Common envelope

A common-envelope phase occurs when unstable binary interaction engulfs both stellar cores in one envelope. Orbital energy and angular momentum may eject the envelope, leaving a compact binary, or the cores may merge.

The common-envelope energy formalism equates a fraction ${\alpha }_{\mathrm{CE}}$ of the released orbital energy to the envelope binding energy, often written $E_{\mathrm{bind}}=G{M}_d{M}_{\mathrm{env}}/(\lambda R_d)$. It estimates the post-envelope separation but compresses uncertain hydrodynamics into ${\alpha }_{\mathrm{CE}}$ and $\lambda$.

The double-degenerate scenario forms a close pair of carbon--oxygen white dwarfs, usually through mass transfer and one or more common-envelope phases. Gravitational radiation then shrinks the orbit until merger or unstable transfer triggers thermonuclear disruption.