Interacting binary star
Binary stars where one star transfers mass to the other.
In an interacting binary star system, at least one of the two stars has expanded to fill or surpass its Roche lobe—a configuration called a semidetached binary. This causes material to stream from the donor star onto its companion, the accretor. If the accretor is a compact object, the inflowing matter may form an accretion disk. These systems are often highly dynamic and variable, and they frequently produce cataclysmic outbursts.
A common setup involves a compact object (well within its own Roche lobe) paired with an evolved giant star. When the compact object is a white dwarf, matter accreting from the giant can push the white dwarf’s mass past the Chandrasekhar limit. This triggers runaway thermonuclear reactions, leading to a Type I supernova.
One example is R Canis Majoris, where the secondary star is thought to have overflowed its Roche lobe, transferring mass to the primary. This has caused the secondary to evolve early onto the subgiant branch and has exposed helium-rich material on the primary’s surface, making it brighter and hotter than typical for its mass.
In close binaries where the primary overflows its Roche lobe before the helium flash, mass loss can produce a helium white dwarf as light as 0.1 solar masses. The same process can occur when the companion is a millisecond pulsar. Evolutionary models suggest that most closely orbiting white dwarf companions in such binaries are helium-based.
- Type
- semidetached binary
- Example
- R Canis Majoris
- Minimum helium white dwarf mass
- 0.1 M☉
Lore & Background
A common type of interacting binary star is one in which one component is a compact object well within its Roche lobe, while the other is an evolved giant star. If the compact object is a white dwarf, accretion of material from the evolved star onto the white dwarf's surface may result in its mass increasing to beyond the Chandrasekhar limit, leading to runaway thermonuclear reactions and a Type I supernova. An example is R Canis Majoris, where the secondary star is thought to have exceeded its Roche lobe and transferred mass to the primary, causing the primary to burn brighter and have a higher effective temperature than expected for its mass, and exposing helium-rich material on its surface. In close binary systems where the first Roche lobe overflow of the primary occurs prior to helium flash, the shedding of mass can leave behind a helium white dwarf with a mass as low as 0.1 M☉. The same scenario works when the companion is a millisecond pulsar. Evolutionary models of binaries suggest that a majority of such closely orbiting white dwarf companions will be helium-based.
Reader's Guide
Interacting binary stars are notable because they are common sources of cataclysmic outbursts, including Type I supernovae when a white dwarf accretes enough mass to exceed the Chandrasekhar limit. The example of R Canis Majoris illustrates how mass transfer can alter the evolution of both stars, causing the primary to appear brighter and hotter than its mass would normally allow. The article also describes how such systems can produce helium white dwarfs with masses as low as 0.1 M☉, and that evolutionary models indicate most closely orbiting white dwarf companions in these binaries are helium-based. These systems thus play a key role in understanding stellar evolution, supernova progenitors, and the formation of compact objects.
Did You Know?
- Interacting binary stars are also known as semidetached binaries.
- If the accretor is a compact star, an accretion disk may form.
- Accretion onto a white dwarf can lead to a Type I supernova.
- R Canis Majoris is an example of an interacting binary star.
More in Binary and Multiple Stars 1-24
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