A cosmic mystery has just been solved through violence.
A pair of stars called HD 148937 doesn’t look like we imagine because there were once three. A new study has found that two of the original star trinaries merged, creating not only the beautiful Dragon Egg Nebula in which the stars are embedded, but also two mismatched stars bound together in a gravitational dance.
“When reading the background information, I was impressed by how special this system seemed,” says astronomer Abigail Frost from KU Leuven in Belgium and the European Southern Observatory in Chile.
“A nebula surrounding two massive stars is a rarity, and we really felt like something cool must have happened in this system.” Looking at the data, the coolness only increased.”
The system, located about 3,800 light-years from Earth, consists of two massive stars in a 26-year orbit. And these are huge – one has 29.9 times the mass of the Sun and the other 26.6 times the mass of the Sun. That’s big enough, the theory goes, that every star forms a black hole when it dies.
The larger of the two stars is really curious. Previous research suggests that it has a magnetic field. This is really strange, because this isn’t usually the case for stars larger than seven times the mass of the Sun – they don’t seem to have the convective interiors needed to maintain a magnetic field, unlike smaller stars.
That piqued the interest of Frost and her team. They analyzed nine years of observational data of the system collected by ESO’s Very Large Telescope, as well as archival data from the MPG/ESO 2.2-meter telescope. This allowed them to characterize the stars and their orbits in detail.
And when they looked closer, the researchers noticed something strange. Two massive stars in a binary like this should be about the same age and formed at about the same time in the same star-forming site.
But when the researchers analyzed the chemical composition of the stars – whose profile helps determine their age – they found that the larger of the two stars appears to be about 2.7 million years old and the smaller one at least 4.1 million years old . That’s not an insignificant age difference.
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And then there’s the nebula, which is estimated to be around 7,500 years old. His presence alone is strange enough; Such nebulae are usually emitted by dying stars, but both stars at the nebula’s core are in the prime of their lives.
And then there’s a lot of nitrogen, carbon and oxygen in there. Normally, these elements are trapped inside stars and do not teem outside of them.
A possible explanation for magnetically massive stars is the merger of two smaller stars. Such an event could also release internal stellar material into the surrounding space. Taken together, the evidence suggests that such a merger did indeed occur in HD 148937.
“We believe this system originally had at least three stars; Two of them had to be close to each other at one point in the orbit, while another star was much further away,” says astronomer Hugues Sana from KU Leuven.
“The two inner stars merged violently, creating a magnetic star and ejecting some material, forming the nebula. The more distant star formed a new orbit with the newly merged, now magnetic star, creating the binary star system we see today at the center of the nebula.”
This would make HD 148937 the first convincing evidence that star mergers actually lead to the magnetic fields that we occasionally observe around massive stars. The expected speed of these mergers is consistent with the percentage of massive stars we see that have magnetic fields.
And since these magnetic fields are believed to be very cosmically volatile, this is also consistent with a relatively young time frame for the merger – in line with the estimated age range of the Dragon Egg Nebula.
The team’s research was published in Science.