Binary Star System found Ejecting Common Envelope

Astronomers have announced the discovery of a tight binary star system that is in the process of ejecting a shell of material that envelopes them both. These new findings are helping complete the picture, and add to the observational evidence, of a stage of evolution between binary stars. 

The system features a compact remnant object, known as a white dwarf star, and a hot class-O Subdwarf companion in a tight orbit that lasts under four hours. As the subdwarf orbits the compact remnant in proximity, it spills matter onto it via an accretion disc that is likely surrounding the white dwarf. 

White dwarfs are what is left behind when stars like our Sun complete the main sequence phase of their lives - that is, they are no longer fusing hydrogen into heavier elements and have puffed away their outer layers. All that is left behind is the hot, cinder core of the former star. The subdwarf is itself a very hot type of star, with the temperature of this one measured at around 54,000 degrees Celsius. Its mass however is low, roughly about half of that of our Sun’s. 

The discovered envelope that surrounds both objects, has a radius of about 80% the diameter of the Sun and a thickness of about 20% of the solar radius. From the study, astronomers believe this blowout happened approximately 10,000 years ago. 

The international research effort, published in the journal Monthly Notices of the Royal Astronomical Society, involved scientists from the Australian National University, including co-author of the study, Associate Professor Christian Wolf who discussed how important it is to be catching the system during this phase of its evolution. 

“In the early stages, two stars often circle around each other with nothing much happening," Associate Professor Wolf said.  

"But when one of the stars grows into a red giant, it does not just claim more empty space the way a single star will do.” 

“Instead, it ’embraces’ or engulfs its companion, and they appear as one star under an opaque envelope. That's when things get really exciting. The friction of their motion inside the envelope profoundly alters what happens next for the stars. It not only causes heat but slows down the stars, so they spiral into an ever-tighter orbit; the envelope finally overheats and gets blown away.” 

Many of the stars we observe across the Galaxy belong in a binary pair - it’s a very common configuration. In many of these cases, as the stars evolve, they have an influence on each other, and can sometimes change the course of their shared future. 

It all begins when the first of the two stars stops burning hydrogen fuel in its core. If the star is massive enough, a series of other elements, like helium and carbon are next in line to begin fusing. This creates such high temperatures that it causes the outer layers of the star to expand into a giant phase. 

This expansion can often engulf both stars, changing their angular momentum and distance from each other, bringing them into tighter, more circular orbits. Eventually, this star finishes its giant phase of evolution, and the only thing left behind is the white dwarf. 

In some cases, the secondary star can also (after some time) undergo the same process of helium burning, causing it to expand into a giant and engulf the white dwarf. This again will change the orbital parameters, as the friction from the envelope changes the momentum of the stars. 

In both these cases, the envelope gets heated up in the process and eventually gains enough energy to blow out, and evaporate. It is during this phase, that this new discovery has been made. 

“The common envelope phase is a missing link in the very long and complex chain of events making up the life of stars. Now we are starting to fix that link," Associate Professor Wolf said. 

"It could even help us better reconstruct gravitational wave events, such as black hole mergers."  

The researchers believe this first glimpse of the phenomenon could lead to the discovery of more stars at this critical stage in their life. 

"It may be easier to recognise them now we have a clearer idea of what to look for. There may be others that have been under our nose the whole time," Associate Professor Wolf said.

The implications of such events can lead to stars that radiate gravitational waves, and in the process are drawn closer together over time. In this latest paper, astronomers have reported that the orbit of this system has shrunk by 0.1 seconds in six years due to the emissions of this gravitational radiation. Whilst this might not seem a lot in our everyday terms, when considering the millions of year timescales that stars live on, this equates to one minute every 600 years.