|
In astronomy, a common envelope (CE) is gas that contains a binary star system. The gas does not rotate at the same rate as the embedded binary system. A system with such a configuration is said to be in a common envelope phase or undergoing common envelope evolution. During a common envelope phase the embedded binary system is subject to drag forces from the envelope which cause the separation of the two stars to decrease. The phase ends either when the envelope is ejected to leave the binary system with much smaller orbital separation, or when the two stars become sufficiently close to merge and form a single star. A common envelope phase is short-lived. Evolution through a common envelope phase with ejection of the envelope can lead to the formation of a binary system composed of a compact object with a close companion. Cataclysmic variables, X-ray binaries and systems of close double white dwarfs or neutron stars are examples of systems of this type which can be explained as having undergone common envelope evolution. In all these examples there is a compact remnant (a white dwarf, neutron star or black hole) which must have been the core of a star which was much larger than the current orbital separation. If these systems have undergone common envelope evolution then their present close separation is explained. Short-period systems containing compact objects are sources of gravitational waves and Type Ia supernovae. Predictions of the outcome of common envelope evolution are uncertain. 〔 〕 〔 〕 〔 〕 A common envelope is sometimes confused with a contact binary. In a common envelope binary system the envelope does not generally rotate at the same rate as the embedded binary system; thus it is not constrained by the equipotential surface passing through the L2 Lagrangian point.〔 In a contact binary system the shared envelope rotates with the binary system and fills an equipotential surface. == Formation == A common envelope is formed in a binary star system when the orbital separation decreases rapidly or one of the stars expands rapidly.〔 The donor star will start mass transfer when it overfills its Roche lobe and as a consequence the orbit will shrink further causing it to overflow the Roche lobe even more, which accelerates the mass transfer, causing the orbit to shrink even faster and the donor to expand more. This leads to the run-away process of dynamically unstable mass transfer. In some case the receiving star is unable to accept all material, which leads to the formation of a common envelope engulfing the companion star.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「common envelope」の詳細全文を読む スポンサード リンク
|