Stripped red giant cores in eclipsing binary star systems

Abstract

Red giant stars can be stripped of their outer layers by stellar collisions or mass transfer in binary star systems such as low mass X-ray binaries. If the star is stripped on or before its first ascent of the red giant branch it will eventually become a very low mass white dwarf composed almost entirely of helium. Very low mass white dwarfs are well known in binary milli-second pulsars and many have recently been found in surveys such as the Sloan Digital Sky Survey, but the precursor phase during which the remnant evolves to higher effective temperature at nearly constant luminosity has rarely been observed. The cooling timescale for very low mass white dwarfs is very uncertain because they are thought to be born with thick hydrogen envelopes which can sustain weak but stable p-p shell burning, but unstable phases of CNO burning (shell flashes) can remove this hydrogen envelope. The predicted number of shell flashes (if any) is dependent on the mass and composition of the star and other details of the models used. In this talk I present new observations of a bright eclipsing binary star recently discovered in the WASP archive in which a stripped red giant is eclipsed by an A-type dwarf star. These observations were used to derive precise masses and radii for both stars and have be used to test the formation scenario outlined above. In addition, I present the main characteristics of 17 new eclipsing binary stars that are also likely to contain the precursors of very low mass white dwarfs.