For decades, models of red giant winds have assumed that radiation pressure on newly formed dust grains is the main driver of mass loss in these evolved stars, which enrich interstellar space with carbon, oxygen, nitrogen and other elements. New observations of R Doradus now indicate that the dust grains in its extended atmosphere are too small to be accelerated outwards strongly enough by starlight alone to escape into interstellar space.
"We thought we had a good idea of how the process worked. It turns out we were wrong. For us as scientists, that's the most exciting result", says Theo Khouri, astronomer at Chalmers and joint leader of the study.
Red giant stars such as R Doradus are cooler, evolved descendants of Sun-like stars that lose mass through dense stellar winds, shedding their outer layers as they approach the end of their lives. These winds supply the raw material for future generations of stars and planets, yet the physical mechanism that launches and maintains them has remained uncertain despite their central role in galactic chemical evolution.
The team used the Sphere (Spectro-Polarimetric High-contrast Exoplanet REsearch) instrument on ESO's Very Large Telescope at Paranal Observatory in Chile to observe light scattered by dust grains in a region around R Doradus roughly comparable in size to the Solar System. By analysing polarised light at different wavelengths, the researchers inferred both the size distribution and composition of the grains and found that they match common stardust species such as silicates and alumina.
These detailed measurements were combined with advanced radiative transfer and dynamical simulations that track how photons from the star interact with dust in the extended atmosphere. "For the first time, we were able to carry out stringent tests of whether these dust grains can feel a strong enough push from the star's light", says Thiebaut Schirmer.
The study shows that the typical grains surrounding R Doradus are only about one ten-thousandth of a millimetre in size, which is too small for radiation pressure to accelerate them sufficiently to account for the observed wind. "Dust is definitely present, and it is illuminated by the star," says Thiebaut Schirmer. "But it simply doesn't provide enough force to explain what we see."
Earlier observations of R Doradus with the ALMA telescope have revealed giant convective structures on its surface, appearing as enormous bubbles that rise and fall through the stellar atmosphere. The new work points to these convective flows, along with stellar pulsations and episodic dust formation events, as likely contributors to the momentum that drives material away from the star.
"Even though the simplest explanation doesn't work, there are exciting alternatives to explore," says Wouter Vlemmings, professor at Chalmers and co-author of the study. "Giant convective bubbles, stellar pulsations, or dramatic episodes of dust formation could all help explain how these winds are launched."
The research forms part of the cross-disciplinary project "The origin and fate of dust in our Universe", funded by the Knut and Alice Wallenberg Foundation and carried out by scientists at Chalmers University of Technology and the University of Gothenburg. The team includes Thiebaut Schirmer, Theo Khouri, Wouter Vlemmings, Gunnar Nyman, Matthias Maercker, Ramlal Unnikrishnan, Behzad Bojnordi Arbab, Kirsten K. Knudsen and Susanne Aalto, with all co-authors based at Chalmers except Gunnar Nyman at the University of Gothenburg.
R Doradus lies about 180 light years from Earth in the southern constellation Dorado and is an asymptotic giant branch (AGB) star born with a mass similar to the Sun's. As an AGB star it is now losing the equivalent of roughly one third of Earth's mass each decade in gas and dust, while some comparable stars shed mass hundreds to thousands of times faster, illustrating the importance of understanding how such winds operate. In several billion years, the Sun is expected to evolve into an AGB star similar to R Doradus, undergoing its own high mass-loss phase and enriching the surrounding interstellar medium.
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Chalmers University of Technology
Stellar Chemistry, The Universe And All Within It
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