Common envelope

In astronomy, a common envelope is a massive cloud of gas that surrounds a binary star system. A binary system consists of two stars orbiting one another. During a common-envelope phase, this shared gas does not rotate at the same rate as the stars inside it. This phase is a critical part of stellar evolution. It can fundamentally change the future of the stars involved. It can lead to the creation of very close binary systems or even single stars.

The process usually begins when one star in a binary system grows rapidly. As the star expands, it reaches a boundary called its Roche lobe. The Roche lobe is the specific region of space around a star where its gravity holds onto its material. When the star overfills this lobe, it begins mass transfer. This means gas spills from the expanding star onto its companion. This often creates a runaway process known as dynamically unstable mass transfer. The orbit shrinks, causing the star to overflow its Roche lobe even more. This cycle accelerates until the gas builds up around both stars, creating the common envelope.

Inside this thick gas cloud, the system contains two distinct objects. These are the core of the original donor star and its companion star. The donor's core does not expand along with the rest of the star's outer layers. As these two objects move through the gas, they experience drag forces. Drag is a force that resists motion. This drag causes the two objects to lose orbital energy. As they lose energy, they begin a spiral-in. This means they move into a much closer orbit. Interestingly, as the orbit shrinks, their orbital velocities actually increase.

The loss of orbital energy has a direct effect on the envelope itself. The energy lost by the orbiting objects is transferred to the gas. This process heats up and expands the common envelope. The entire common-envelope phase is quite short-lived compared to the total lifetimes of the stars. The phase eventually reaches an end through one of two primary outcomes. The first outcome is the ejection of the envelope into space. If the gas is expelled, the two stars remain as a binary system with a much smaller orbital separation. The second outcome is a merger. In a merger, the two objects become close enough to combine into a single star.

Scientists have identified several types of systems that likely resulted from this process. These include cataclysmic variables and X-ray binaries. We also see systems composed of close double white dwarfs or neutron stars. These systems all contain a compact remnant. A compact remnant is the dense core left behind after a star changes. These might be white dwarfs, neutron stars, or black holes. In these cases, the core was originally much larger than the current distance between the stars. The common-envelope phase explains how they reached such a close separation.

Observing these events is difficult because they happen so quickly. However, astronomers can infer their existence through specific signals. A common-envelope event should be brighter than a typical nova but fainter than a supernova. The photosphere, or the visible surface, of the envelope is relatively cool, around 5,000 K. This temperature causes the system to emit a red spectrum. Because the envelope is so large, it has a high luminosity. It can be as bright as a red supergiant. These events are sometimes called luminous red novae, or LRNe. They are a subset of intermediate-luminosity red transients.

There are several notable examples that help us understand this phenomenon. The event V1309 Scorpii is a possible example of the ejection of a whole envelope. Other observations, like M31 RV and V838 Monocerotis, are considered possible star mergers. We have also seen a planetary nebula that likely came from a common-envelope binary. These close-separation systems are also important for other areas of science. Short-period systems containing compact objects are sources of gravitational waves. They can also lead to Type Ia supernovae. Understanding the common envelope helps us understand the violent and complex life cycles of stars.