more of these peculiar objects (Rappaport et al., 2014). In each case, the orbital periods are on the order of a few days and the WDs have surface temperatures of approximately 10,000 to 20,000K. What makes them so unusual is that they have radii as large as 0.3 Rsun and thus are enormously bloated relative to their fully-degenerate radii. We believe that the primordial binaries were comprised of primary components with masses of ~2 Msun (the progenitors of the hot WDs), that the secondaries originally had masses of ~1.5 Msun, and that the system evolved via stable Roche-lobe overflow. We present detailed calculations of the evolution of the primordial binaries and the thermal evolution of the WDs and conclude that the WDs should have masses in the range of approximately 0.16 to 0.25 Msun (i.e., helium WDs). We further show that the discovery probability is very much skewed to the detection of these very low-mass WDs.

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