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7 Jul 2022

A stellar embrace ending 10,000 years ago

Half of all stars are born into couples, called binaries. But unfortunately, we cannot compute their lives. This may be about to change, owing to a new discovery revealing the signature of re-born binaries. 

Single stars, such as the Sun, we understand just fine: they are round and simple, and long timesteps in computer calculations are enough so we can use a computer to fast-forward into the late stages of a star's life. Binary stars, in contrast, are difficult. They may be born wide and perform a distant orbital dance around each other during the early lives. But when one of them becomes a red giant and engulfs the other, they enter a “common-envelope phase”, where two stars orbit inside an opaque envelope. We have never seen this and we cannot compute it. Time steps need to be on scales of minutes instead of millennia, the physics is too complex, and calculations rapidly lose their connection with reality.

The common-envelope (CE) phase was postulated to explain the abundance of implausibly tight binaries in the Galaxy. Inside the CE, friction slows down the two stars and their orbit spirals in. Friction produces heat, and eventually the envelope overheats and is blown away: the “CE ejection”. Revealed from within is a much tighter binary than nature would make without CEs. Here is the newborn seed population for stellar mergers, neutron-star mergers, gravitational-wave events, supernovae, kilonovae, contact binaries, cataclysmic variables, and the list goes on.

Reconstructing anything from all those tight binaries is a problem. We understand single stars and we can put a clock on their evolutionary path. But as we do not know what exactly the CE does to a binary orbit, all the tight binaries we see are at “some” time after the CE ejection, whether that’s 10 million years or 10 billion years ago, and they have evolved since then. So, not only was the clock on the lives of the star pair reset, when they were reborn tight from a CE phase, but we even lost the clock. If we knew just how they get reborn, we could connect that to their present appearance and work out where they are along their way. But we can’t. This may start to change, owing to a new discovery.

In June 2019, A. Prof. Christian Wolf (ANU) searched for a rare kind of pulsating star and selected a few for more detailed observations. Technically, a simple query was used in the data interface of the SkyMapper Survey, which is underpinned by the National Computational Infrastructure. Then, Prof. Mike Bessell (ANU) observed the stars with the ANU 2.3m telescope and noticed that one of them was unusual. It had strong Calcium absorption from gas around the star. Dr. Christopher Onken (ANU) monitored the star with SkyMapper; its light curve revealed a compact eclipsing binary in a 3.5-hour orbit. As such a tight orbit can only have been made with a CE ejection, might the surrounding gas just be the leftovers of the CE? Mike Bessell confirmed the Calcium line did not move over time, while the Balmer lines followed the orbit.

The key to a full analysis of the system were our collaborators at Yunnan Observatories in China, with Prof. Zhanwen Han an expert on binary star evolution. Supervised by him, the PhD student Jiangdan Li analysed all available data and modelled the system. She found that the binary is a "subdwarf O star" with a white dwarf companion and an accretion disk, and nearly a solar mass between them. Crucially, the Calcium line shows the shell is expanding with 170 km/sec. Luckily, the Kepler satellite and the Zwicky Transient Facility had observed the object as well. As a result, we see the orbital period shrinking by 0.1 sec over six years, which requires friction on residual material. The best model now is that a common envelope of over a solar mass was probably ejected about 10,000 years ago. 

This means that now we know a tight binary that has been reborn from a common-envelope ejection just moments ago by astronomy's standards. If we can find more such systems, then we will learn the typical features and diversity of re-born tight binaries and will be able to understand much better where on their journey the already known tight binaries are. A missing link in the chain of events in the lives of binary stars will be fixed.

This discovery is now published in a paper led by Jiangdan Li (Yunnan Observatories).