Common envelope binary
A binary system embedded in a non-rotating common gaseous envelope.
A common envelope binary is a binary-star system embedded within a shared gaseous envelope that does not rotate at the same rate as the embedded binary. This configuration, known as a common-envelope phase, is notable because drag forces cause the orbital separation to shrink, potentially leading to envelope ejection or stellar merger. The phase is short-lived relative to the lifetimes of the stars involved and is thought to explain the formation of close binary systems containing compact objects, such as cataclysmic variables, X-ray binaries, and double white dwarfs or neutron stars.
- Envelope temperature
- about 5,000 K
- Expansion velocity range
- 200–1000 km/s
- Total radiated energy range
- 10^38–10^40 J
- Luminosity comparison
- on the order of that of a red supergiant
- Brightness comparison
- brighter than typical novae but fainter than typical supernovae
Lore & Background
A common envelope forms when the orbital separation of a binary decreases rapidly or one star expands rapidly, leading to dynamically unstable mass transfer. The donor star overfills its Roche lobe, causing the orbit to shrink further and accelerating mass transfer in a runaway process. If the receiving star cannot accept all the material, a common envelope engulfs the companion. The donor's core does not participate in the envelope expansion; the envelope contains two objects: the donor's core and the companion star. These objects lose orbital energy due to drag, spiraling inward and heating the envelope. The phase ends when the envelope is expelled or the objects merge.
Observational manifestations of common-envelope events are difficult to detect directly. Their existence is inferred from binary systems that cannot be explained otherwise. Such events should begin with a sharp rise in luminosity, followed by a months-long plateau of constant luminosity powered by hydrogen recombination, then a rapid decline. The photosphere is relatively cool at about 5,000 K, emitting a red spectrum, but the large size yields a luminosity comparable to a red supergiant. Several observed events, called luminous red novae, fit this description, including M85 OT2006-1 (possible envelope ejection), V1309 Scorpii (possible merger), M31 RV, V838 Monocerotis, and Ou 5 (a planetary nebula with a common-envelope progenitor).
Reader's Guide
Common-envelope evolution is significant because it provides a mechanism to explain the existence of close binary systems containing compact objects, such as white dwarfs, neutron stars, or black holes, where the current orbital separation is much smaller than the original donor star's size. Without this phase, such tight orbits would be difficult to account for. Systems that may have undergone common-envelope evolution include cataclysmic variables, X-ray binaries, and close double white dwarfs or neutron stars. These short-period systems are important sources of gravitational waves and Type Ia supernovae. However, predictions of the outcome of common-envelope evolution remain uncertain. The phase is distinct from a contact binary: in a common-envelope binary, the envelope does not generally rotate with the binary and is not constrained by the equipotential surface through the L2 Lagrange point, whereas in a contact binary the shared envelope rotates with the system and fills an equipotential surface. The legacy of common-envelope studies lies in explaining the formation of compact binaries that are key to modern astrophysics, though direct observational confirmation remains challenging.
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