On the alpha formalism for the common envelope interaction

Abstract

The {\alpha}-formalism is a common way to parametrize the common envelope interaction between a giant star and a more compact companion. The {\alpha} parameter describes the fraction of orbital energy released by the companion that is available to eject the giant star's envelope. By using new, detailed stellar evolutionary calculations we derive a user-friendly prescription for the {\lambda} parameter and an improved approximation for the envelope binding energy, thus revising the {\alpha} equation. We then determine {\alpha} both from simulations and observations in a self consistent manner. By using our own stellar structure models as well as population considerations to reconstruct the primary's parameters at the time of the common envelope interaction, we gain a deeper understanding of the uncertainties. We find that systems with very low values of q (the ratio of the companion's mass to the mass of the primary at the time of the common envelope interaction) have higher values of {\alpha}. A fit to the data suggests that lower mass companions are left at comparable or larger orbital separations to more massive companions. We conjecture that lower mass companions take longer than a stellar dynamical time to spiral in to the giant's core, and that this is key to allowing the giant to use its own thermal energy to help unbind its envelope. As a result, although systems with light companions might not have enough orbital energy to unbind the common envelope, they might stimulate a stellar reaction that results in the common envelope ejection.