When stars like our Sun end their lives, they throw off their outer layers through a dramatic stellar wind.

This stellar wind is a key source of dust in the universe and enriches the interstellar medium of the Galaxy with heavy elements that are produced during the star’s death throes.

W Aquilae (W Aql) is one star going through this process. Unlike our future Sun, however, W Aql is not dying alone, with a longer-lived sunlike star keeping it company as they orbit around each other. Before work led by ASTRO 3D Associate Investigator Taïssa Danilovich, little was known about the orbit of the two stars, except that it was probably centuries long. Now, thanks to evidence of the two stars passing very close to each other — a process that left behind a chemical imprint — researchers can determine that the orbit has a very elliptical shape, as well as taking around a millennium to complete one cycle.

A variety of molecules and dust make up the stellar wind of the dying star, and the researchers found that some of these were, unexpectedly, found only on one side of the system. Such an arrangement could only have happened with the aid of another hotter star.

“Once we saw the silicon nitride emission off to one side, we knew something unusual was going on,” says lead author Dr Taïssa Danilovich.

The research team ran hydrodynamical simulations to understand how the sunlike star was shaping the stellar wind of the dying star. These showed that if the W Aql system was viewed side on, then something like concentric rings would form in the wind. These are visible in data taken with the ALMA telescope in Chile, and combined with observations from the SPHERE instrument on the Very Large Telescope and older observations from the Hubble Space Telescope, the team was able to understand the orbit well.

Movie of the hydrodynamic model by second author, Jolien Malfait, who is a PhD student in Belgium.

Now that the study of W Aquilae has shown that the chemical signatures of past stellar flybys can be detected with ALMA, a similar technique can be used to find the hidden stellar companions of the dustiest dying stars. Even though our Sun is a single star, about half of the stars like it in our galaxy are found in pairs (or even triple systems). This study on the W Aquilae system is a key step in understanding how stellar companions change the lives and deaths of their nearest neighbours and what that might mean for the Galaxy as a whole.

The study has been published in Nature Astronomy.

Image: The ALMA telescope shows how the gas around the dying star in W Aquilae has been shaped by interactions with a sunlike companion star. Both stars are located in the bright central region of this image. Credit: ALMA

Media releases: Monash University, KU Leuven