Convection Reconciles the Difference in Efficiencies Between Low-Mass and High-Mass Common Envelopes

TL;DR

This paper explains the differing efficiencies of common envelope evolution in low-mass and high-mass stars through the role of convection, reconciling observational discrepancies in binary systems.

Contribution

It introduces convection as a key factor that explains the efficiency differences in common envelope phases across stellar masses.

Findings

Convection has minimal impact on high-mass stars' orbital decay.

In low-mass stars, convection enables radiative cooling, reducing efficiency.

Including convection aligns theoretical models with observed binary properties.

Abstract

The formation pathways for gravitational-wave merger sources are predicted to include common envelope (CE) evolution. Observations of high-mass post-common envelope binaries suggest that energy transfer to the envelope during the CE phase must be highly efficient. In contrast, observations of low-mass post-CE binaries indicate energy transfer during the CE phase must be highly inefficient. Convection, a process present in low-mass and high-mass stars, naturally explains this dichotomy. Using observations of Wolf-Rayet binaries, we study the effects of convection and radiative losses on the predicted final separations of high-mass common envelopes. Despite robust convection in massive stars, the effect is minimal as the orbit decays well before convection can transport the liberated orbital energy to the surface. In low-mass systems, convective transport occurs faster then the orbit decays, allowing the system to radiatively cool thereby lowering the efficiency. The inclusion of convection reproduces observations of low-mass and high-mass binaries and remains a necessary ingredient for determining outcomes of common envelopes.