Common envelope

In astronomy, a common envelope (CE) is gas that contains a binary star system.

A common envelope phase is short-lived relative to the lifetime of the stars involved.

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.

[2] 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.

Their existence has been mainly inferred indirectly from presence in the Galaxy of binary systems that can not be explained by any other mechanism.

They are subset of a broader class of events called intermediate-luminosity red transients (ILRTs).

Key stages in a common envelope phase. Top: A star fills its Roche lobe. Middle: The companion is engulfed; the core and companion spiral towards one another inside a common envelope. Bottom: The envelope is ejected or the two stars merge.
Stages in the life of a binary system as a common envelope is formed. The system has mass ratio M1/M2=3. The black line is the Roche equipotential surface. The dashed line is the rotation axis. (a) Both stars lie within their Roche lobes, star 1 on the left (mass M1 in red) and star 2 on the right (mass M2 in orange). (b) Star 1 has grown to nearly fill its Roche lobe. (c) Star 1 has grown to overfill its Roche lobe and transfer mass to star 2: Roche lobe overflow. (d) Transferred too fast to be accreted, matter has built up around star 2. (e) A common envelope, represented schematically by an ellipse, has formed. Adapted from Fig. 1 of Izzard et al. (2012). [ 6 ]