by Massive stars gain their magnetism by colliding and merging with other stars, according to evidence from a strange binary system surrounded by a dusty, element-rich nebula.
Inside that bipolar nebula, which has the dual designation NGC 6164/6165, is the star system HD 148937. Located about 3,800 light year out, in the southern hemisphere constellation Norma, HD 148937 has two monsters stars in orbit around each other. One of these stars is magnetic and is, in fact, the brightest and hottest massive star known to have magnetic field. That’s interesting because, based on what we know about the interiors of stars, massive stars should no have magnetic fields.
We are used to thinking the sun as a magnetic star, which drives phenomena like sun spotsflares, prominences and the solar wind. The magnetic dynamics is generated inside the sun, at the boundary between the inner radiative layer and the outer convection layer. In the radiative layer, energy is transported from the star’s core via gamma ray photons; at the convective layer, this energy is converted into a stream of hot plasma that rises up to the sun visible surfacewhere the energy is released as light and heat.
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The less massive a star is, the greater proportion of its volume is taken up by the convective layer. The smallest one red dwarf they are almost entirely convective, and are therefore very magnetically active. However, the more massive the star, the smaller the convective layer. This means that the most massive stars are all radiating, and without a convection layer, they cannot generate a magnetic field.
But, somehow, about 7% of the most massive stars have been observed to have a magnetic field. The question that is bothering astronomers is: How?
The answer to this mystery may lie within HD 148937.
“When I was doing some background reading, I was struck by how special this system seemed,” said Abigail Frost of the European Southern Observatory in her press release.
Frost and Hugues Sana of KU Leuven in Belgium led efforts to study HD 148937 in more detail, using nine years’ worth of observations from ESO. A very large telescope Interferometer, which combines the powers of four eight-meter telescopes in Chile.
“After a detailed analysis, we could determine that the giant star appears to be much younger than its companion, which makes no sense because they should have formed at the same time,” said Frost.
The more massive of the two stars, with between 50 and 60 times the the mass of the sun, is the magnetic one. Based on its temperature, it appears to be 1.5 million years younger than its companion. This is a significant age difference for massive stars, which typically only live for a few million years before disappearing supernova.
There is also the nebula to consider; its dipole shape is a reflection of its origin in some kind of explosion from one of the stars in HD 148937 in the last 7,500 years. The IS nebula containing high concentrations of carbon, nitrogen and oxygen, which are elements normally found inside stars, but not outside them.
Frost and Sana’s comments were able to put this star puzzle together. One star in a binary system younger than its companion is a sure sign that a merger of two stars has occurred in the last few thousand years.
“We think this system originally had at least three stars,” Sana said. “Two of them had to be close together at one point in their orbit and another star much further away.”
We can picture HD 148937 as it once was: a close binary on a short orbit, surrounded by a third star. The two stars were closely spiraled together, merging into one. They ejected excess material along their new axis of rotation, forming the nebula. Meanwhile, inside the newly merged star was, and still is, in a state of flux. The stellar material from both stars is ejected, giving the merged star a younger and more youthful complex, and creating a turbulent and convective interior capable of generating the dynamics that give the kilo magnetic field strength. for the star (for comparison, the sun’s magnetic field has an average strength of 1 gauss).
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“The two inner stars combined violently, creating a magnetic star and throwing out some material, which formed the nebula,” Sana said. “The more distant star formed a new orbit with the newly merged, now magnetic star, creating the binary we see today in the center of the nebula.”
The magnetic field will not last; The interior of the star will eventually become completely mixed and settle down to become fully radiative. From there, the magnetic dynamo will be turned off. Not only is this additional evidence that the merger must have occurred recently, but it also puts into context the figure of 7% of massive stars with magnetic fields, suggesting the merger rate in compact binaries .
The findings are reported in a paper published on April 12 in the journal Science.
