When a Trio of Stars Becomes a Duo: A Cataclysmic History Unraveled

Published by Adrien - Friday, April 19, 2024 - Other Languages: FR, DE, ES, PT
Source: ESO

When astronomers examined a stellar pair at the heart of a stunning cloud of gas and dust, they were in for a surprise. Star pairs are usually very similar, like twins, but in the case of HD 148937, one of the stars seemed younger and, unlike its counterpart, was magnetic.

New data from the European Southern Observatory (ESO) suggest that there were originally three stars in the system, until two of them collided and merged. This violent event created the surrounding cloud and forever changed the system's fate.

The nebula (NGC 6164/6165) surrounding HD 148937 seen in visible light.

“In my research, I was struck by the uniqueness of this system,” states Abigail Frost, an astronomer at ESO in Chile and the lead author of the study published today in Science. The system, HD 148937, is located about 3800 light years from Earth, in the direction of the Norma constellation.

It comprises two stars much more massive than the Sun and is enveloped by a beautiful nebula, a cloud of gas and dust. “A nebula surrounding two massive stars is a rarity, and it made us think that something cool must have happened in this system. The more we examined the data, the more that feeling grew.”

“After a detailed analysis, we were able to determine that the more massive star seems much younger than its companion, which doesn't make sense as they should have formed at the same time!” explains Abigail Frost. The age difference – one star appears to be at least 1.5 million years younger than the other – suggests that something must have rejuvenated the more massive star.

Another piece of the puzzle is the nebula surrounding the stars, known as NGC 6164/6165. It is about 7,500 years old, which is hundreds of times younger than the two stars. The nebula also contains very large amounts of nitrogen, carbon, and oxygen, which is surprising, as these elements are normally expected inside a star, not outside; it's as if a violent event had released them.

Wide-field view of the sky region around the nebula NGC 6164/6165.

To solve this mystery, the team gathered nine years of data from the PIONIER and GRAVITY instruments, both installed on the Very Large Telescope Interferometer (VLTI) of the ESO, located in the Atacama Desert in Chile. They also used archival data from the FEROS instrument at the ESO's La Silla Observatory.

“We believe this system originally had at least three stars; two of them were close to each other at some point in the orbit, while another star was much farther away,” explains Hugues Sana, a professor at KU Leuven in Belgium and lead observer. “The two inner stars merged violently, creating a magnetic star and throwing off matter, which lead to the formation of the nebula. The more distant star formed a new orbit with the newly merged, now magnetic, star, thus creating the binary system we see today at the center of the nebula.”

“The merger scenario was already on my mind in 2017 when I was studying observations of nebulae obtained with the European Space Agency's Herschel Space Telescope,” adds co-author Laurent Mahy, currently a senior researcher at the Royal Observatory of Belgium. “The discovery of an age difference between the stars suggests that this scenario is the most plausible, and it was only with the new ESO data that it was possible to demonstrate it.”

This scenario also explains why one of the stars in the system is magnetic while the other is not – another peculiar feature of HD 148937 spotted in the VLTI data.

At the same time, it helps resolve a long-standing mystery in astronomy: how massive stars acquire their magnetic fields. While magnetic fields are a common feature of low-mass stars like our Sun, more massive stars cannot maintain magnetic fields in the same way. Yet, some massive stars are indeed magnetic.

Astronomers had long suspected that massive stars could acquire magnetic fields during the merger of two stars. But this is the first time researchers find direct evidence of this phenomenon. In the case of HD 148937, the merger must have occurred recently. “Magnetism in massive stars should not last very long compared to the star's lifespan, so it seems we observed this rare event not long after it happened,” adds Abigail Frost.

The Extremely Large Telescope (ELT) of the ESO, currently under construction in the Atacama Desert in Chile, will allow researchers to understand in more detail what happened in the system, and perhaps reveal other surprises.
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