Stars are plasma, gas ionized as the result of extreme internal temperatures. A solitary star will be mostly spherical under the force of its own gravity. However, when stars are in close binaries, their mutual attraction distorts their shapes. The extreme version of this is the common envelope stage, wherein the stars' outer regions merge to make a single, huge double star. According to theory, that is. While nobody seriously doubts this model, all the observational evidence for common envelope binaries is indirect.

A new Science paper proposes that a class of violent astronomical events that we've observed may be due to common envelope stars, providing more direct evidence for their existence. These cataclysms are known as "red transient outbursts," and in brightness terms, they're somewhere between novas (flares of nuclear activity at the surfaces of white dwarfs) and supernovas, the violent deaths of stars. N. Ivanova, S. Justham, J. L. Avendado Nandez, and J. C. Lombardi Jr. identified a possible physical model for these outbursts, based on the recombination of electrons and ions in the plasma when the stars' envelopes merge.

The most famous red transient outburst came from the star euphoniously known as V838 Monocerotis. Before 2002, nobody had noticed the star at all, but for a brief period of time, it expanded hugely, flared brightly, and shed an impressive amount of gas and dust into surrounding space. The Hubble Space Telescope (HST) tracked the outburst over the intervening years, but despite the regular check-ins, there is no widely accepted explanation for it.

On the theoretical side of the coin, common envelope events (CEEs) are a standard part of stellar astrophysics, yet all observational evidence for them is indirect at best. According to the models, CEEs occur when one star transfers a lot of its mass onto another star, or a stellar remnant like a white dwarf or neutron star. When this happens, the recipient object is sometimes unable to accept all the mass, and the gas forms a shared envelope between the two bodies.

The shared envelope produces a huge drag on one or both stars, reducing the separation between them drastically. At the end of the process, the binary sheds a lot of its mass into space and, at least in some cases, the stars can merge catastrophically. CEEs are one possible cause of type Ia supernova explosions. But it's very hard to actually image any of this happening. The CEE stage of binary evolution is relatively brief, and since the separation between the stars is small, it's very difficult to even tell there are two stars there.

Enter the new study. The researchers examined a model for the mass ejected during a CEE, including the light it should emit. The shared envelope initially would be plasma: a neutral gas of free electrons and the positively charged atoms they were stripped from. In the model, as the mass was swept away from the binary, it would rapidly cool. This would allow some of the electrons to rejoin their atoms, a process known as recombination. The authors' model showed this happening along a roughly spherical surface within the extended shell of gas.

Recombination releases a lot of energy in the form of light, making a spherical glowing surface not unlike the surface of a star—but far larger. That could explain the impressive apparent growth of V838 Monocerotis, for example. Similarly, the temperature of recombination would result in predominantly red light emission, explaining another characteristic of red transient outbursts.

The authors compared their model both to V838 Monocerotis and V1309 Scorpii, another outburst event long believed to be a binary star merger. Their predictions matched both the luminosity and duration of the events, lending strong support to the idea that these were caused by CEEs. While their model did not predict that all red transient outbursts are CEEs, it does provide enough testable parameters to compare to the rates of outbursts and CEE-triggered supernovas.

Science, 2013. DOI: 10.1126/science.1225540 (About DOIs).