Stars with a mass of less than 8 solar masses end their lives as planetary nebulae, structures of ionized gas thrown off by the star and heated by the stellar core. Planetary nebulae are often bipolar in shape or contain complex morphological features such as rings or spirals. Astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA) have mapped the stellar winds blowing out from red giant stars and found that their stellar winds have distinct non-spherical geometries with morphological similarities to planetary nebulae.
These ALMA images show the stellar winds around red giant stars; the blue color represents material that is coming towards us; red is material that is moving away from us. Image credit: ALMA / ESO / NAOJ / NRAO / Decin et al.
“Dying stars swell and cool to eventually become red giants. They produce stellar winds, flows of particles that the star expels, which causes them to lose mass,” said Professor Leen Decin from the Institute of Astronomy at KU Leuven and the University of Leeds and colleagues.
“Because detailed observations were lacking, astronomers have always assumed that these winds were spherical, like the stars they surround.”
“As the star evolves further, it heats up again, and the stellar radiation causes the expanding ejected layers of stellar material to glow, forming a planetary nebula.”
The astronomers observed the stellar winds of 14 stars during their red giant phase of stellar evolution and found morphologies in these winds similar to planetary nebulae.
“We noticed these winds are anything but symmetrical or round. Some of them are quite similar in shape to planetary nebulae,” Professor Decin said.
“Some stellar winds were disk-shaped, others contained spirals, and we identified cones in a third group.”
“We were very excited when we explored the first images,” said Dr. Miguel Montargès, also from the Institute of Astronomy at KU Leuven.
“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.”
This ALMA image shows the stellar wind around R Aquilae. Image credit: ALMA / ESO / NAOJ / NRAO / Decin et al.
The team proposed that a process known as binary interaction is responsible for the shape of the stellar winds of red giant stars.
As the name suggests, binary interaction involves two objects. The theory is that the stellar winds achieve their shape due to the influence of another star, or a giant planet.
“Just like how a spoon that you stir in a cup of coffee with some milk can create a spiral pattern, the companion sucks material towards it as it revolves around the star and shapes the stellar wind,” Professor Decin said.
“Our findings change a lot. 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,” she added.
“Cool aging stars are considered boring, old and simple, but we now prove that they are not: they tell the story of what comes after,” Professor Decin concluded.
“It took us some time to realize that stellar winds can have the shape of rose petals (see, for example, the stellar wind of R Aquilae).”
“But, as Antoine de Saint-Exupéry said in his book Le Petit Prince: ‘C’est le temps que tu as perdu pour ta rose, qui fait ta rose si importante’ — ‘It’s the time you spent on your rose that makes your rose so important’.”
The team’s paper was published this week in the journal Science.
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L. Decin et al. 2020. (Sub)stellar companions shape the winds of evolved stars. Science 369 (6510): 1497-1500; doi: 10.1126/science.abb1229
