A stellar finding about shapes in space

Astronomers have proposed a new explanation for the varied shapes of planetary nebulae, based on a unique set of observations of stellar winds around ageing stars. 

Contrary to common consensus, they say, stellar winds are not spherical, but have a shape similar to that of planetary nebulae. And, they suggest in a paper in the journal Science, both winds and nebulae are shaped through interaction with an accompanying star or exoplanet. 

Dying stars swell and cool to eventually become red giants, producing stellar winds (flows of particles that the star expels) that cause them to lose mass. For want of evidence otherwise, astronomers have assumed these winds are spherical, like the stars they surround. 

However, when an international team led by Leen Decin from Belgium’s KU Leuven observed the stellar winds of 14 AGB stars using the ESO’s Atacama Large Millimeter / submillimeter Array (ALMA) in Chile, they found evidence to the contrary.

200918 r aquilae
The stellar wind R Aquilae resembles the structure of rose petals. Credit: L Decin, ESO/ALMA

“We noticed these winds are anything but symmetrical or round,” Decin says. “Some of them are actually quite similar in shape to planetary nebulae.” 

The team was able to gather a large, detailed collection of observations made using the exact same method. From these they could even identify different categories of shapes. 

Some were disc-shaped, others contained spirals or cones – a clear in indication, the researchers say, that the shapes weren’t created randomly, and suggesting that they were caused by low-mass stars or even heavy planets in the vicinity of the dying star. 

They put this theory into models, and these showed that the shape of stellar winds can indeed be explained by the companions that surround them, and the rate at which the cool evolved star is losing its mass due to the stellar wind is an important parameter. 

“Our findings change a lot,” Decin says. “Since the complexity of stellar winds was not accounted for in the past, any previous mass-loss rate estimate of old stars could be wrong by up to a factor of 10.” 

The team is now doing further research to see how this might impact calculations of other crucial characteristics of stellar and galactic evolution.

They say their findings also help paint a picture of what the Sun might look like when it dies in around 7000 million years from now. 

“Jupiter, or even Saturn, because they have such a big mass, are going to influence whether the Sun spends its last millennia at the heart of a spiral, a butterfly, or any of the other entrancing shapes we see in planetary nebulae today,” Decin notes. 

“Our calculations now indicate that a weak spiral will form in the stellar wind of the old dying Sun.”

“We were very excited when we explored the first images,” adds her colleague Miguel Montargès, a co-author. “Each star, which was only a number before, became an individual by itself. Now, to us, they have their own identity. This is the magic of having high-precision observations: stars are no longer just points anymore.”

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