Astronomers have observed for the first time a rare and important life event in the evolution of binary star pairings.
The team discovered a tight binary surrounded by an expanding shell of matter. This envelope is matter left over from a stage in stellar evolution called the common envelope phase.
This phase occurs when material from one star swells and envelops the other in a cosmic “embracing.” This leads to a mass transport from the swollen star to its companion, which can get out of control. The aftermath of this phase was something astronomers had not previously anticipated.
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“The common envelope phase is a missing link in the very long and complex chain of events that make up stellar life,” said Christian Wolf, associate professor at the Australian National University (ANU), and part of the team that made the observations has, in a Expression. (opens in new tab) “Now we’re going to start repairing that connection.”
Half of all the stars in the universe occur in binary pairs, and while the early stages of partnerships can be uneventful, things get interesting for the pairing when a star runs out of hydrogen for nuclear fusion.
The first step in these events is the collapse of the star’s hydrogen-depleted core while its outer layers “bulge out” – a process the Sun will experience in about 5 billion years – creating a red giant star. But stars in binary pairs do things differently than our lone star.
“When one of the stars grows into a red giant, it doesn’t just take up more empty space like a single star would,” Wolf said. “Instead, it ’embraces’ or devours its companion, and they appear as a star under an opaque shell. Then it gets really exciting.”
Wolf explains that the friction in the shell caused by the motion of the stars within it has profound implications for the next step in binary star evolution. “Not only does it cause heat, but it slows the stars down so that they orbit ever closer together; the hull eventually overheats and is blown away,” he said.
As a result, by the end of the shared envelope phase, the stars can be over 100 times closer together than they were at the beginning, after the heat of the process causes surrounding matter to be expelled in a violent ‘blow-out’. ‘
The outburst observed by Wolf and colleagues for the binary stars occurred about 10,000 years ago. The researchers predict that the binary stars they are observing, which are now a white dwarf and a hot subdwarf that will eventually evolve into a white dwarf, will continue to spiral together and eventually merge.
The team’s findings and the first glimpse into the aftermath of the shared envelope phase of stellar evolution could help other researchers discover more binary stars in the critical phase of their lives.
“It might be easier to spot them now that we have a clearer idea of what to look for. There might be others that have been under our noses all along,” Wolf said, adding that the results also have implications might have on other cosmic unions . “It could even help us better reconstruct gravitational-wave events like black hole mergers.”
The team’s research was published in the journal Monthly Bulletins of the Royal Astronomical Society. (opens in new tab)
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