Astronomers get a stellar wind of unprecedented detail

Astronomers have come up with explanations for the shapes of planetary nebulae. The discovery is based on a set of observations of stellar winds around the stars of old age. Contrary to popular belief, the team found that the stellar winds are not spherical, but have the same shape as the planetary nebula. The team concludes that interactions with stars or exoplanets shape both stellar winds and planetary nebulae. The findings were published in Science.

Dead stars melt away and eventually become red giants. They produce stellar winds, flow particles that blow them out of the stars, causing them to lose mass. Due to the lack of detailed observations, astronomers have always assumed that these winds were spherical, like the stars around them. As the star develops more, it heats up again and the radiation of the stars causes the extended protruding layers of stellar material to glow, creating a nebula of planets.

For centuries, astronomers have been in the dark about the extraordinary variety of shapes of planetary nebulae that have been observed. The nebulae all seem to have a certain symmetry but they are almost never round. “The sun – which will eventually become a red giant – is as round as a billiard ball, so we wondered: how could such a star produce all these different shapes?” Says the corresponding author Lane Dassin (KU Leuven).

His team watched the stellar wind around the cold red giant stars with the ALMA Observatory in Chile, the world’s largest radio telescope. For the first time, they collected a large, detailed collection of observations, each of which he made using the same method. This was crucial in comparing the data directly and excluding biases.

Astronomers were amazed at what they saw. “We saw that these winds are nothing but symmetrical or spherical,” says Professor Desin. “Some of them are very similar in shape to the nebulae of the planets.”

Allies

Astronomers can also identify different categories of shapes. “Some of the stars had wind disk shapes, others had spirals, and in the third group, we identified the cones.” This is a clear indication that the shapes are not randomly drawn. The team realized that other, lower-mass stars, or even heavier planets, were causing different causes around the dead star. This companion is too small and vague to find directly. “Just like you stir a teaspoon into a cup of coffee with some cups to create a spiral pattern, the companion sucks the material towards it as it revolves around the star and shapes the stellar wind,” explains Desin.

The team put this theory into models, and indeed: the shape of a stellar wind can be explained by the allies around it, and the fact that a cold-formed star is losing its mass due to stellar winds is an important parameter. Verdict: “All our observations can be explained by the fact that they are stellar companions.”

Until now, calculations about the evolution of stars have been based on the assumption that stars, like the aging sun, have stellar winds that are spherical. “Our findings change a lot. In the past, stellar wind complexity was not taken into account, so any previous mass-loss rate estimate of older stars could be incorrect up to a factor of 10.” The team is now doing more research to see how this affects the calculations of stellar and other critical characteristics of galactic evolution.

The future of the sun

This study also helps to visualize what the sun will look like when it dies in 7000 million years. “Jupiter or Saturn – because they have such a large mass – will affect whether the Sun spends its last millennium in the center of a spiral, butterfly or other entrance that can be seen in the planetary nebula today,” Noto decides. “Our calculations now indicate that a weak spiral will form in the stellar winds of the old dying sun.”

“We were very excited when we first discovered the images,” says co-author Miguel Montargus (KU Leuven). “Every star, which used to be just numbers, became a single person. Now, we have our own identity. This is the magic of making high-precision observations: stars are no longer just points.”

The study is part of the Atomium project, which aims to learn more about the physics and chemistry of old stars. “Cool old stars are considered boring, old and simple, but now we prove that it is not: it tells the next story. It took us a while to understand that the wind of the stars could be the shape of rose petals (see, for example, R. Aquile no stellar wind), but, as Anton de Saint-Exupery said in his book Le Petit Prince: “Si’st le temps que tu jem pardu pourvani ta gulab, qi fit ta gulab se important.” (“This is the time you spend on your rose that makes your rose so important.”))