ALMA resolves close companion orbiting giant red star

Close companions can influence stellar evolution in many ways. While some companions can be detected around young stellar objects, direct observational evidence of companions around asymptotic giant branch (AGB) stars or aging stars, has remained elusive.

Unveiling the mysteries of π¹ Gruis

At some 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 aging 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 Gruis 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.

ALMA reveals a hidden companion

In a paper published in 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, Ph.D. 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.”

Challenging previous models of stellar evolution

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 circularization rates may have been underestimated. Researchers believe this will open avenues for our understanding of tidal interaction physics and binary evolution.