The Formation of EL CVn-type Binaries

TL;DR

This paper investigates the formation mechanisms of EL CVn-type binaries, demonstrating that they form via stable mass transfer in close binaries rather than common envelope evolution, and predicts their properties and population in the Galaxy.

Contribution

It provides a comprehensive analysis of the formation of EL CVn binaries through stable mass transfer, supported by extensive binary evolution simulations, and aligns theoretical predictions with observations.

Findings

EL CVn binaries cannot form via rapid common envelope evolution.

Stable mass transfer explains the formation of EL CVn binaries.

Predicted properties match observed systems and suggest many remain undiscovered.

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 ($M_{\rm WD}-P$) 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.