EL CVn-type binaries - discovery of 17 helium white dwarf precursors in bright eclipsing binary star systems
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The star 1SWASP J024743.37-251549.2 was recently discovered to be a binary star in which an A-type dwarf star eclipses the remnant of a disrupted red giant star (WASP 0247-25 B). The remnant is in a rarely observed state evolving to higher effective temperatures at nearly constant luminosity prior to becoming a very low mass white dwarf composed almost entirely of helium, i.e. it is a pre-helium white dwarf (pre-He-WD). We have used the photometric database from the Wide Angle Search for Planets (WASP) to find 17 eclipsing binary stars with orbital periods P = 0.7-2.2 d with similar light curves to 1SWASP J024743.37-251549.2. The only star in this group previously identified as a variable star is the brightest one, EL CVn, which we adopt as the prototype for this class of eclipsing binary star. The characteristic light curves of EL CVn-type stars show a total eclipse by an A-type dwarf star of a smaller, hotter star and a secondary eclipse of comparable depth to the primary eclipse. We have used new spectroscopic observations for six of these systems to confirm that the companions to the A-type stars in these binaries have very low masses (≈0.2MΘ). This includes the companion to EL CVn which was not previously known to be a pre-He-WD. EL CVn-type binary star systems will enable us to study the formation of very low mass white dwarfs in great detail, particularly in those cases where the pre-He-WD star shows non-radial pulsations similar to those recently discovered in WASP0247-25 B.
Maxted , P F L , Bloemen , S , Heber , U , Geier , S , Wheatley , P J , Marsh , T R , Breedt , E , Sebastian , D , Faillace , G , Owen , C , Pulley , D , Smith , D , Kolb , U , Haswell , C A , Southworth , J , Anderson , D R , Smalley , B , Cameron , A C , Hebb , L , Simpson , E K , West , R G , Bochinski , J , Busuttil , R & Hadigal , S 2014 , ' EL CVn-type binaries - discovery of 17 helium white dwarf precursors in bright eclipsing binary star systems ' Monthly Notices of the Royal Astronomical Society , vol 437 , no. 2 , pp. 1681-1697 . DOI: 10.1093/mnras/stt2007
Monthly Notices of the Royal Astronomical Society
© 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society
This work was supported by funding from the European Research Council under the European Community’s Seventh Framework Programme (FP7/2007–2013)/ERC grant agreement no 227224 (PROSPERITY), as well as from the Science and Technology Facilities Council (grant numbers ST/I001719/1, ST/J001384/1)
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