2024-04-19 04:00:12
When astronomers observed a stellar pair at the heart of an impressive cloud of gas and dust, they were surprised. Pairs of stars are usually very similar, like twins, but in the case of HD 148937, one of the stars appears younger and, unlike the other, is magnetic.
New data from theEuropean Southern Observatory (ESO) suggest that there were originally three stars in the system, until two of them came into collision and merge. This violent event created the cloud that surrounds it and forever changed the destiny of the system.
“During my research, I was struck by the particularity of this system,” explains Abigail Frost, astronomer at ESO in Chile and lead author of the study published today in Science. The system, HD 148937, is located approximately 3800 light years from Earth, in the direction of constellation Norma. It is made up of two stars much more massive than the Sun and surrounded by a magnificent nebula, a cloud of gas and dust. “A nebula surrounding two massive stars is a rarity, and it gave us the impression that something cool must have happened in this system. As we looked at this data, this impression did not than grow.”
“After a detailed analysis, we were able to determine that the most massive star appears much younger than its companion, which is not logical since 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 made the more massive star younger.
Another piece of the puzzle is the nebula that surrounds the stars, known as NGC 6164/6165. It is 7,500 years old, hundreds of times younger than the two stars. The nebula also has very large amounts of nitrogen, carbon, and oxygen, which is surprising because these elements are normally expected inside a star, not outside; it is as if a violent event had freed them.
To unravel this mystery, the team gathered nine years of data from the PIONIER and GRAVITY instruments, both installed on ESO’s Very Large Telescope Interferometer (VLTI), located in Chile’s Atacama Desert. They also used archival data from the FEROS instrument at ESO’s La Silla Observatory.
“We think that this system originally had at least three stars; two of them must have been close to the orbit at some point, while another star was much further away,” explains Hugues Sana, professor at KU Leuven, Belgium, and principal responsible for the observations. “The two inner stars merged violently, creating a magnetic star and releasing matter, which gave birth to the nebula. The more distant star formed a new orbit with the newly merged star, which became magnetic, creating the binary we see today at the center of the nebula.”
“The merger scenario was already running through my head in 2017 when I was studying observations of nebulae obtained with the Herschel space telescope of theEuropean Space Agency“, adds co-author Laurent Mahy, currently searcher senior at theRoyal Observatory of Belgium. “The discovery of an age difference between the stars suggests that this scenario is the most plausible, and only with the new data from ESO has it been possible to demonstrate this.”
This scenario also explains why one of the system’s stars is magnetic while the other is not – another peculiar feature of HD 148937 spotted in the VLTI data.
At the same time, it helps solve a long-standing mystery in astronomy: how massive stars obtain 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. However, some massive stars are indeed magnetic.
Astronomers have suspected for some time that massive stars might acquire magnetic fields when two stars merge. But this is the first time that researchers have found direct evidence of this phenomenon. In the case of HD 148937, the merger must have occurred recently. “Magnetism in massive stars is not expected to last very long compared to the lifespan of the star, so it appears that we observed this rare event very soon following it occurred,” adds Abigail Frost.
ESO’s Extremely Large Telescope (ELT), currently under construction in the Chilean Atacama Desert, will allow researchers to understand in more detail what happened in the system, and perhaps reveal other surprises.
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