For the first time, researchers have directly observed a star orbiting very close to a giant star at the end of its life. This discovery provides the first conclusive evidence that such companions do indeed exist and that they follow a perfectly circular orbit around their host star.
AI-generated illustration of a binary star system (credit: Mats Esseldeurs)
At the end of their life cycle, stars like our Sun transform into pulsating red giants that expel enormous amounts of gas and dust. In doing so, they contribute to the creation and dispersion of new chemical elements in the Universe, thus becoming the engine of the "cosmic cycle". This final stage of stellar evolution is still partially understood.
A key question remains: how does a close companion star influence the mass loss and evolution of the giant? Hidden by a dense and dusty environment, these companions have long escaped detection. Thanks to new observations, scientists have now discovered one of these companions and revealed how it moves around the giant star.
The study focuses on π¹ Gruis, an aging giant star more than 7,000 times more luminous than our Sun. This star ejects large amounts of gas and dust into space via a powerful stellar wind. Indirect clues had long suggested the presence of a close companion star, but it remained hidden by the dusty environment and the strong turbulence surrounding the giant.
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Giant stars are difficult to observe. Their environment is so turbulent and dusty that even close companion stars become practically invisible", explains Mats Esseldeurs, a PhD student jointly supervised by KU Leuven and CEA-Paris Saclay. "
Thanks to new techniques, we have been able to directly track their motion for the first time."
Images of the stellar system ? π¹ Gru, taken with the ALMA observatory in July 2019 and October 2023. The aging and expanding red giant is marked with a white cross, and its companion star with a gray cross. The orange and yellow ellipses at the bottom indicate the resolution (or "sharpness") of the ALMA observations in 2019 and 2023. The colors in the images represent the intensity of the detected signal. ALMA telescope pierces the dust
The team used the ALMA telescope in Chile, a network of 66 radio telescopes ideal for imaging gas and dust structures in detail. π¹ Gruis was observed in 2019 and again in 2023. These repeated measurements allowed scientists to confirm, for the first time, the presence of a companion star moving in an almost perfectly circular orbit around the giant.
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It's as if we were able to watch the dance of two stars live", explains Professor Leen Decin, an astronomer at KU Leuven. "
This direct observation changes everything. Until now, we could only suspect the existence of a companion; now, we actually see it orbiting the giant."
Trajectory of the evolved red giant and its companion, projected onto the sky plane. The trajectory of the giant (= M1) is represented by a white dashed line, and that of its companion by a dotted line. Time progresses from right to left, as indicated at the top (in years). The relative oscillations of the two trajectories are due to orbital motion. The observation data appear in color and include measurements from the Hipparcos and Gaia space missions, as well as from the ground-based ALMA telescope in Chile. New perspectives on evolved stars
The result is surprising: theory predicted an elliptical orbit for the companion star, but we observe an almost perfectly circular orbit. This indicates that the orbit evolves faster than previously thought due to the combined action of the giant star's mass loss and tidal interactions with its companion.
This result calls for adjustments to existing models describing the final life phase of giants with companions. "
Our Sun will one day go through a similar stage", adds Mats Esseldeurs. "
Understanding how close companion stars behave under these conditions helps us better predict what will happen to the planets around the Sun, and how the companion influences the evolution of the giant itself."