Constraining the Common Envelope Evolution Using Short-Orbital Hot Subdwarf Binaries

Common envelope evolution (CEE) is vital for forming short-orbital-period compact binaries. It covers many objects, such as double compact merging binaries, Type Ia supernovae progenitors, and short-orbital period hot subdwarf binaries. The standard energy description of constraining the CEE must include accurate binding energy and common envelope (CE) ejection efficiency. However, this knowledge still needs to be improved, though progress has been made recently. Short-orbital-period hot subdwarf B star (sdB) plus white dwarf (WD) binaries are the most straightforward samples to constrain CEE physics. We apply the known orbital period-WD mass relation to constrain the sdB progenitors of seven sdB+WD binaries with a known inclination angle. The binding energies of the progenitors of sdBs are calculated using a new method that considers the remnants’ response after the CE ejection. The average CE efficiency parameter is 0.32. This is consistent with previous studies. However, the CE efficiencies need not be constant; they are a function of the initial mass ratio based on well-constrained sdB progenitor mass and evolutionary stage. Our results can be used as physical inputs for future binary population synthesis simulations of related objects.