Key points
- An international team of scientists have been able to calculate the orbit of companions around asymptotic giant branch (AGB) star - π1 Gruis
- Scientists used the resolving power of the Atacama Large Millimeter/submillimeter Array (ALMA) to conduct this research
- This discovery furthers our understanding of the Keplerian motion of close companions to giant stars.
Close companions can influence stellar evolution in many ways. Whilst some companions can be detected around young stellar objects, direct observational evidence of companions around asymptotic giant branch (AGB) stars or ageing stars, has remained elusive.
530 light-years from Earth, a red giant star called π1 Gruis (affectionately known as pi-one-Gru by astronomers and scientists) has long been a mystery for scientists. Known as an asymptotic giant branch (AGB) star, π1 Gruis, was once like the Sun but is now an ageing star at the end of its life, having cooled as it swelled to over 400 times the size of the sun, it has now become a red giant star.
These stars can make new elements, undergo pulsations on the timescale of days to years, and lose a lot of their mass – throwing off an Earth-mass equivalent of material over a four year period – before ending their lives as planetary nebulae or in layman's terms, ionized gas ejected from red giant stars late in their lives.
π1 Gru shines a few thousand times more brightly than our Sun. Which makes detecting any close companion objects to these stars exceptionally difficult, since they can outshine them and also vary in brightness.
In a paper recently published by Nature Astronomy, an international team of scientists have been able to directly show the orbit of a companion around π1 Gruis using the resolving power of the Atacama Large Millimeter/submillimeter Array (ALMA) – an astronomical interferometer of 66 radio telescopes in the Atacama Desert of northern Chile.
Yoshiya Mori, PhD Candidate in Astrophysics at Monash University was responsible for making detailed comparisons between the observed properties of π1 Gruis and state-of-the-art Monash University stellar evolution models, along with models from existing literature that predict how these stars pulsate.
“A key part of understanding the orbit of the companion is knowing the mass of the AGB star. Our team helped better constrain this mass by using its observed luminosity and pulsation characteristics to find the best suited stellar model,” Mr Mori said.
“This research is especially interesting, as throwing a close companion into the mix could possibly wreak further havoc on the already complicated processes surrounding these stars.”
Despite earlier predictions of an elliptical orbit for the companion star, the research has observed an almost perfectly round orbit. This suggests the orbit evolves faster than previously thought. The result calls for adjustments to existing models of the final life stage of giant stars with companions.
Project lead, Mats Esseldeurs from KU Leuven says our Sun will one day go through such a stage as well.
“Understanding how close companions 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 star itself,” said Mr Esseldeurs.
The analysis suggests that model predicted circularisation rates may have been underestimated. Researchers believe this will open avenues for our understanding of tidal interaction physics and binary evolution.
This project was a collaboration between KU Leuven, Monash University, CEA Paris-Saclay, and other international partners.
Read the research paper: https://doi.org/10.1038/s41550-025-02697-2
RESEARCHERS
Yoshiya Mori, PhD Candidate in Astrophysics at Monash University
Dr Taïssa Danilovich, Senior Research Fellow and ARC DECRA Fellow in the School of Physics and Astronomy at Monash University
Professor Amanda Karakas, School of Physics and Astronomy at Monash University
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