The Formation of EL CVn-type Binaries

Abstract

EL CVn-type binaries contain an A-type dwarf star and a very low mass ($\sim 0.2M_{\odot }$) helium white dwarf precursor (pre-He WD). Recent surveys and observations have discovered dozens of them, and showed that some pre-He WDs have multi-periodic pulsations, opening up the possibility of using asteroseismology to study the interior structure of these stars. We have studied the formation of EL CVn-type binaries to understand the physics of their formation and give the properties and the space density of this population of stars in the Galaxy. We use a simple analysis to show that EL CVn binaries cannot be produced by rapid common envelope evolution because this process leads to merging of the components when a giant has such a low-mass core. We find that EL CVn-type binaries can be produced by long-term stable mass transfer between low-mass stars in close binary systems. We have comprehensively studied this formation channel and the characteristics of the resulting population of EL CVn-type binaries from more than 65,000 runs of binary evolution with primary star masses between 0.9 and $2.0M_{\odot }$ and assuming composition appropriate for Population I stars. The theoretical evolutionary tracks, mass ratios and the WD mass-period ($M_{\mathrm{WD}}-P$) relation are generally in agreement with observations. We conclude that many more EL CVn-type binaries remain to be discovered and that these binaries will be predominantly low-mass systems in old stellar populations.

17 pages, 20 figures

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• WASP 1628+10 - an EL CVn-type binary with a very-low-mass stripped-red-giant star and multi-periodic pulsations
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